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To the Editor: Dr Singh and colleagues¹ report that a negative colonoscopy is associated with a standardized incidence ratio (SIR) of 0.55 to 0.69 for subsequent colon cancer relative to the general population. However, in reaching conclusions about the predictive value of a negative colonoscopy, there are 2 other factors that must be considered.

First, by design the authors have created a low-risk cohort by excluding individuals at highest risk for CRC (eg, those with a history of inflammatory bowel disease, prior CRC, or colonoscopy within the past 5 years). Thus, the SIR would already be expected to be below 1 (ie, below average).

Second, studying persons who have all had colonoscopy may also introduce a bias, although the direction would be unclear. It could result in a high-risk group because many may have been symptomatic or had a family history of colon cancer. Conversely, colonoscopy may be a marker . . . [Full Text of this Article]

Douglas J. Robertson, MD, MPH
douglas.robertson@med.va.gov

Brenda E. Sirovich, MD, MS
White River Junction VAMC
White River Junction, Vt

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