Statistical analysis
Statistical analysis was performed with SPSS 18.0 software for Windows (SPSS, Chicago, IL, USA). The Kolmogorov-Smirnov test was used to determine whether variables were normally distributed. Normally distributed data are described as means±standard deviations (SDs). Group differences were analyzed by Mann-Whitney tests (for non-normally distributed data) or analysis of variance (ANOVA). Differences between categorical variables were assessed by the χ2 test. Simple linear regression analysis was performed by Spearman’s rank correlation test. A p-value < 0.05 was considered as statistically significant.

Sample size determination

The minimum sample size was computed with respect to a two-tailed, one-sample Student t-test considering, on the basis of data from a previous pilot study (data not shown): a difference of 8 pg/ml for H2O2 levels between children affected by VKC and controls, 5.5 as SD, 0.05 (α) as type I error probability and 0.95 as power 1 − β. The sample size was n = 10 patients/group.