An assessment of occult blood testing to determine which patients with large bowel symptoms require urgent investigation.

One hundred and fifty-two consecutive patients with symptoms suggestive of colorectal disease were offered occult blood testing before undergoing barium enema examination or colonoscopy; one hundred and thirty-nine successfully completed the test. Thirty-four had positive results of whom thirteen had a cancer and eight an adenomatous polyp (diagnostic yield for neoplasia of 59 per cent). No false negative results occurred, a sensitivity of 100 per cent, and only 21 false positives occurred, a specificity for malignancy of 84 per cent. Subjects attending outpatients should be offered occult blood testing; those with a positive test should undergo colonoscopy. The cost-benefit of such a scheme is emphasized.