Antipsychotics and mortality: adjusting for mortality risk scores to address confounding by terminal illness.

OBJECTIVES: To determine whether adjustment for prognostic indices specifically developed for nursing home (NH) populations affect the magnitude of previously observed associations between mortality and conventional and atypical antipsychotics.
DESIGN: Cohort study.
SETTING: A merged data set of Medicaid, Medicare, Minimum Data Set (MDS), Online Survey Certification and Reporting system, and National Death Index for 2001 to 2005. PARTICIPANTS: Dual-eligible individuals aged 65 and older who initiated antipsychotic treatment in a NH (N=75,445). MEASUREMENTS: Three mortality risk scores (Mortality Risk Index Score, Revised MDS Mortality Risk Index, Advanced Dementia Prognostic Tool) were derived for each participant using baseline MDS data, and their performance was assessed using c-statistics and goodness-of-fit tests. The effect of adjusting for these indices in addition to propensity scores (PSs) on the association between antipsychotic medication and mortality was evaluated using Cox models with and without adjustment for risk scores.
RESULTS: Each risk score showed moderate discrimination for 6-month mortality, with c-statistics ranging from 0.61 to 0.63. There was no evidence of lack of fit. Imbalances in risk scores between conventional and atypical antipsychotic users, suggesting potential confounding, were much lower within PS deciles than the imbalances in the full cohort. Accounting for each score in the Cox model did not change the relative risk estimates: 2.24 with PS-only adjustment versus 2.20, 2.20, and 2.22 after further adjustment for the three risk scores.
CONCLUSION: Although causality cannot be proven based on nonrandomized studies, this study adds to the body of evidence rejecting explanations other than causality for the greater mortality risk associated with conventional antipsychotics than with atypical antipsychotics.