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Response to Dr. Hilsden

TO THE EDITOR: We appreciate Dr. Hilsden's concern regarding the inherent limitations of the retrospective cohort study design and the Department of Veterans Affairs administrative databases, which were addressed in our discussion (1). We also agree that a comparison of colon cancer–specific mortality and all-cause mortality would be useful to support the contribution of surveillance colonoscopy to the mortality benefit found. Unfortunately, there is no reliable method to determine cause of death in this data set. In addition, there are no specific ICD-9-CM codes available to distinguish recurrent colorectal cancer from a personal history of colorectal cancer. Thus, we were not able to directly measure recurrence or investigate its impact on performance of repeat colonoscopy, as Dr. Hilsden suggests. Realizing that those who present with distant metastases would have a higher mortality and decreased likelihood of receiving follow-up colonoscopy, we excluded those patients with ICD-9-CM codes at the index hospitalization indicating metastatic disease. This method has been previously used (2) but not rigorously validated. Therefore, we took the additional precaution of excluding those patients who died within 1 yr of diagnosis, because they also had a high probability of being inappropriate surveillance candidates, owing to metastatic disease or severe comorbidity. We adjusted for baseline comorbidities using the Charlson index. High comorbidity burden was associated with decreased survival, with a hazard ratio of 1.35 (95% CI = 1.18–1.56) for one major comorbidity compared with none and with a hazard ratio of 1.90 (95% CI = 1.65–2.20) for two or more major comorbidities compared with none. Whether measurement of new comorbidities during the follow-up period would impact our findings is an interesting question. We therefore performed a sensitivity analysis to investigate the impact of such a confounder on our model.