Alcohol-based antisepsis: A hand sanitizer monograph

The antimicrobial properties of alcohol were reported first for ethyl alcohol (EA) in 1888 and subsequently for isopropyl alcohol (IA) and N-propyl alcohol (NA) in 1904.^1,2 Although alcoholic antimicrobial activity increases with increasing carbon chain length, these alcohols—also referred to as ethanol, isopropanol and N-propanol—are preferred for their solubility in water.^2,3 The bactericidal activity of these compounds is typically ranked N-propanol > isopropanol > ethanol; however, only ethanol and isopropanol are approved in the United States for use in healthcare antisepsis and disinfection.^2-6 In light of this, although N-propanol is widely used in Europe, this article focuses primarily on ethyl and isopropyl alcohol and attempts to provide an in-depth discussion of the attributes, efficacy, and role of these alcohols in preventing infection.

Mechanism of action

Alcohols are believed to exert their antimicrobial effects primarily by denaturing and coagulating proteins, leading to disruption of cellular metabolism and membrane lysis.^2,6-10 The attraction between the carbon tails of alcohols and hydrophobic protein residues enables alcohols to insert themselves into proteins, where the polar presence of the alcoholic oxygen atom weakens lipophilic interactions between non-polar residues and increases the internal affinity of the structure for water. This decreases the stability of the 3-dimensional protein structure and causes enzymes to fall apart at physiologically relevant temperatures.^11-16

Alcohols appear to permeabilize the cytoplasmic membrane through a similar mechanism.¹⁷ Although this mechanism of action has not been intensively studied, the microbicidal effects of cytoplasmic disruption and protein denaturation are validated by reports linking cell death with loss of protein viability.^18,19 Ultimately, alcohol exposure destroys proteins and disorganizes the cell membrane, causing catastrophic damage that rapidly kills a variety of microbes.

Efficacy

Not surprisingly, the devastating protein denaturation induced by alcohol exposure renders ethanol and isopropanol highly effective antiseptic agents. In vitro, alcohols rapidly eliminate a wide variety of microorganisms and this activity has been shown to extend to in vivo tests as well, where alcohols are generally recognized as the most effective form of hand antisepsis.² While they offer no protection against post-application contamination, alcohols have been shown to reduce bacterial counts on inanimate surfaces as well, providing solid rationale for their use as general healthcare disinfectants and antiseptics.

Unfortunately, while studies indicate that alcohols potently reduce microbial contamination, they have not been shown to have an independently superior effect on infection rates in either the community or healthcare setting. However, alcohols are easy to use, can be placed directly at point-of-care sites, and are equal to or better than other antiseptics at reducing microbial load, making them excellent options for effective, accessible antisepsis and disinfection.

In vitro spectrum

Ethyl and isopropyl alcohol are rapidly microbicidal against vegetative bacteria, fungi, and most viruses.^2,6,7-9,20 This potency is highly concentration-dependent, with antimicrobial effectiveness beginning at concentrations of 30% and maximum efficacy typically achieved at concentrations of 60-95% ethanol and 70-91.3% isopropanol, though decreases in activity towards the higher end of these ranges have been observed.^2,5-9,21 Activity is also enhanced as temperature increases from 10° C to 40° C.²² Against Gram-positive and Gram-negative bacteria, the 15-30 second in vitro reduction factors (RFs) of alcohols antiseptics typically range from 4-8 log₁₀ colony-forming units (CFUs).^23-30 Seventy-percent ethanol is highly effective against mycobacteria—including Mycobacterium tuberculosis—after 30 seconds of exposure and eliminates fungi within 15-20 minutes.^23,31-34 High alcohol concentrations are active against enveloped and non-enveloped viruses, including human immunodeficiency virus (HIV), influenza A, SARS-coronavirus, and hepatitis B/C, though hepatitis A, poliovirus, and MS2 bacteriophage appear to be relatively insusceptible to alcoholic inactivation.^{2,23,30,35-47} Despite this broad antimicrobial spectrum, alcohols are not considered high-level disinfectants due to their lack of relevant sporicidal activity.^7,48-54

In vivo results

In addition to their potent in vitro efficacy, alcohols also demonstrate significant antimicrobial activity in vivo. Since hands are the most significant target for reducing nosocomial infection, dozens of studies have explored the effects of alcohols on both transient and normal bacterial hand flora.⁶ In these studies, ethanol and isopropanol are associated with multilog RFs at a variety of concentrations and exposure times. Alcohols are also effective as disinfectants on a wide variety of surfaces, though specific RFs have not been extensively reported for inanimate surfaces.⁷  Unfortunately, this antiseptic and disinfecting efficacy is highly dependent on exposure time, and since reported in-use application times are typically far shorter than those used in most in vivo studies, log₁₀ RFs seen in clinical practice may not reflect those observed in vivo.⁶ However, while the clinical relevance of most study times is debatable, alcohols are generally superior to other biocides in vivo and are widely considered the standard of reference for antimicrobial efficacy.

Antisepsis

The antiseptic effects of ethyl and isopropyl alcohol are well-established in the literature. After a comprehensive literature review, Kampf et al (2004) estimated that ethanol and isopropanol reduce transient bacterial flora by 2.6-4.5 and 4-6.81 log₁₀, respectively, and resident flora by 1.5-2.4 log₁₀.² An independent analysis of more than 30 studies and over 2,000 data points generally corroborates these ranges (Tables 1 and 2), which appear to be accurate regardless of the method used to contaminate hands.^55-57 Alcohols are also effective against viruses in vivo, and are associated with RFs of 0.9-4.64 log₁₀ at 10-30 second exposure times.^58-62 Although the results of in vivo trials are not entirely consistent, the bacterial and viral RFs of alcohols are generally superior to the RFs of other antiseptics, leading the CDC to recommend alcohols as the preferred agents for general hand antisepsis.^6,63

Like most biocides, the efficacy of alcohols is highly dependent on exposure time. Unfortunately, clinical handwashing duration is typically less than 10 seconds, and most in vivo studies evaluate the efficacy of alcohols at antisepsis times ranging from 30-60 seconds.⁶ To address this discrepancy, Sickbert-Bennett et al evaluated the effects of 61% ethanol on transient Serratia marcescens after 10 seconds of exposure.⁶⁴ Unlike the 2-4 log₁₀ RFs seen at 30-60 seconds of antisepsis, 10 seconds of 61% ethanol use was associated with a 1.55 log₁₀ RF in bacterial load and actually performed worse than 4% chlorhexidine (1.89 log₁₀ RF), 1% triclosan (1.9 log₁₀ RF), plain soap (1.87 log₁₀ RF) and tap water (2 log₁₀ RF). Similarly, another trial reported a 2.15 log₁₀ RF for 70% isopropanol on Escherichia coli at 10 seconds of exposure, although isopropyl alcohol outperformed chlorhexidine (1.96 log₁₀ RF), plain soap (0.5 log₁₀ RF) and tap water (1 log₁₀ RF).⁶¹

These findings indicate that alcoholic antisepsis may not provide the same superior antisepsis observed in most in vivo tests under in-use application times. In contrast, the results of a study by Dharan et al suggest that 15-30 seconds of exposure to alcohols may be associated with even greater effects on transient bacteria (5.03-7.18 log₁₀ RF) than previously described.⁶⁵ However, due to differences in the contamination, disinfection and recovery techniques employed by Dharan and colleagues, these results were excluded from the mean RF value calculations in the following table. Inclusion of this study increases the mean RFs of ethanol and isopropanol to 5.74 (range 2.7-6.8) and 5.66 (range 5.03-6.05) log₁₀ RF, respectively, at 15 seconds application time and 5.37 (range 1.97-6.88) and 4.43 (range 2.9-6.81) log₁₀ RF, respectively, after 30 seconds. Although this study paints alcoholic antisepsis in an even more favorable light, the aberrant nature of the reported RFs also highlights the risk of under or overestimation of in vivo efficacy inherent in current testing methodology.

Further exposing the intrinsic variability of in vivo tests, several studies report unusually low RFs associated with alcoholic antisepsis. In one trial, 61% ethanol was completely ineffective (-0.2 to 0 log₁₀ RF) in eliminating the spores of a Bacillus anthracis surrogate, though this result is not surprising since alcohols lack sporicidal activity in vitro.⁶⁶ The results of two other studies comparing ethanol with a surfactant, allantoin and benzalkonium chloride (SAB) antiseptic are more difficult to explain. After a 2-minute application time on hands artificially contaminated with Serratia marcescens, both SAB antiseptic and 62% ethanol were associated with a 2.6-2.8 log₁₀ RF, while 62% ethanol gel was associated with a 2.1 log₁₀ RF.^67,68 Since ethanol and ethanol gel are associated with a 3-4 log₁₀ reduction of transient bacteria after 60 seconds of exposure, these results seem largely incongruous with established literature.^4,69 The authors fail to address this inconsistency in their article, calling the quality of their study into serious question. Aside from these few studies, however, alcohols perform quite well in most trials and the weight of evidence seems to favor their use as antiseptics at 30 second exposure times.⁷⁰

In addition to their proven antiseptic efficacy, alcohols have significant disinfecting activity. Alcohols are highly flammable and are not recommended for disinfection of large environmental surfaces; however, they can be safely used to disinfect a variety of inanimate objects.^62,103,104 The efficacy of alcoholic disinfection been examined on a variety of medical devices as well as a slew of more routine objects, where ethanol and isopropanol generally demonstrate excellent antimicrobial activity against bacteria and fungi.^62,105 Unfortunately, alcohols have poor activity against sporulated bacteria and some viruses, leading to the recommendation that ethanol and isopropanol only be used to disinfect noncritical healthcare objects.¹⁰⁴

Medical devices are one of several target healthcare vectors for alcohol-based disinfection. In several trials, 70% alcohol outperformed iodine and quaternary ammonium compounds (QACs) and was equivalent or superior to phenolics and formaldehyde for disinfecting oral thermometers, reducing contamination rates by 79.6-97%.^106-108 In contrast, coliform bacteria persisted on 71.2% of contaminated rectal thermometers wiped down with 95% ethanol and soap.¹⁰⁹ Alcohols have also been tested for use in the disinfection of endoscopes, which have been linked to more infection outbreaks than any other medical instrument.^104,110-115 Following 5 minutes of immersion in benzalkonium chloride, immersion of endoscopes in 96% alcohol for 5 minutes is associated with complete elimination of vegetative bacteria.¹¹⁶ However, due to the poor activity of alcohols against spores, alcoholic disinfection of endoscopes is limited to rinsing after high-level disinfection.^114,117,118

Further studies have evaluated the efficacy of alcohols for disinfection of intravascular transducers and ophthalmologic tonometers, both of which have been implicated in nosocomial infection outbreaks.^119,120 For transducers, wiping with 70% alcohol sterilizes 98% of the blood pressure monitors heads and is equivalent to ethylene oxide in terms of positive culture risk and patient outcomes.^121,122 Alcohols also eliminate over 3 log₁₀ of type 1 human immunodeficiency virus (HIV), coronavirus 229E, adenovirus type 5, and parainfluenzavirus type 3 from tonometers and other non-porous surfaces.^123,124 However, 70% isopropanol is ineffective at eliminating adenovirus 8 (0.47-1.07 log₁₀ RF)—a common cause of epidemic keratoconjunctivitis—from inanimate objects and is not recommended for disinfection of tonometers or other ophthalmologic devices.^120,125,126

Alcohols have also been evaluated as disinfectants for a variety of noncritical healthcare objects.  In a study of hospital staff pagers, swabbing with 70% ethanol was associated with a 1.22 log₁₀ RF in bacterial contamination.¹²⁷ Similarly, wiping ultrasound probe heads with 70% ethanol-moistened paper reduced bacterial load by 2.31 log₁₀ CFUs.¹²⁸ Other studies have reported that 89.3% of contaminated scissors and 100% of contaminated stethoscopes swabbed with 70% ethanol are sterile after disinfection.^129,130 Wipe down with 70% alcohol has also been shown to reduce overall herpes simplex virus recovery from CPR manikin heads by 84%, completely eliminating the virus from all but the least accessible test sites.¹³¹ Although limitations in scope of effect prevent alcohols from being considered high-level disinfectants, these studies indicate an important role for alcohols in the disinfection of a wide variety of potential fomites.

Persistent activity

One often-mentioned weakness of alcoholic antisepsis is a lack of persistent activity. Defined by the CDC as “prolonged or extended antimicrobial activity that prevents or inhibits the proliferation or survival of microorganisms after application of the product,” persistent activity is facilitated by the presence of residual biocide on a surface after the initial application.⁶ This persistence is characterized by either continued suppression of surviving bacteria or activity against post-application inoculation. Alcohols are highly volatile and quickly evaporate under normal conditions, making them ideal for rapid antisepsis or disinfection; however, they leave no residue and provide no protective long-term antimicrobial effect. Despite this lack of residual presence, alcohols mildly suppress the regrowth of resident hand flora, but they offer no defense against exogenous recontamination. Unfortunately, since clean skin is quickly reinoculated by contact with contaminated surfaces, the antiseptic effects of alcohols are short-lived under most conditions.

Suppression of normal flora

While antisepsis substantially reduces the number of viable bacteria on the hand, it does not leave skin sterile. Even without external contamination, normal flora regrowth leads to recovery of bacterial load within hours of antisepsis. Bacterial regrowth is particularly important under surgical conditions, where contamination from skin surrounding the surgical site or glove perforation can lead to contamination of the surgical site and subsequent infection. To prevent this, the FDA has mandated that surgical scrubs must maintain hand flora below baseline for at least 6 hours after application.⁵ Alcohols meet this criterion through two mechanisms: 1) they have a mild post-antiseptic effect in vivo, and 2) they produce a decrease in bacterial load that cannot be overcome within 6 hours. Additionally, chlorhexidine gluconate (CHG) is sometimes added to ethyl or isopropyl alcohol in an effort to increase the longevity of their effects on hand flora, though the superiority of this combination is debatable. However, even in the absence of chlorhexidine, alcohols appear to independently inhibit bacterial regrowth long after their initial application.

Resident hand flora recovers by 0.49 log₁₀ CFUs (weighted mean, range -0.29 to 1.3) within 3 hours of a 180-second application of an alcoholic surgical scrub.^84,88,93 Further, a trial of the long-term effects of 61% ethanol reported that despite an initial 2.22 log₁₀ RF, bacterial counts returned to baseline levels within 8 hours.¹⁰¹ In contrast, a trial by Mulberry et al found that 61% ethanol use was associated with a 1.1 log₁₀ immediate RF in general hand flora and a 1.4 log₁₀ RF three hours after antisepsis. Six hours after exposure, however, bacterial counts were only 0.5 log₁₀ lower than baseline.¹⁰⁰ Similarly, another study reported an increased RF (2.77 vs 2.48 log₁₀) on gloved hands 3 hours after application of an 85% ethanol gel.⁸⁸ This post-antiseptic effect may be due to sublethal damage to surviving bacteria that can be overcome on a culture medium, but results in further bacterial death on skin, a hypothesis that may also explain the slow initial regrowth rates observed in these studies.¹³²

Chlorhexidine gluconate, a biguanide antiseptic with very low volatility, is frequently added to alcohol-based products in an effort to improve both immediate and long-term RFs. Problematically, while 93% of applied chlorhexidine remains on the skin for 5 hours, studies of alcoholic chlorhexidine at a 3 minute application time indicate that resident hand flora recovers by approximately 0.83 log₁₀ CFUs within 3 hours of exposure and by 1.6 log₁₀ after 6 hours.^84,94,133 In 1 study, Mulberry et al reported initial, 3-hour and 6-hour RFs of 2.5 log₁₀, 2.9 log_10, and 2.5 log₁₀, respectively, against general hand flora; however, it should be noted that many chlorhexidine studies suffer from inadequate validation of utilized neutralizing agents, potentially causing a 2 log₁₀ overestimation of antiseptic efficacy.^{4,100,134-137} Regardless of neutralization validation, alcoholic chlorhexidine appears to lack any advantage over plain alcohol in terms of immediate effect and is often associated with a more rapid recovery of bacterial load, seriously undermining arguments favoring the use of this antiseptic combination.

Alcohols, alone or in combination with chlorhexidine, have also been shown to suppress general flora on other clinically relevant skin sites. Since baseline microbial loads for these sites are several log₁₀ lower than typical hand counts, alcoholic antisepsis results in near-complete bacterial elimination, making recovery more difficult and thus amplifying the long-term effects of alcohols. In 4 trials, inguinal or abdominal antisepsis with 70% isopropanol led to a mean 2.78 log₁₀ initial RF, 2.58 log₁₀ 6-hour RF and a 2.09 log₁₀ 24-hour RF. Addition of 2% chlorhexidine increased these values to 2.87 log₁₀ initial RF, 2.76 log₁₀ 6-hour RF and 2.89 log₁₀ 24-hour RF.^138-140 By comparison, using 89.5% N-propanol on the arms or back caused a mean 2.1 log₁₀ initial RF and 0.35 log₁₀ 72-hour RF, whereas general skin flora remained 1.27 log₁₀ lower than baseline at 72 hours when N-propanol was supplemented with 2% chlorhexidine.¹⁴¹ In contrast, while 60% isopropanol initially reduced inguinal carriage of Proteus mirabilis by 2.15 log₁₀, 4% chlorhexidine had no meaningful effect and bacterial load for both agents returned to baseline within 4 hours.¹⁴² The combined results of these trials indicate that alcohols have a temporary suppressive effect on normal flora that may be mildly improved in combination with chlorhexidine.

Contamination prevention

Unfortunately, while alcohols appear to exert suppressive effects on surviving flora, the long-term effects of alcohols are also countermanded by exogenous contamination. Studies of fingertip recontamination after hand antisepsis report that healthcare staff acquire an average of 16-20 CFUs/minute during routine patient care.^143,144 Assuming an average fingertip surface area of 1 cm², the recontamination rate for the palmar surface of a whole hand may exceed 150 CFUs/minute, an estimate that is supported by reports that nurses may acquire from 100-6000+ CFUs during brief, ostensibly “clean” activities such as lifting, touching or taking a pulse from an infected patient.^145,146 Baseline hand counts range from 3.9 x 10⁴ to 4.6 x 10⁶ total bacteria and should be reduced to 10-400 bacteria by alcoholic antisepsis.^2,147 Using these values, regrowth times for general flora and a linear contamination rate of 150 CFUs/minute, hand flora will recover by 0.74-2.19 log₁₀ CFUs within 10 minutes and reach 3.9 x 10⁴ bacteria approximately 90 minutes after antisepsis.⁹⁴

This rapid recontamination rate highlights the primary weakness of alcohol-based antisepsis: a lack of protective effect. Lowbury et al reported greater post-application inhibition of an inoculum of 5.1 x 10³ Staphylococcus aureus after plain soap handwashing (0.12 log₁₀ RF) than after use of 70% ethanol (0.03 log₁₀ RF). Inoculation with 78,500 bacteria resulted in a 0.25 log₁₀ RF for plain soap and a -0.01 log₁₀ RF for ethanol.⁸² Similarly, Herruzo et al described a mean post-application RF of 0.3 log₁₀ for 60% isopropyl alcohol against inoculums of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. A combined alcohol-quaternary ammonium compound-chlorhexidine product, however, had a mean 3.9 log₁₀ residual RF.⁷⁸  Alcoholic chlorhexidine was also associated with a 4 log₁₀ post-application RF in another study that showed no residual activity for 60% isopropanol, although both trials failed to sufficiently validate their neutralizing agents.¹⁴⁸ These studies demonstrate the limitations of alcoholic antisepsis and indicate the potential benefits to supplementation with a less volatile antimicrobial.

Clinical trials

Despite their lack of protective effect, in vivo studies generally indicate that alcohols are superior to most other antiseptics. In an effort to link this efficacy to clinical outcomes, alcohols have been studied in a variety of trials evaluating their effects on hand contamination and infection rates in surgical, general healthcare and community settings.⁶ Although many of these studies suffer from limitations in design, size and scope, they generally fail to directly link alcoholic antisepsis to the ultimate goal of hand hygiene: reduced infection.¹⁴⁹ Instead, the evidence indicates that under general use conditions, alcohols may reduce infection rates chiefly by improving handwashing compliance. Unlike soaps, alcohols are effective without water and can be strategically placed to facilitate point-of-care antisepsis.  As a result, while the superiority of alcoholic antisepsis is unproven, alcohols increase ease of use and, by extension, handwashing compliance. This explanation helps to explain the conflicting results of efficacy trials and offers rationale for the continued use of alcohols despite disappointing clinical outcomes.

Surgical scrubbing

While numerous trials verify the in vivo effects of isopropanol and ethanol on perioperative bacterial counts, there is a lack of evidence to prove that alcoholic antisepsis reduces infection rates to a greater extent than other surgical scrubs. Since conducting a placebo-controlled trial on the effects of alcohols on nosocomial infection rate is ethically unreasonable, most studies compare antiseptic agents for effects on either microbial load or infection rate.¹⁵⁰ However, in a test of antiseptic activity on murine surgical sites inoculated with highly virulent pneumococcal bacteria, ethanol and isopropanol decreased mortality from 100% to less than 15%.¹⁵¹ Unfortunately, clinical results are far less conclusive in humans. Although alcoholic antisepsis appears to provide superior perioperative reductions in hand contamination, the long-term effects of alcohols on either baseline bacterial load or surgical site infection rates appear to be equivalent to other antiseptic agents.

As preoperative antiseptics, the clinical effects of alcoholic antisepsis on bacterial load appear to be generally superior to most antiseptic agents. In one study, rubbing with 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate for 3 minutes reduced hand colonization by 2.05 log₁₀ CFUs immediately and maintained a 1.66 log₁₀ RF through the end of a 15-60 minute surgery, as compared to a 1.58 log₁₀ immediate and 1.12 log₁₀ post-operative RF using 7.5% povidone-iodine (P = 0.02-0.04).¹⁵² A separate study reported a 2.4 log₁₀ initial and 1.8 log₁₀ post-surgery RF for the same alcohol-preservative combination, whereas 4% chlorhexidine was associated with a mere 1.3 log₁₀ initial and 0.9 log₁₀ post-operative RF (P < 0.001).⁷⁷ In another study, alcohols demonstrated better post-operative dampening of bacterial regrowth than 7.5% povidone-iodine or 4% chlorhexidine (4.7 vs 5.2 log₁₀ CFUs, P = 0.017) at surgery times > 3 hours.¹⁵³ These results show that despite a lack of persistent presence on the skin, alcohols are clinically equivalent or superior to other surgical scrubs in terms of perioperative bacterial counts.

In spite of their superior effects on hand counts, alcohols do not offer a clear benefit over other antiseptic agents in terms of baseline bacterial load or the incidence of surgical-site infections. Three weeks of antisepsis using 1% chlorhexidine in 61% ethanol has been shown to reduce prescrub flora by 0.54 log₁₀ CFUs; however, use of 4% chlorhexidine also reduced baseline counts by 0.51 log₁₀ CFUs (P = 0.08).¹⁵⁴ In another hospital study, changing to a 3-minute, 70% ethanol-based rub slightly increased the incidence of surgical site infection (1.33% vs < 1%).¹⁵⁵ The clearest data stems from a 16-month clinical trial comparing the 30-day incidence of surgical site infection (SSI) after antisepsis with 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate or either 4% povidone-iodine or 4% chlorhexidine. In this crossover study, infection occurred after 53 of 2,135 surgeries (2.44%) when hands were sanitized using the alcohol-preservative combination and after 55 of 2,252 surgeries (2.48%) using standard surgical scrubs.¹⁵⁶ The incidence of SSIs was statistically equivalent for alcohols, povidone-iodine and chlorhexidine (P < 0.01). Based on these results, there appears to be poor correlation between reduction of hand flora and decreases in infections. Alcohols are more effective on bacterial load than other antiseptics; however, that efficacy does not directly translate into fewer surgical infections.

Hygienic hand antisepsis

Alcohols have also been widely studied as general antiseptics in both the healthcare and community setting. As general healthcare antiseptics, the effects of alcohols on bacterial load and nosocomial infection rates are similar to those reported for their use as surgical antiseptics. Hand antisepsis with alcohol-based products is associated with lower baseline bacterial load than reported for plain soap; however, alcohols have not been shown to independently reduce hospital-acquired infections.¹⁴³ Several clinical trials have also shown benefit to implementation of an alcohol-based hand hygiene program, but these results appear to be largely due to increased handwashing compliance rather than improved antiseptic efficacy. Similarly, alcohols also appear to reduce community-acquired infections by facilitating quick, accessible hand sanitization, although the quality of most community-based studies is debatable and comparison studies between alcohols and other antiseptics are lacking.¹⁵⁷ Overall, while alcohols reduce hand contamination to a greater extent than plain soap, their impact on healthcare and community infection rates is mild and appears to derive mainly from their beneficial effects on handwashing compliance.

Contamination

In terms of bacterial elimination, alcoholic antisepsis has repeatedly demonstrated superiority over plain soap, although proof that alcohols are more efficacious than other antiseptics is limited. Alcohol is more effective than soap at ridding hands of Gram-negative bacteria, and has been shown to prevent transfer of Gram-negative bacteria from contaminated hands to sterile catheters in 10/12 experiments, while plain soap was effective in only 1/12 tests.^146,158 Prior to training in antiseptic technique, glycerol in 70% isopropanol gel and plain soap were associated with respective 0.52 log₁₀ and 0.05 log₁₀ RFs, which increased to 0.68 log₁₀ and 0.4 log₁₀ RFs, respectively, with technique standardization.¹⁵⁹ In a Russian neonatal intensive care unit (ICU), use of a 79% ethanol/0.1% quaternary ammonium compound hand rub instead of plain soap reduced the Klebsiella pneumoniae colonization rate from 21.5 colonized patients/1,000 patient days to 3.2 patients/1,000 patient days. The colonization rate for other pathogens—especially Candida albicans—was also reduced, albeit to a lesser extent.¹⁶⁰ In other studies, alcoholic antisepsis reduced pathogen isolation from 68% to 25% on hands with artificial fingernails and substantially reduced transient organism recovery on the hands of nurses wearing rings.^161,162 These results validate the current preference for alcoholic antisepsis over handwashing with plain soap.⁶

In addition to the preceding studies, 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate has proven superior to plain soap antisepsis in multiple European hand contamination studies. In 4 trials, this combined alcohol-preservative product was associated with a 0.93-1.7 log₁₀ RF on fingertips, which was significantly greater than the 0.3-0.74 log₁₀ RF decrease in contamination observed with soap (P ≤ 0.0003).^163-166 Another trial actually reported a 0.12 log₁₀ increase in bacterial load for plain soap versus a 0.34 log₁₀ decrease for the alcohol-antiseptic, while a study of infection control professionals using 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate reported a mean RF of 2 log₁₀ (range 0-3.85 log₁₀).^167,168 For both soap and the alcohol-based product, however, bacterial counts appear to return to baseline within 10-30 minutes of antisepsis.¹⁶³ Although N-propanol is not approved for use in the United States, these well-designed trials are included here as further evidence of the superiority of alcohol-based antisepsis on bacterial load.

Despite their generally accepted superiority over plain soap, alcohols do not offer a clear clinical advantage over other antiseptics. For example, a 4-week study reported no effect on baseline nurse hand flora for either 60% isopropanol or 4% chlorhexidine.¹⁶⁹ Alcohols are also non-superior to other antiseptics for reducing false-positive rates during blood culture sampling and are statistically equivalent to other antiseptics in interrupting the transfer of bacteria from contaminated hands to sterile fabric.^170,171 Furthermore, 1 study reported a 1.4 log₁₀ fingertip RF following use of 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate, whereas handwashing with 10% povidone-iodine or 4% chlorhexidine resulted in a 1.13-1.2 log₁₀ RF. This difference was statistically irrelevant.¹⁶⁴ In contrast, a trial of hand antisepsis during patient care reported a significantly greater (P = 0.012) median reduction in bacterial load after a 30-second application of this alcohol-antiseptic than after use of 4% chlorhexidine (0.77 log₁₀ vs 0.49 log₁₀ RF).¹⁷² Although alcohols do not appear to offer a definitive advantage over other antiseptics, it should be noted that there is a conspicuous lack of quality trials comparing the effects of various agents on hand contamination. Based on the available data, however, alcoholic antisepsis appears to be superior to plain soap handwashing, but its place in the hierarchy of antiseptic efficacy has yet to be established.

Healthcare infection rates

Similar to their effects on hand contamination, there is a lack of clear evidence that alcohols independently reduce infection rates to a greater extent than other forms of hand antisepsis. While many studies show an infection-rate benefit to alcohol-based antisepsis, the results of these studies are complicated by a simultaneous increase in handwashing compliance. Since a 12-40% improvement in handwashing compliance—independent of changes in antiseptic regimen—has been shown to decrease nosocomial infections by 41-45%, the direct effects of alcoholic antisepsis on infection rates are difficult to extricate from an increase in compliance.^173,174 A 5-year trial reported a 57% reduction in cases of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia 3 years after initiating a hand hygiene program utilizing alcoholic chlorhexidine (P = 0.01).¹⁷⁵ However, hand hygiene compliance doubled (21% vs 42%) during the first 12 months of the study. Improved compliance (56% to 95%) also confounded the results of another trial, where conversion from 2% chlorhexidine to an alcohol foam reduced the rate of Clostridium difficile colitis from 4.96 infections/10,000 patient days to 3.98 infections/10,000 patient days (P = 0.0036).¹⁷⁶ While these studies demonstrate that alcoholic antisepsis can positively affect infection rate, this benefit may be due to improved handwashing compliance rather than superior effects on bacterial load.

The hypothesis that increased compliance is the primary factor driving decreased infection rate is further supported by the results of trials with stable handwashing compliance. In a 2-year crossover trial of 2,826 infants in 2 neonatal ICUs, alcoholic antisepsis was associated with 12.1 infections/1,000 patient days. Handwashing with 2% chlorhexidine was associated with 9.5 infections/1,000 patient days. However, after adjusting for birth weight, study site, surgery and follow-up time, both biocides were statistically equivalent.¹⁷⁷ Similarly, an 8-month crossover trial of 1,894 patients in 3 ICUs reported 38 infections/1,000 patient days when 4% chlorhexidine was used and 50.7 infections/1,000 patient days with the use of 60% isopropanol. This difference, however, was statistically nonsignificant and did not affect length of stay or mortality.¹⁷⁸ During a 1-year interventional study with stable handwashing compliance, MRSA infections decreased by less than 5% and vancomycin-resistant Enterococcus (VRE) infections actually increased by 12-13% despite a doubling in use of an alcohol foam product.¹⁷⁹ On the other hand, 2  trials reported no difference in hospital-acquired infection rate between alcoholic antisepsis and plain soap handwashing despite near-doubled compliance.^158,180 These results indicate that compliance may be a better, albeit imperfect, independent predictor of nosocomial infections than use of alcohol-based products.

Internationally, the increased handwashing facilitated by alcohol-based antiseptics has been shown to dramatically reduce nosocomial infections under conditions where soap-and-water handwashing is impractical. Nosocomial infections fell from 13.1% to 2.1% in a Vietnamese trial after implementation of an alcohol-centered handwashing program as handwashing compliance increased from approximately 0% to 43-71%.¹⁸¹ In another Vietnamese trial, SSIs decreased from 8.3% to 3.8% with use of a 70% isopropanol/0.5% chlorhexidine antiseptic, whereas SSIs increased from 7.2% to 9.2% in the control group. Although compliance rates were not reported, baseline hand antisepsis rates were likely low, since only 1 sink was available in the 60-bed surgical ward.¹⁸² By comparison, during an 8-year Norwegian trial of 27,284 patients, the prevalence of nosocomial infections only fell from 7.9% to 7.1% while use of an ethanol antiseptic increased 7-fold (P = 0.04).¹⁸³ Taking these findings into account, it seems likely that alcohols reduce nosocomial infections primarily by increasing the speed and accessibility of antisepsis, an effect that becomes more apparent when handwashing compliance is low.

Community infection rates

Although less research has been conducted regarding the effects of alcoholic antisepsis on infection rates in the community than in healthcare, available evidence indicates that regular use of alcohols may decrease illness. As in the healthcare environment, this benefit appears to stem largely from increased handwashing compliance. The effects of alcoholic antisepsis on community-acquired infections have been studied most in elementary schools, where transmission of infectious pathogens increases student and parental absenteeism from school and work.¹⁸⁴ Despite repeated instruction and encouragement, handwashing compliance is remarkably poor among school-age children. For instance, only 54% of middle- and high-school students wash their hands after using the restroom, 37% wash for > 5 seconds and a mere 18% use soap.¹⁸⁵ By facilitating improved handwashing compliance among students, alcoholic antisepsis has the potential to substantially reduce scholastic infection rates. For example, in a study of 290 kindergarten-to-3rd grade students in 5 schools, classrooms equipped with a bottle of 62% ethanol reported 1.05 illness-related absences/100 student days, whereas classes without hand sanitizer had only 2.08 absences/100 student days.¹⁸⁶ Similarly, a controlled study of 138 students reported a 28% reduction in absences due to illness after installation of an alcohol dispenser.¹⁸⁷

Despite the positive finding of these trials, increased access to alcohol-based hand sanitizers is not conclusively associated with decreased illness.¹⁵⁷ The strongest evidence comes from a trial incorporating 6,080 students from 4 states, 5 school districts, and 18 schools. In this trial, classroom use of 62% ethanol reduced year-long student absences by 19.8% and teacher absenteeism by 10.1%. While these overall results were statistically significant, analysis of individual school districts showed that only 3 of 5 districts actually reached a statistically relevant reduction in student absenteeism. One district actually reported a nonsignificant 3.75% increase in absences with use of the alcohol product.¹⁸⁸ Similarly underwhelming results have also been reported in several other trials. In a study of 253 students, classes with alcohol dispensers averaged 3.45 absences/100 student days, while control classes averaged 3.75 absences/100 student days.¹⁸⁹ A study of 360 second-to-third grade students reported no difference in absenteeism between plain soap and alcoholic antisepsis.¹⁹⁰ Further, while an 8-week study of 285 students reported a statistically significant 8% decrease in school absences due to gastrointestinal illness with use of 70% ethanol, it also reported a nonsignificant 2% increase in respiratory illness for the test group.¹⁹¹ Based on these results, alcoholic antisepsis does not appear to be the cure for the common cold; however, increases in hand hygiene facilitated by alcohol use may help reduce community infection rates.¹⁹²

Hindering factors

A variety of factors may help to explain the lack of correlation between the in vitro, in vivo and clinical efficacy of alcohol-based antisepsis. As mentioned previously, in-use application times are typically too short to eliminate hand flora to a greater extent than plain soap handwashing.^6,64 Applied volume and inoculum size are also important considerations, since 3 mL of alcohol has been shown to cause a 0.27-1.28 log₁₀ greater RF than 1 mL of alcohol and increasing bacterial load from 10³ to 10⁶ CFUs/fingertip reduces alcoholic RFs by 0.24-1.83 log₁₀.^72,73,91 Furthermore, practical application technique rarely resembles the regimented procedures used during in vivo tests. One recent study reported that only 31% of tested healthcare workers properly applied an alcohol hand rub, resulting in a mere 1.4 log₁₀ RF. After hand rub training, antibacterial efficacy rose to 2.2 log₁₀ RF.¹⁹³ Other personnel-related factors include artificial fingernails or ring wearing, which have been shown to significantly decrease the effectiveness of alcoholic antisepsis.^161,162 Overall, these factors seriously undermine the efficacy of alcohols and may limit their clinical effectiveness.

In addition to personnel-related factors, alcohols have several inherent limitations. Alcohols affect microbes by disrupting proteins and are inhibited by extraneous protein, leading to recommendations against their use on visibly dirty surfaces.^{6-9,20,32,33,103} Product formulation may also reduce efficacy, since alcohol gels appear to be about 1-2 log₁₀ less effective in vivo than their rinse counterparts (see Table 1).^{64,76,77,87,166,194} Furthermore, dried bacteria are less susceptible to alcohols, potentially enabling desiccated microbes to better survive surface disinfection.^21,49 Infection outbreaks have been linked to alcohol solutions contaminated with Bulkholderia cepacia—which has an unusual ability to metabolize alcohols—and sporulated Bacillus cereus.^195,196 Lastly, while not related to clinical efficacy, the flammability of alcohols also limits their use as disinfectants.¹⁰³ At concentrations ≥ 70%, ethyl and isopropyl alcohol have flash points lower than 21° C and are considered “easily flammable;” however, the reported incidence of alcohol-related fires is extremely low.^6,70,197 Failure to account for these limitations may also help to explain the disconnection between in vivo and clinical results.

Biofilms

Another hindering factor of alcoholic antisepsis and disinfection is poor efficacy against biofilms. Biofilms form readily on almost any surface and are associated with an estimated 65% of nosocomial infections, making them a significant consideration in infection prevention.^198-200 Unfortunately, an in vitro study reported that 1 minute of exposure to 70% alcohol reduced the number of viable biofilm Pseudomonas aeruginosa and Staphylococcus aureus by a mere 0.59-0.96 log₁₀ and 0.96-1.22 log₁₀ CFUs, respectively. After a full hour, ethanol decreased both strains by 1.4-2 log₁₀ CFUs and isopropyl alcohol reduced Staphylococcus aureus by only 1.9 log₁₀ CFUs. However, 70% isopropanol was relatively effective against Pseudomonas aeruginosa, causing a > 5 log₁₀ RF after 30 minutes of exposure.²⁰¹ Isopropanol also appears to be efficacious against Staphylococcus epidermidis biofilms, killing 5.3 log₁₀ bacteria within 30 seconds.²⁰² In contrast, 70% ethanol only reduces sessile Salmonella Typhyimurium by about 2 log₁₀ CFUs after 1 and 5 minutes of exposure.²⁰³ Furthermore, fungal biofilms also demonstrate decreased susceptibility to ethanol.²⁰⁴ Given the ubiquitous nature of biofilms, the generally poor efficacy of alcohols against sessile microorganisms may seriously undermine the clinical impact of these disinfectants.

Toxicity

While the clinical efficacy of alcohols is somewhat limited by personnel-related factors, the presence of organic material and biofilms, toxicity does not appear to be a significant hindrance to their use. Unfortunately, despite their excellent safety profile, alcohols suffer from the popular belief that they damage skin.²⁰⁵ This stigma is likely derived from the burning sensation experienced when alcohols are applied to pre-damaged skin.²⁰⁶ Alcohols do solubilize sebum and lipids in the epidermis; however, the majority of studies indicate that alcohol-based antisepsis is less drying and better tolerated than plain soap or other antiseptics.^{23,67,154,167,169,177,207} Additionally, alcohols can be combined with a variety of emollients to reduce skin irritation and most healthcare workers consider them to be acceptable for routine use.^{155,163,166,208,209} Since the Environmental Protection Agency (EPA) considers ethanol to be practically non-toxic to both humans and animals, alcohols appear to be environmentally and clinically safe antimicrobial agents.²¹⁰

Conclusion

After more than a century of scrutiny and research, alcohols remain among the safest, most effective biocides available and are used for a remarkably wide variety of disinfecting and antiseptic purposes. Given their potent, largely irresistible mechanism of action, broad antimicrobial spectrum and superior in vivo efficacy, alcohols appear to deserve their status as the preferred antiseptic agent. Unfortunately, in spite their many strengths, ethanol and isopropanol have not been shown to independently reduce infection rates. Rather, they appear to mitigate infection primarily by improving handwashing compliance. Since the greatest weakness of alcohol-based antisepsis and disinfection appears to be a lack of residual presence, efforts to further reduce community and nosocomial infection rates should focus on providing inter-application antimicrobial activity. Until a product with protective, persistent activity reaches the market, however, alcohols will likely remain the most effective and accessible antimicrobial agent available.

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