REPLY
Fecal Occult Blood Tests for Colorectal Cancer
D. James B. St. John and
Graeme P. Young
15 March 1993 | Volume 118 Issue 6 | Page 472
IN RESPONSE:
Drs. Singh and Gallo raise several practical concerns about occult blood testing, including the need to restrict diet and medications, to train the staff properly, and to avoid testing after dental extraction, during menstruation, or when hemorrhoids are bleeding. Nonetheless, they are unduly critical of the tests. In most large screening programs, Hemoccult positivity has ranged from 1% to 2.5% [1-3]. The special problem with nonhydrated Hemoccult is not its specificity [3] but its rather low sensitivity for colorectal cancer [1, 2]. Also, the predictive value of a positive test for cancer may be only 10%, but another 15% to 30% of middle-aged screenees prove to have large adenomas.
Alternatives to Hemoccult tests include guaiac tests with a higher level of sensitivity for hemoglobin, the heme-porphyrin assay, and immunochemical tests specific for human hemoglobin. The latter especially merit careful evaluation because diet and medication need not be changed. In addition, they have a high sensitivity for hemoglobin and are able to differentiate between bleeding from the upper and lower gastrointestinal tracts [4].
Furthermore, flexible sigmoidoscopy is not as advantageous as Singh and Gallo suggest because of the anatomical distribution of cancers in the large bowel. The theoretical sensitivity of flexible sigmoidoscopy for colorectal cancer can be no greater than 55% to 65% and may well be less in practice. Flexible sigmoidoscopy and fecal occult blood testing probably have complementary roles in screening. Meanwhile, the search for better occult blood tests should continue.
1. Hardcastle JD, Thomas WM, Chamberlain J, Pye G, Sheffield J, James PD, et al. Randomised, controlled trial of faecal occult blood screening for colorectal cancer. Results for first 107,349 subjects. Lancet. 1989; 1:1160-4.
2. Kronborg O, Fenger C, Olsen J, Bech K, Sondergaard O. Repeated screening for colorectal cancer with fecal occult blood test: a prospective randomized study at Funen, Denmark. Scand J Gastroenterol. 1989; 24:599-606.
3. Allison JE, Feldman R, Tekawa IS. Hemoccult screening in detecting colorectal neoplasm: sensitivity, specificity, and predictive value. Ann Intern Med. 1990; 112:328-33.
4. Young GP, St John DJ. Selecting an occult blood test for use as a screening tool for large bowel cancer. In: Rozen P, Reich CB, Winawer SJ; eds. Frontiers of Gastrointestinal Research. Advances in Large Bowel Cancer: Policy, Prevention, Research and Treatment. Basel: Karger; 1991:135-56.
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