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Effect of Water Exposure on Contact Lens Storage Case Contamination in Soft Lens Wearers

Arshad, Memoona BSc, MPhilOptom, PhD1; Carnt, Nicole BOptom, PhD, FAAO1; Tan, Jacqueline PhD, BOptom1; Stapleton, Fiona BSc, MSc, PhD, FAAO1∗

Author Information
doi: 10.1097/OPX.0000000000001772


A contact lens storage case is the most frequently contaminated contact lens care accessory,1,2 with contamination rates of used storage cases ranging from 50% to as high as 85%.1,3–5 Microbial contamination of contact lens storage cases is associated with contact lens disease ranging from sterile infiltrates to microbial keratitis, including Acanthamoeba keratitis.6–10 The causative organism in microbial keratitis including Acanthamoeba and Pseudomonas aeruginosa has been recovered from used contact lens and/or storage case.8,11,12 Pathogenic microorganisms from the storage case may contaminate the contact lens, which then transfers these microorganisms to the eye, initiating contact lens–related adverse events.12,13

Hygiene compliance–related risk factors for storage case contamination include poor hand hygiene,12,14 failure to clean the storage case properly,15–17 and exposing the storage case to nonsterile water.18,19 Identical strains of free-living amoebae including Acanthamoeba have been recovered in the household tap water supply and the contact lens storage cases of patients with Acanthamoeba keratitis.20 Water exposure during contact lens handling can transfer free-living protozoa Acanthamoeba and water-borne gram-negative microorganisms such as Pseudomonas aeruginosa to the contact lens storage case.18,19 In addition, toxins produced by microorganisms such as endotoxin may cause inflammatory adverse events.21

Previous studies have found an association between demographic factors, such as years of lens-wearing experience, and contact lens hygiene habits, such as not washing hands before handling lenses, mismatched disinfecting solution and storage case, and the use of peroxide solution, with increased levels of storage case contamination.13,16 However, no recent prospective study has reported on associations between contact lens wearers' water contact behaviors and storage case contamination in soft lens users, specifically gram-negative contamination. Water-borne gram-negative bacteria have been associated with sight-threatening contact lens–related adverse events.6,22,23 This study aimed to explore associations between water contact behaviors of community-based soft contact lens wearers using frequent replacement lenses and overall level of storage case microbial bioburden, as well as endotoxin levels of storage cases, considered a surrogate for gram-negative bacterial contamination.


This study was a subanalysis of a single-center clinical trial conducted between June 2017 and December 2018, as approved by the University of New South Wales Human Research Ethics Committee (approval reference number HC16735), and all procedures were conducted in accordance with the tenets of the Declarations of Helsinki. In the clinical trial, 200 healthy contact lens wearers using frequent replacement lenses (defined as regular replacement of lenses on either of a 1-, 3-, or 6-monthly schedule) were enrolled. For this study, data from 165 daily wear soft lens users have been analyzed and reported. Written informed consent was obtained from a convenience sample of 165 healthy daily-wear soft contact lens wearers 18 years and older who were using frequent replacement lenses (defined as regular replacement of lenses on a 1-, 3-, or 6-monthly schedule). All other lens types including daily disposable lens users, rigid gas-permeable lens wearers, and those wearing lenses because of keratoconus and/or other ocular pathologies were excluded from this analysis. Participant recruitment was conducted at the University of New South Wales Optometry Clinic by disseminating the study advertisement in the clinic and other university catchment areas.

All potential participants underwent a preliminary ocular assessment including visual acuity measurement and slit-lamp biomicroscopy by the study investigator (same investigator for all participants) to rule out any exclusion criteria specific to the study, including any ocular pathology such as keratoconus. Study participants (N = 165) completed a self-administered questionnaire (Appendix 1, available at regarding contact lens–related hygiene habits and water exposure during contact lens wear.24,25 A composite lens self-reported hygiene compliance score was used based on a similar protocol to that reported previously.24,26,27 Briefly, four key areas of lens hygiene were scored including lens disinfection (0 to 20), hand hygiene (0 to 8), storage case hygiene (0 to 6), and storage case replacement schedule (0 to 6; Appendix Table A1, available at Lens hygiene compliance was classified as “excellent” for a score higher than 35, indicating that participants reported that they “always disinfected their lenses after use, washed hands before handling contact lenses, cleaned storage case and replaced the case every month.” An overall score between 28 and 35 was classified as moderate compliance and indicated that participants reported that they “always disinfected their lenses and partially maintained storage case hygiene,” and a score higher than 28 was classified as poor compliance because it indicated that participants reported that they “sometimes” disinfected their lenses or skipped disinfection, and/or missed handwashing/lens care steps before handling their contact lenses.

Water exposure was considered an important variable for evaluation, and a separate scoring system was devised, including questions related to the use of tap water during contact lens care, use of wet hands to handle lenses, and showering or swimming while wearing lenses with/without swimming goggles. A total score of 0 to 8 was calculated based on the individual water contact behaviors, where all possible behaviors within a task category were considered, and the highest score was assigned to the riskiest behavior, whereas a score of zero was assigned to the behavior considered to have the lowest risk (Appendix Table A2, available at Water contact behavior was subsequently categorized such that no/minimal water exposure (≤1 score) was classified as good water contact behavior, and all other water exposures (>1 score) were classified as poor water contact behavior. All these variables including wearers' demographics, contact lens and disinfection system types, lens hygiene compliance, and water contact behaviors were analyzed to explore their relationship and association with contact lens case contamination.

Microbial Analysis of Storage Cases

The storage cases were collected after study enrollment and administration of research questionnaire and were stored at 2 to 4°C immediately, and microbial analysis was performed within 48 hours as described previously.27 Briefly, the lid of the storage case was removed under aseptic conditions, and any residual solution in the storage case was discarded. The right well of the case was used for the adenosine triphosphate (ATP) assay (Bactiter Glo; Promega, Sydney, New South Wales, Australia) for overall bacterial bioburden of the storage case as per the manufacturer's guidelines.28 A standard curve of the ATP assay was developed in a pilot experiment (unpublished data) using Pseudomonas aeruginosa 6294 (a well-characterized strain from a corneal ulcer),29Serratia marcescens ATTC 13880 (strain used in ISO testing), and Staphylococcus aureus 031 (retrieved from the contact lens of a patient with sterile corneal ulcer). These organisms are known storage case contaminants, and the strains have been implicated in contact lens adverse events and widely used during in vitro testing of lens care products.30 The luminescence values obtained from the ATP assay were converted into log colony-forming units (CFU)/mL from the standard curve. The ATP assay showed better repeatability for low numbers of bacterial cells in the pilot experiment (lowest detection limits of 2.19 log CFU/mL, coefficient of variance of 5.1%; unpublished data).

The left well was used for conducting the limulus amebocyte lysate (LAL) assay (Pyrochrome; Associates of Cape Cod, Inc., East Falmouth, MA) to determine endotoxin levels of the storage case, as a surrogate for gram-negative bacterial contamination. For all one-well basket storage cases used for hydrogen peroxide–based disinfection systems, the biofilm removal and assays followed the same protocol as described previously, except that the biofilm was removed in the LAL reagent water instead of sterile phosphate-buffered saline with 1% Luria broth solution, and after inoculation of the sample for the LAL assay, 1% Luria broth was added to the storage case to prepare the sample for the ATP assay. The inner basket and inside rims of the storage case were considered as the “well” of the case. The LAL assay was performed per the manufacturer's guidelines (Pyrochrome; Associates of Cape Cod, Inc.),31 and the average absorbance values from three repeats for each sample were converted into endotoxin units (EU) per milliliter.

Safe endotoxin limits for sterile medical devices are considered to be less than 0.5 EU/mL32; however, tissue culture–based and in vivo animal-based studies with endotoxin contamination less than 2 EU/mL showed minimum or no adverse response.33,34 Because contact lens storage cases are not generally considered sterile devices because of their repetitive handling, the endotoxin levels ≤2 EU/mL were considered minimal and safe, and cases with these endotoxin levels were categorized as “low endotoxin levels” and those cases with endotoxin levels higher than 2 EU/mL were categorized as “high endotoxin levels.”

In a subset of 85 storage cases of all 165 soft lens cases, Acanthamoeba identification was performed using real-time polymerase chain reaction. For polymerase chain reaction, the solution from right and left wells of the storage case was combined to extract DNA using Chelx resin (MB Chelex-100 resin; Bio-Rad Laboratories, Hercules, CA) following the method described by Iovieno et al.35 Briefly, 500 μL of the solution from each case was centrifuged at 10,000g for 5 minutes to form cyst suspension, and 200 μL of Chelex solution (10% [wt/vol]) in 0.1% Triton X-100 and 10 mM Tris buffer (pH 8.0) was added to cyst suspensions. This solution was vortexed for 10 seconds, centrifuged at 10,000g for 10 seconds, and then heated at 95°C for 20 minutes and finally centrifuged at 10,000g for 20 seconds. This supernatant was used as the substrate for a polymerase chain reaction (4 μL). Acanthamoeba castellanii ATCC 30868 was used as a positive control. The Acanthamoeba genus-specific primers used for polymerase chain reaction were as follows:

  • forward primer 5′-TCTCACAAGCTGCTAGGGCGTCA-3′
  • reverse primer 5′-GTCAGAGGTGAAATTCTTGG-3′

Statistical Analysis

Software used for data management and analysis included Microsoft Excel 2016 (Microsoft Corporation, Redmond, WA) and SPSS, version 24.0 (IBM, New York, NY). Hygiene habits such as storage case cleaning, handwashing, showering, swimming, using wet hands to handle lenses, and using tap water to clean lens/storage case were investigated using descriptive statistics (frequencies and percentages). For microbial contamination, an independent t test compared the overall bacterial bioburden between low and high endotoxin levels. Linear and logistic regressions (univariate and multivariate) were used to examine whether the level of storage case contamination (overall and endotoxin) was associated with contact lens wearers' hygiene habits and water contact behaviors. Based on previous studies in a similar area,36,37 all variables with P ≤ .20 in the univariate analyses were included in the multivariate model. Stepwise linear regression and forward-conditional logistic regression models were conducted to determine independent risk factors for overall storage case bacterial bioburden and endotoxin levels. The final multiple regression model provided the power of 80% (α = 0.05). Lens hygiene compliance and water exposure score were two separate scoring systems consisting of individual variables already present in the model. Therefore, these two variables were not added into the multivariate models.


Of the 165 participants wearing soft contact lenses, the average age (standard deviation) of participants was 28 (13.5) years (range, 18 to 78 years). Sixty-five percent of the participants were female. Of the 165 participants, 129 (78.2%) were using a 1-monthly soft lens and 27 (16.4%) were using a 6-monthly replacement plan (9 participants did not provide this information in the self-reported questionnaire). Ninety-three participants (56.4%) reported using conventional hydrogel lens materials, and 36 (21.8%) reported using silicone hydrogel lens materials, whereas 36 participants (21.8%) did not mention lens material/brand in the self-reported questionnaire. One hundred fifty-seven participants (85.2%) were using a multipurpose disinfection system, whereas eight participants (4.8%) were using a hydrogen peroxide-based disinfection system during study enrollment period. Eighty-five participants (51.6%) reported having more than 5 years of lens wear experience, and 80 participants (48.4%) reported lens wear experience of 5 years or less before study enrollment. The detailed information about the participants' demographics and contact lens care history is provided in Appendix Table A3, available at

Based on the self-administered risk factor questionnaire response and overall lens hygiene compliance scoring, 29 participants (17.6%) were rated as having excellent compliance, whereas 78 (47.3%) had moderate, and 58 (35.2%) had poor self-reported hygiene compliance. Because all participants were using a daily wear soft lenses, 41 participants (24.8%) reported incidental sleeping overnight while wearing their contact lenses.

Seventy-five participants (45.5%) reported showering while wearing their contact lenses, and 82 participants (49.7%) reported swimming while wearing their contact lenses during the past 12 months. Of these, 53 of 82 participants (65.4%) did not wear swimming goggles (Table 1). When combined as an overall water contact behavior, 81 participants (49.1%) were rated as having good and 84 participants (50.9%) had poor water contact behavior (Fig. 1).

TABLE 1 - Frequency distribution of water contact behaviors as reported in the self-administered study questionnaire by participants
Category Variable Soft lens wearer (total N = 165), n (%)
Showering with lenses No 90 (54.5)
Yes 75 (45.5)
Swimming with lenses No 83 (50.3)
Yes 82 (49.7)
Use of swimming goggles* Yes 28 (34.6)
No 53 (65.4)
Use of wet hands to handle lenses No 112 (70.0)
Yes 46 (29.1)
Rinsing storage case with tap water No 136 (82.4)
Yes 29 (17.6)
Rinsing/storing lens in tap water No 163 (98.8)
Yes 2 (1.2)
*Only those participants who reported swimming with their lenses answered this question, and one participant did not answer this question.

Frequency of water contact behaviors as reported in the self-administered study questionnaire by participants (N = 165).

Overall average (standard deviation) contact lens case bioburden was 3.10 (0.98) log CFU/mL (range, 1.09 to 5.94 log CFU/mL), whereas median endotoxin contamination was 1.37 EU/mL with interquartile range ±4.47 EU/mL. There was no detectable endotoxin retrieved from 39 storage cases (23.6%), whereas 61 storage cases (37.0%) had equal to or less than 2 EU/mL and 65 storage cases (39.4%) had more than 2 EU/mL. Overall, 100 participants (60.6%) had low endotoxin levels, whereas 65 participants (39.4%) had high endotoxin levels as per the endotoxin level criteria defined previously. Contact lens storage cases with high endotoxin levels had significantly higher overall levels of bacterial bioburden compared with those with low endotoxin levels (P < .001; Fig. 2). There was no detectable Acanthamoeba in all 85 storage cases undergoing real-time polymerase chain reaction procedure (Fig. 3).

Overall storage case bioburden (ATP assay) by endotoxin levels (LAL assay).
Gel electrophoresis image for real-time polymerase chain reaction for Acanthamoeba identification. −ve C = negative control (DNA-free water); +ve C = positive control (Acanthamoeba castellanii ATCC 30868); S1–S6 = samples from participants' storage cases.

The statistical analysis for general contact lens and self-reported hygiene factors associated with higher overall bacterial bioburden and endotoxin levels of storage cases is presented in Table 2. In the multivariate analysis, wearing lenses for more than 5 years (P = .007) compared with less lens-wearing experience, using a 6-monthly replacement frequency lenses compared with a monthly replacement frequency lenses (P = .05), napping while wearing lenses (P = .004), not rinsing the storage case at all after use compared with rinsing lens case with the disinfecting solution or saline/water (P = .02), and replacing a storage case longer than 3 months (P = .03) were independently associated with higher overall storage case bioburden. Interestingly, female sex (P = .02; odds ratio [OR], 2.54; confidence interval, 1.11 to 5.18), purchasing lenses online compared with purchase from a practitioner (P = .03; OR, 2.31; CI, 1.06 to 5.01), and using a storage case older than 1 month (P < .02; OR, 2.48; CI, 1.13 to 5.44) were independently associated with high endotoxin levels. Of the water contact behaviors, showering (P = .001) was independently associated with higher overall storage case bioburden, whereas using wet hands to handle lenses (P = .01; OR, 2.41; CI, 1.19 to 4.86) was independently associated with high endotoxin levels (Table 3). Overall water exposure score showed an ordered correlation with contact lens storage case bioburden (Spearman R = 0.197; P = .01; Fig. 4).

TABLE 2 - Multiple regression model to determine the association between demographics and hygiene habits and contact lens case contamination (overall microbial bioburden and endotoxin levels)
Storage case bioburden Storage case endotoxin
Univariate model Multivariate model Univariate model Multivariate model
Variable Category Total (N = 165), n (%) Log CFU/mL, mean (SD) P P Low, total n = 100 (60.6) High, total n = 65 (39.4) P OR CI P OR CI
Age 18–25 y 87 (52.7) 2.96 (0.96) .049 NS 57 (57.0) 30 (46.2) .17 1.54 0.82–2.89 NS
>25 y 78 (47.3) 3.26 (0.99) 43 (43.0) 35 (53.8)
Sex Female 108 (65.0) 3.12 (0.98) .98 NI 60 (61.2) 48 (73.8) .10 0.55 0.28–1.11 .02 2.54 1.11–5.18
Male 55 (35.0) 3.12 (0.96) 38 (38.8) 17 (26.2)
Soft lens material Silicon hydrogels 93 (56.3) 3.06 (1.03) .85 NI 57 (72.2) 36 (72.0) .99 1.00 0.45–2.21 NI
Hydrogels 36 (21.8) 3.10 (1.10) 22 (27.8) 14 (28.0)
Type of disinfectants Peroxide system 8 (4.8) 2.73 (1.07) .28 NI 7 (7.0) 1 (1.5) .11 4.81 0.57–40.1 NS
MPDS system 157 (95.2) 3.12 (0.98) 93 (93.0) 64 (98.5)
Years of lens wear <5 80 (48.4) 2.91 (0.99) .01 .007 52 (52.0) 28 (43.1) .26 1.43 0.76–2.68 NI
>5 y 85 (51.6) 3.29 (0.95) 48 (48.0) 37 (56.9)
Soft lens replacement frequency* Every month 129 (78.2) 3.01 (1.00) .02 .05 84 (85.7) 47 (75.8) .11 1.91 0.85–4.30 NS
Every 6 mo 27 (16.4) 3.51 (0.82) 14 (14.3) 15 (24.2)
Days of lens wear ≤4 d/wk 43 (26.1) 2.91 (0.97) .14 NS 29 (29.3) 14 (21.5) .27 1.50 0.72–3.14 NI
>4 d/wk 121 (73.4) 3.17 (0.99) 70 (70.7) 51 (78.5)
Hours of lens wear per day ≤8 h 53 (32.2) 3.02 (0.92) .44 NI 33 (33.0) 20 (30.8) .76 1.10 0.56–1.69 NI
>8 h 112 (67.8) 3.14 (1.01) 67 (67.0) 45 (69.2)
Mode of purchase From practitioner 115 (69.6) 3.05 (1.01) .49 NI 77 (77.8) 38 (61.3) .02 2.21 1.10–4.43 .03 2.31 1.06–5.01
Via Internet 46 (27.8) 3.17 (0.93) 22 (22.2) 24 (38.7)
Cleaning storage case after use Rinses with disinfecting solution 45 (27.2) 2.93 (0.98) .06 .02 31 (31.3) 14 (21.9) .13 1.84 0.82–4.13 NS
Rinses with saline/water 29 (17.5) 2.80 (1.09) 14 (14.1) 15 (23.4)
Does not rinse/unsure 89 (53.9) 3.31 (0.90) .003 54 (54.5) 35 (54.7) .99 0.99 0.53–1.85 NI
Frequency of contact lens case replacement ≤1 mo 79 (47.8) 2.97 (0.94) .79 .03 59 (59.0) 20 (30.8) .17 1.58 0.82–3.03 .02 2.48 1.13–5.44
≤3 mo 58 (35.1) 3.07 (0.97) 31 (31.0) 27 (41.5)
>3 mo 28 (16.9) 3.53 (1.03) .01 10 (10.0) 18 (27.7) .003 3.44 1.47–8.06 .002 4.99 1.82–13.67
Hand hygiene Always washes/dries hands 95 (57.6) 3.11 (0.79) .98 NI 63 (63.0) 32 (49.2) .44 1.33 0.64–2.77 NI
Washes hands but does not dry 38 (23.0) 3.11 (1.20) 21 (21.0) 17 (26.2)
No/unsure 32 (19.4) 3.06 (1.23) .79 16 (16.0) 16 (24.6) .17 1.71 0.78–3.73 NS
Overall lens hygiene compliance Excellent 29 (17.6) 2.71 (0.97) .55 NI 23 (23.0) 6 (9.2) .93 1.02 0.55–1.92 NI
Moderate 78 (47.3) 3.15 (0.97) 47 (47.0) 31 (47.7)
Poor 58 (35.2) 3.24 (0.98) .20 30 (30.0) 28 (43.1) .09 1.76 0.92–3.38
Napping while wearing lenses No 74 (44.8) 2.87 (1.07) .007 .004 50 (50.0) 24 (36.9) .10 1.70 0.90–3.23 NS
Yes 91 (55.2) 3.29 (0.87) 50 (50.0) 41 (63.1)
Sleeping while wearing lenses† No 123 (74.5) 3.12 (1.02) .64 NI 74 (74.7) 49 (75.4) .93 0.96 0.46–1.99 NI
Yes 41 (24.8) 3.03 (0.86) 25 (25.3) 16 (24.6)
The first variable in each category was the referent variable. P < .05, bold and italicized; P < .10, italicized; P > .20, not included in the multivariate analysis model. The OR and CI were not analyzed for variables with P > .001. *This variable was excluded from the multivariate analysis, as it excludes hard (rigid gas-permeable) lens wearers whose lens replacement frequency was yearly or less frequently. †Orthokeratology lens wearers were excluded from this analysis. CI = confidence interval; MPDS = multipurpose disinfection solution; NI = not included in the multivariate analysis; NS = not significant; SD = standard deviation.

TABLE 3 - Multiple regression model to determine the association between water contact behaviors and CL case contamination (overall microbial bioburden and endotoxin levels)
Storage case bioburden (ATP assay) Storage case endotoxin (LAL assay)
Univariate model Multivariate model Univariate model Multivariate model
Variable Category Total (N = 165),
n (%)
Log CFU/mL, mean (SD) P P Low, total n = 100
High, total n = 65
Showering while wearing lenses No 90 (54.5) 2.87 (0.99) .001 .001 58 (58.0) 32 (49.2) .27 1.42 0.76–2.66 NI
Yes 75 (45.5) 3.38 (0.91) 42 (42.0) 33 (50.8)
Swimming while wearing lenses* No 83 (50.3) 2.93 (0.93) .02 NS 57 (57.0) 26 (40.0) .03 1.98 1.05–3.75 NS
Yes 82 (49.7) 3.28 (1.01) 43 (43.0) 39 (60.0)
Use of swimming goggles† Yes 28 (34.6) 3.27 (0.92) .96 NI 13 (30.2) 15 (39.5) .38 0.66 0.26–1.66 NI
No 53 (65.4) 3.28 (1.08) 30 (69.8) 23 (60.5)
Use of wet hands before lenses handling No 112 (70.9) 3.12 (0.85) .68 NI 75 (78.1) 37 (59.7) .01 2.41 1.19–4.86 .01 2.41 1.19–4.86
Yes 46 (29.1) 3.19 (1.21) 21 (21.9) 25 (40.3)
Use of tap water to rinse CL storage case No 136 (82.4) 3.17 (0.95) .07 NS 86 (86.0) 50 (76.9) .13 1.84 0.82–4.13 NS
Yes‡ 29 (17.6) 2.80 (1.09) 14 (14.0) 15 (23.1)
Use of tap water to rinse/store lenses No 163 (98.8) 3.11 (0.99) .32 NI 99 (99.0) 64 (98.5) .76 1.54 0.09–25.71 NI
Yes 2 (1.2) 2.42 (0.26) 1 (1.0) 1 (1.5)
The first variable in each category was the referent variable. P < .05, bold and italicized; P < .10, italicized; P > .20, not included in the multivariate analysis model. The OR and CI were not analyzed for variables with P > .05 in the multiple regression model. *Includes surfing and swimming while wearing lenses. †Includes those who swam while wearing lenses. ‡Percentage of those who rinsed the case. CI = confidence interval; CL = contact lens; NI = not included in the multivariate analysis; NS = not significant; SD = standard deviation.

Association between water exposure score and storage case bacterial bioburden (ATP assay).


In this cross-sectional study, the relationship and association between water exposure and contact lens storage case contamination were explored among a convenience sample of daily soft contact lens wearers. Showering, swimming, and using wet hands to handle lenses were common water contact behaviors and showed a significant association with higher overall storage case bioburden and endotoxin levels, respectively. Poor contact lens storage case hygiene, namely, not rinsing storage cases after use and using a storage case older than 3 months, were associated with higher overall storage case contamination. In addition, lens wear experience of more than 5 years and the use of 6-monthly lens replacement frequency contact lenses increased the risk of higher overall storage case contamination.

Water exposure during contact lens wear can transfer environmental microorganisms such as Pseudomonas aeruginosa to the contact lens and the storage case,18,19 increasing the risk of contact lens–related disease. Of the water contact behaviors evaluated in this study, showering while wearing lenses significantly increased the risk of higher overall levels of storage case bioburden, regardless of any other behavior reported by study participants. Previously, showering in lenses has been linked with contact lens–related sterile6 and microbial keratitis.36,38 The link between showering while wearing lenses and increased storage case contamination has been discussed in the literature,6,39 and previous research on water exposure and Acanthamoeba keratitis suggests that environmental pathogens such as Acanthamoeba may aerosolize during showering, exposing the contact lens and ocular surface to these pathogens.39,40

In the current study, those who reported using wet hands to handle lenses had twice an increased risk of having high endotoxin levels in their storage cases (Table 3, multiple regression model). This has been consistently associated with microbial keratitis38,41 including Acanthamoeba keratitis in previous studies.36,42 Although the current study did not find Acanthamoeba contamination in the subset of lens cases, pathogenic strains of Acanthamoeba have been found in previous studies in the household tap water8,19 and swimming pools.8 Overall, the findings in the current study suggest a direct association between water exposure and higher storage case contamination.

Other risk factors for higher storage case bioburden included not rinsing storage cases after use, using a case less than or equal to 3 months (compared with a 1-month-old case), and poor overall lens hygiene compliance. These findings are in agreement with previous research where irregular replacement of storage case was associated with storage case bioburden,15,16,43 and rubbing and rinsing with disinfecting solution, followed by air drying–reduced storage case bioburden.44–46 In an epidemiological study, storage case replacement frequency (less frequent than every 3 months) increased the risk of microbial keratitis by sixfold.17 In addition, noncompliance to proper lens and storage case hygiene has been consistently associated with contact lens–related adverse events,47–49 possibly because of the storage cases acting as a reservoir for potential pathogens.17

In this study, more than 5 years of lens wear experience was a significant risk factor for a higher level of storage case bacterial bioburden. Similar findings were reported by Wu et al.16 where contact lens wearers who had less than 2 years of lens wear experience had significantly lower levels of storage case contamination compared with those who had more than 2 years of lens wear experience. This finding indicates that established contact lens wearers may become less vigilant over time50 resulting in noncompliance to recommended lens and storage case hygiene, and higher storage case contamination. This highlights the importance of careful messaging to present risks associated with noncompliance and regular reinforcement of appropriate lens care guidelines at follow-up visits for experienced contact lens wearers. Other factors including demographics, type of lens and disinfection system used, and incidental sleeping while wearing lenses had no significant impact on contact lens case contamination.

In the current study, the level of bacterial bioburden of storage cases using the ATP assay was like previous studies using viable culture techniques.15,51,52 This suggests that this assay may be a useful alternative for quantification of microbial contamination in contact lens storage cases. In the current study, the ATP assay was efficient for retrieving even small traces of microbial organisms compared with viable culture technique, which is growth medium selective and is unable to detect nonviable/damaged microbial cells.53 Higher overall levels of storage case bioburden were associated with high endotoxin levels. Endotoxin levels were considered a surrogate for gram-negative bacterial contamination in this study.54,55 Many contact lens–related disease-causing pathogens are environmental gram-negative bacteria such as Pseudomonas aeruginosa and Serratia marcescens,56,57 and determining endotoxin levels may provide information on environmental microbial contamination of storage cases.

In this study, half of the participants (50.9%) reported some form of water exposure including swimming and showering while wearing lenses, using wet hands to handle lenses, and using tap water to rinse the lens and/or storage case. These findings are consistent with previous reports where up to two-thirds of habitual contact lens wearers report some form of water exposure.40,51 In 2017, a study in the United States reported that 86% of soft contact lens wearers showered, and 62% wore lenses when swimming.40 Given the high frequency of occurrence of water contact behaviors among contact lens wearers, it is important to consider lens hygiene compliance education strategies such as the use of visual infographics on lens care accessories.27,58

The limitations of this study include the data collection from a convenience sample population using a self-administered questionnaire. The self-administered nature of the questionnaire not only may allow participants to skip some parts of the questionnaire but also can create bias toward a more complaint attitude, as some wearers may have not reported actual behavior. In this study, participants' level of refractive error and motivation for contact lens wear was not measured, which may also influence behavior. In addition, level of refractive error and contact lens prescription is an important factor in the context of swimming with lenses, and subjects with higher levels of refractive errors may have a greater tendency to swim while wearing their lenses. Further research to understand relationships between level of refractive error and swimming while wearing lenses may provide useful information to control the compliance of water exposure during swimming.

In summary, this study reported the association of water exposure during lens wear (showering, swimming, and using wet hands to handle lenses) and poor storage case hygiene (not rinsing storage case after use and infrequent replacement of storage case) with higher overall and endotoxin case contamination. These findings suggest a direct link between water exposure and increased contact lens storage case contamination and call for strategies to improve contact lens wearers' compliance toward water-related risk factors. Practitioners may improve contact lens education to reduce water exposure and storage case contamination, which may in turn reduce the risk of lens-related adverse events.


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