Limitations of the faecal occult blood test in screening for colorectal cancer.

BACKGROUND: A screening test should be acceptable, safe, simple, accurate, reliable, effective, and inexpensive. Screening for colorectal cancer with the faecal occult blood test is done to reduce the incidence and mortality from colorectal cancer. How does this test measure up to these requirements?

METHODS: The characteristics of faecal occult blood test are described by means of clinical epidemiology, analysing its compliance, sensitivity, specificity, positive predictive value, and its test performance in Bernoulli trials. A decision tree and a Markov model are used to compare the cost-effectiveness of screening strategies involving faecal occult blood test and colonoscopy.

RESULTS: After 5-10 years, patient compliance in a faecal occult blood test screening programme falls below 50%. Over 80% of the patients are likely to leave the programme before its completion. Although the test itself may seem safe and simple, the high rate of false-positive outcomes exposes many subjects to the potential complications of colonoscopy. The high rate of false-negative tests gives patients with colorectal cancer a false sense of security and delays their proper diagnostic work-up. In populations with low prevalence rates of colorectal cancer, faecal occult blood test becomes very inaccurate in diagnosing colorectal cancer, as its positive predictive value falls below 5%. Its long-term test performance is unreliable in that it comes to depend on the frequency, with which the test is repeated. Any negative or positive test result can be achieved by varying the frequency of test repetition. Screening by colonoscopy every five or ten years is more cost-effective than screening by annual faecal occult blood test in preventing the occurrence of colorectal cancer and its associated mortality.

CONCLUSIONS: Screening strategies for colorectal cancer involving faecal occult blood test should be abandoned.