To the Editor: Dr Chan and colleagues1 reported the results of an observational study of nonsteroidal anti-inflammatory drug (NSAID) use in the Nurses' Health Study (NHS), in which a significant reduction in colorectal cancer–specific mortality was observed with regular postdiagnostic aspirin use (odds ratio, 0.71; 95% confidence interval, 0.53-0.95), but not with prediagnostic use.
We have concerns about the interpretation of these results because of patient losses to the study population. Research investigating the association between NSAID use and colorectal cancer survival should include all incident colorectal cancer cases to validly generalize results to the larger patient population. All study participants in the study by Chan et al1 belonged to the NHS cohort; in the colorectal cancer survival study, an NHS cohort member was identified as a diagnosed case of colorectal cancer via the biennial mailed questionnaire. After noting diagnosis of colorectal cancer in the biennial questionnaire, study participants then had to consent to release of medical and pathology records for use in the present study. Due to the potential for significant lag time between diagnosis of colorectal cancer in NHS cohort members and entry into the present study as a participant, a number of colorectal cancer patients may have died before being detected. The result is that information, including both NSAID exposure and vital status, may have been lost from the study.