The Mismeasurement of Quality by Readmission Rate: How Blunt Is too Blunt an Instrument? A Quantitative Bias Analysis

Background: The rate of readmission is widely used as a measure of hospital quality of care, often with funding implications for outlying facilities. Objectives: This study explored the plausibility of readmission as a proxy for health care quality with quantitative bias analysis and the application of a structural Directed Acyclic Graph framework. It applies this paradigm to observed ethnic differences in the odds of readmission in a sample of New Zealand hospital patients. Research Design: Ethnicity was defined as the exposure, readmission rate as the proxy outcome, and quality of care as a missing mediator. Using data from 89,090 surgical patients from New Zealand, and estimates from the literature of the prevalence of poor quality and the strength of the quality-of-care readmission association, a series of sensitivity analyses were performed to calculate an odds ratio of the ethnicity-readmission association corrected for the missing mediator quality. Results: Given the assumptions applied, potentially only 29% of the excess odds of readmission for Maori compared with Europeans were due to poor quality of care. Conclusions: This investigation finds substantial error when using readmission as a marker of quality, and suggests that differences in readmission between populations are more likely to be due to factors other than quality of care.