Analyses were performed using SAS version 9.3 (SAS Institute, Inc., Cary, NC, USA). We examined the association between serum levels of homocysteine, vitamin B12, and folate (study factors), and PSC prevalence and incidence (dichotomous outcomes). Multivariable-adjusted logistic regression models were used to estimate the associations, expressed as ORs and 95% confidence intervals (CI). Homocysteine was assessed continuously using the unit of each SD increase in serum levels, while vitamin B12 and folate were assessed continuously using the unit of each SD decrease in serum levels. The following potential confounders were included in the logistic regression model: age, sex, smoking status, hypertension, diabetes, education, myopia, and ever use of inhaled or oral steroids. We tested for interaction between homocysteine and vitamin B12 for PSC. When a significant interaction (effect modification, P < 0.05) was detected, we performed subgroup analyses stratified by the particular effect-modifying factor.