Case-mix adjustment for evaluation of mortality in cystic fibrosis

Abstract

Comparison of patient mortality rates in cystic fibrosis (CF) obtained from different institutions requires the use of case-mix adjustment methods to account for baseline differences in patient and disease characteristics. There is no current professional consensus on the use of case-mix adjustment methods for use in comparing mortality rates in CF. Characteristics used for this case-mix adjustment should include those that are different across institutions and are associated with patient survival. They should not include characteristics of disease severity that may be a consequence of effectiveness of treatment. The goal of these analyses was to identify a set of these characteristics of patients or disease that would be useful for case-mix adjustment of CF mortality rates. Data from the Cystic Fibrosis Foundation Patient Registry and from the United States Census of the Population (1990) were used in these analyses. Kaplan-Meier techniques, the log-rank test, and Cox proportional hazards regression were used to estimate survivorship, calculate hazard ratios (HR), 95% confidence intervals (Cl95%), and to conduct tests of statistical significance. The data set included all 30,469 CF patients seen at CF Care Centers from 1982-1998. There were 5,906 deaths during 508,721 person-years of follow-up. In multivariate analyses, female gender (HR 1.30, Cl95% (1.16, 1,47), P<0.001), nonwhite race (HR 1.48, Cl95% (1.07, 2.04), P=0.018), Hispanic ethnicity (HR 1.85, Cl95% (1.42, 2.43), P<0.001), and symptomatic presentation (respiratory, gastrointestinal, respiratory and gastrointestinal, meconium ileus, and other symptomatic presentations; H Rs 1.38-1.83; P values, 0.028 to < 0.001) were associated with higher risk of death. The homozygous ΔF508 genotype (HR 1.36, Cl95% (1.19, 1.55), P < 0.001) and neither mutation being ΔF508 (HR 1.40, Cl95% (1.15, 1.71), = 0.001) were also associated with higher risk of death. Patients diagnosed after 36 months of age had aimost 50% reduction in risk of death compared to those diagnosed before 6 months of age (HR 0.52 Cl95% (0.44, 0.61), P<0.001). When patients living in zip codes with a median household income