We also analysed results with a propensity score approach, which used all demographics, drug use at baseline, and identified medical conditions as confounders. Propensity scores were estimated using logistic regression with the outcome being the indicator of treatment and the covariates being all confounders considered. For those with no missing
data, all covariates were used; whereas for those with missing data (for body mass index, systolic blood pressure, or smoking), we used separate logistic regression models, which excluded the missing covariates, to estimate propensity scores. Analysis stratified by the propensity scores balances the treated and untreated groups with respect to the observed covariates used in estimating the propensity scores. We
determined outcome hazard ratios for each fifth of the propensity score and combined the five hazard ratios to determine an overall hazard ratio using a Cox model
treating the fifths as strata with different baseline hazards. The propensity score thereby accounts for missing confounders in a different fashion from the multiple imputation method used with the Cox analysis. The matched database study for Syst-Eur,
our first study, was analysed only with propensity score analysis.