Median Household Income and Mortality Rate in Cystic Fibrosis

Abstract

Context. Poverty has been shown to be a determinant of health outcomes in many epidemiologic studies. Objective. The goal of this study was to assess the association between household income and the mortality rate in cystic fibrosis (CF) patients. Design, Setting, and Patients. We selected white patients diagnosed before 18 years old and having 1 or more records in the Cystic Fibrosis Foundation Patient Registry since 1991. These 23 817 patients were linked to the 1990 US Census by their zip code of residence. The median household income was adjusted for state level differences in cost of living using the 1998 Consumer Price Index. Interventions. None. Main Outcome Measures. We examined the association between categories of the median household income and the mortality rate. We examined the association between income categories and age-related changes in pulmonary function and body weight as well as specific nutritional and pulmonary therapies. Results. We found a strong monotonic association between the median household income and the mortality rate. The test of trend was significant, and this effect was maintained after adjustment for a variety of patient and disease characteristics. When the lowest income category (<$20 000) is compared with the highest (≥$50 000), the adjusted incidence rates were 90.3 and 62.6 per 10 000 person years, respectively; this represents a 44% increased risk of death in the lowest income category. Patients living in areas with lower median household income also had consistently lower pulmonary function and body weight than did those living in higher income areas. The differences in weight percentiles and forced expiratory volume in 1 second are substantial in magnitude, they appear at an early age, and they persist into adulthood for these CF patients. Prescribed nutritional treatments and screening for CF-related diabetes were significantly higher among patients living in areas with lower median household income. Prescription of deoxyribonuclease and inhaled tobramycin was not significantly associated with median household income. Conclusion. There was a strong association between lower household income and increased mortality rate among CF patients. Additional understanding of this effect will require more complete and direct measurement of socioeconomic status and a better understanding of treatment adherence, local environmental conditions, and especially the care of CF patients during the early years of life.