Inclusion criteria of study need clarification

EDITOR—Wauters et al report on the diagnostic value of rectal bleeding in terms of subsequent development of colorectal cancer.1 We feel that this study requires clarification for several reasons.

Firstly, the authors do not report the pre-test probability of colorectal cancer in age specific categories in their population. The diagnostic value of a symptom such as rectal bleeding and the impact on post-test probability and subsequent referral threshold are maximised when the pre-test probability of the disease is known.2

Secondly, they fail to mention that less than half of patients with rectal bleeding have no other symptoms.3 More often it is associated with other bowel symptoms that have higher diagnostic value than rectal bleeding alone.4

Thirdly, the reported positive likelihood ratio of 68.3 “rules in” a diagnosis of colorectal cancer, irrespective of the pre-test probability of the disease.2 The reported specificity of 99.5% has the same effect of ruling in the target disorder of colorectal cancer. These findings imply that any patients attending their general practitioner with rectal bleeding need referral and further evaluation. Our own clinical experience and other community based studies of rectal bleeding indicate that such a high specificity and likelihood ratio is unlikely and may well be misleading. 4 5

Finally, the most likely explanation for the results relates to general practitioners underreporting rectal bleeding in the prospective arm of the study. Wauters et al chose “rectal bleeding as the reason …