Possible biases when estimating the outcome of colorectal cancer screening
The UK Colorectal Cancer Screening Pilot Group writes it is a fact
that “Population based randomised controlled trials have shown that
screening by faecal occult blood testing for colorectal cancer can reduce
One of the studies this fact is based on is the Funen study from
Denmark (1) and this study has some biases that favours the effect of
Persons in the control-group were not told about the study and
continued to use health-care facilities as normal. Between 1985 and 1995
the usual practice in Denmark was for the general practitioner to perform
a rectoscopy on a patient with symptoms of colorectal cancer and to refer
the patient for an X-ray examination of the colon at the local hospital.
In contrast, nearly all participants in the Funen study with a positive
FOB test had a colonoscopy. Consequently, the outcome of the Funen study
is not only related to the FOB test but also to the fact that colonoscopy
is a better diagnostic method than X-ray of the colon (2).
The age of the participants was 45 – 84 years, but the suggested age
group is 50 – 69 or 50 - 74 years. Since the incidence of colorectal
cancer is higher in the elderly, the positive predictive value of the FOB
test plus colonoscopy was higher than would be obtained in practice, and
rates of false positive tests lower. The adverse effects of screening
could therefore be underestimated in the Funen study.
In the intervention group all patients received their operation at
the University Hospital, where it is likely that the surgeons were more
competent than at local hospitals where the control group underwent
surgery. Since surgery now is centralised the benefits of a colorectal
screening program might be smaller today.
In the Funen study, experienced staff probably conducted most of the
colonoscopies. In a real life situation non-specialists in training would
undertake some of the colonoscopies and, consequently, it would be
expected that more cancers would be missed and more false positive tests
occur, as has been the case with breast screening (3).
It is likely that some of these biases are also relevant for other
studies of colorectal cancer screening e.g. the Nottingham study (4).
1. Kronborg O, Fenger C, Olsen J, Jorgensen OD, Sondergaard.
Randomised study of screening for colorectal cancer with faecal-occult-
blood test [see comments]. Lancet 1996;348:1467-71.
2. Rex DK, Rahmani EY, Haseman JH, Lemmel GT, Kaster S, Buckley JS.
Relative sensitivity of colonoscopy and barium enema for detection of
colorectal cancer in clinical practice. Gastroenterology 1997;112:17-23.
3. Sickles EA, Wolverton DE, Dee KE. Performance parameters for
screening and diagnostic mammography: specialist and general radiologists.
4. Hardcastle JD, Chamberlain JO, Robinson MH, Moss SM, Amar SS,
Balfour TW et al. Randomised controlled trial of faecal-occult-blood
screening for colorectal cancer [see comments]. Lancet 1996;348:1472-7.
Competing interests: No competing interests