Exploring the methodological challenges of investigating comparison groups with different underlying characteristics: a case study.

Abstract

OBJECTIVE: To examine challenges in forming accurate conclusions from claims data when comparing different patient groups, and to stress the need to adjust for confounding variables through application of statistical methods. METHODS: Patients who were 60 or older and began amlodipine or felodipine in 1997 or 1998 were identified from a proprietary managed care claims database. Patients had 6 months of medical and outpatient pharmacy benefits coverage before and after initiating therapy. They were stratified on varying degrees of hypertension severity and on all-condition clinical complexity, and quantified with a proprietary Burden of Illness (BOI) score, defined as the expected resource use for the patient.s combined conditions during the 6 months before therapy. RESULTS: Of the 17,667 patients, 10,469 (59.3%) were female. The 12,389 amlodipine users were similar in age to felodipine users (n=5,278), 73 versus 74, respectively. More amlodipine users were assigned to the 2 most clinically complex quintiles of the BOI distribution (45.5% to 26.9%), with 24.3% of patients in the top quintile (9.7% for felodipine) and twice the prevalence of ischemic heart disease/angina (42.4% versus 19.4%). High-severity hypertension was found among 33.6% of amlodipine users, contrasted with 17.9% for felodipine. A significant difference in the mean average daily dose (ADD) between the amlodipine (5.93 mg) and felodipine (5.8 mg) groups (P=0.0007) was noted. CONCLUSION: These results stress the importance of assessing the impact of potentially confounding variables and the need to address differing patient characteristics through either risk adjustment or stratification in making head-to-head comparisons.