Background: Studies of cancer survivors usually report positive correlations between income and health-related quality of life (QoL). These correlations cannot necessarily be interpreted as income disparities because earnings and income are affected by health, as well as the reverse.
Objectives: The goal of this study was to quantify income disparities in QoL among cancer survivors by using instrumental variables (IV) to assess and, if necessary, correct for reverse causality.
Methods: We constructed an instrumental variable for income from home ownership, sources of unearned income, marital status at diagnosis, and spousal characteristics. Then, we examined income's effect on QoL in regressions controlling for other clinical and demographic predictors of QoL. The data were from interviews in 2002 with a cohort of cancer survivors who were 25 to 62 years of age when diagnosed during 1997 to 1999.
Measures: The Functional Assessment of Cancer Therapy-General (FACT-G) and the SF-12 measured QoL in multiple domains. Questions adapted from the Health and Retirement Study ascertained the ratio of annual family income to the poverty threshold in 2001.
Results: Endogeneity tests were sensitive to assumptions of linearity for the income-QoL relationship and the choice of QoL measure. Consistently estimated income disparities were significant in all QoL models. The income elasticity of QoL ranged from 2% to 10%.
Conclusions: There are income-related disparities in the QoL of cancer survivors that cannot be explained away by the effect of health on earnings. High-income patients are not only more likely to survive cancer, but they enjoy better QoL as survivors.