Table 1

Distribution of cases and controls for the variables that were potential risk factors for AK chosen for inclusion in the MV analyses

VariableControlCaseTotalOR*P values95% CI for OR
Lens disinfectant: classified by the principal active ingredient (PAI)†
 1: Polyhexanide (0.00 005–0.0001)9011101Referent
 2: PQ+ALDOX/PQ+Alex‡269352.830.0381.06 to 7.57
 3: Hydrogen peroxide 3%125173.410.0481.01 to 11.51
 4: Polyquad-1(0.0001%)+polyaminopropyl biguanide112131.490.6330.29 to 7.60
 5: Oxipol5529844.31<0.0012.00 to 9.33
 6: Rigid gas permeable lens solution115163.720.0361.09 to 12.71
CL materials classification: simplified from the ANSI classification§
 Groups I+II+III186241.950.2330.65 to 5.83
 Group IV137203.150.0391.06 to 9.37
 Group VA761389Referent
 Groups VB+VC73291022.320.0231.12 to 4.81
 Rigid gas permeable materials115162.660.1130.79 to 8.91
Hygiene score: categories split between the top (worst) quartile of the sample and the rest¶
 1: Good–moderate (score 1.75–5.08)17430204Referent
 2: Poor (score 5.09–8.08)3933724.91<0.0012.68 to 8.98
Hand washing before handling CLs
 1: No/unsure2721483.71<0.0011.90 to 7.22
 2: Yes18639225Referent
Showering when wearing CLs
 1: No14125166Referent
 2: Yes72381102.98<0.0011.67 to 5.31
Water activity using CL’s: categories combined
 1: None11420134Referent
 2: Ocean/sea/river/lake429511.220.6490.52 to 2.89
 3: Public or private pool/hot tub5731883.100.0011.63 to 5.91
Ethnic group: for categories**
 4: British (white)14154195Referent
 6: Other726780.220.0010.09 to 0.53
Occupation: for categories††
 1–3 Professional/director/manager/associate professional and technical/senior officials16336199Referent
 4–9 Combined5022721.990.0291.07 to 3.70
  • *ORs are not adjusted for confounding effects of other variables.

  • †Classification of CL solutions is given in online supplementary table 2.

  • ‡PAI category 2: PQ +ALDOX/PQ +Alex=Polyquad-1 (0.001%)+ALDOX (0.0005%) or Polyquad-1 (0.0003%)+alexidine (0.00016%)—the latter for 4 controls and 1 case only.

  • §American National Standard Institute (ANSI) standard Z80.20-2016. American national standard for ophthalmics—contact lenses—standard terminology, tolerances, measurements and physicochemical properties (ophthalmic).

  • ¶Hygiene scores were calculated for the 14 hygiene variables listed in online supplementary table 5 using the rules described in that table. The mean score for each patient was then used to divide the patient sample into a simple binary classification for MV analysis in which the worst quartile (the quartile with the highest score) of the sample was categorised as having “Poor” hygiene and compared with the rest categorised as having “Good-Moderate” hygiene. “Good-Moderate” mean scores were 1-75-5.08 and “Poor” mean scores were 5.09-8.08. Hand washing before handling CLs and showering while wearing CLs were kept as separate variables and and analysed as such.

  • **Ethnic categories (UK census categories): 1: Asian or Asian British (Bangladeshi, Indian, Pakistani); 2:black or black British (African, Caribbean, other); 4: British (white); 6: other.

  • ††Occupation: 1: managers, directors and senior officials; 2: professional occupations; 3: associate professional and technical occupations; 4: administrative and secretarial occupations; 5: skilled trades occupations; 6: caring, leisure and other service occupations; 7: sales and customer service occupations; 8: process, plant and machine operatives; 9: occupations requiring no specific training or skills and student: categorised by parents’ occupation.

  • AK, Acanthamoeba keratitis; CL, contact lens; MV, multivariate.