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Rectal bleeding and colorectal cancer in general practice: diagnostic study

BMJ 2000; 321 doi: https://doi.org/10.1136/bmj.321.7267.998 (Published 21 October 2000) Cite this as: BMJ 2000;321:998

Rapid Response:

Rectal bleeding: study requires clarification

Sir,

Hans Wauters and colleagues 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:

1. They do not report on the prior probability of colorectal cancer
in age specific categories in their population. The diagnostic value of a
symptom such as rectal bleeding and its impact on post-test probability
and subsequent referral threshold is most useful when the prior
probability of disease is known.[2]

2. They fail to mention that less than half of rectal bleeding occurs
in isolation.[3] More often it is associated with other bowel symptoms
which have higher diagnostic value than rectal bleeding alone.[4]

3. The reported positive likelihood ratio of 68.3 "rules in" a
diagnosis of colorectal cancer, irrespective of the pre-test probability
of colorectal cancer.[2] Similarly, the reported specificity of 99.5% has
virtually the same impact of ruling in the target disorder of colorectal
cancer. These findings suggest that any patient attending their general
practitioner with rectal bleeding requires referral and further diagnostic
evaluation. Our own clinical experience and other community based studies
of rectal bleeding suggests that such a high specificity and likelihood
ratio are unlikely and may well be misleading.[4] [5]

4. We feel that the most likely explanation for their results relates
to general practitioners under-reporting rectal bleeding in the
prospective arm of their study. Wauters et al "chose rectal bleeding as
the reason for visiting a general practitioner" as their inclusion
criterion for their study. They should clarify whether this means that
patients in whom rectal bleeding was not the primary reason for consulting
their general practitioner were excluded. A prospective study in the
Netherlands showed that in patients presenting with rectal bleeding, only
51% stated this as the primary reason for consulting their general
practitioner; 49% consulted for a different reason but rectal blood loss
was subsequently mentioned during the consultation.[4] In another
prospective study it was found that 3% (95% CI 1.4% to 5.8%) of patients
with rectal bleeding subsequently develop rectal cancer, and even in this
study there was an over-representation of patients with "clinically
relevant rectal bleeding".[5] The number of patients with rectal bleeding
who subsequently developed colorectal cancer in Wauters et al's study was
reported as 27 in their table (7%, 95% CI 4.6% to 10%).

In summary it seems likely that there was a systematic bias in
excluding less severe forms of rectal bleeding which may have not been the
primary reason for consulting with a general practitioner. This has
resulted in inflated values for specificity and positive likelihood ratio.
Before their results are incorporated into clinical practice Wauters et al
should clarify their inclusion criteria and provide age specific 2x2
tables so that readers can judge for themselves the diagnostic value of
isolated rectal bleeding in general practice.

Yours sincerely,

Tom Fahey (Senior Lecturer in General Practice)

Alan Montgomery (MRC Training Fellow)

Knut Schroeder (MRC Training Fellow)

We declare no conflict of interest.

References

1. Wauters, Van Casteren V, Buntinx F. Rectal bleeding and colorectal
cancer in general practice: diagnostic study. British Medical Journal
2000;321:998-999.

2. Black ER, Bordley D, Tape TG, Panzer RJ. Diagnostic Strategies
for Common Medical Problems. Philadelphia: American College of Physicians,
1999.

3. Douek M, Wickramasinghe M, Clinton JJ. Does isolated rectal
bleeding suggest colorectal cancer? Lancet 2000;354:393

4. Fijten G, Starmans R, Muris J, Schouten H, Blijham G, Knottnerus
JA. Predictive value of signs and symptoms for colorectal cancer in
patients with rectal bleeding in general practice. Family Practice
2000;12:279-286.

5. Fijten G, Muris J, Starmans R, Knottnerus JA, Blijham G, Krebber
T. The incidence and outcome of rectal bleeding in general practice.
Family Practice 2000;10:283-287.

Competing interests: No competing interests

02 November 2000
Tom Fahey
Senior Lecturer in General Practice
University of Bristol