Immunochemical Detection of Fecal Occult Blood

Abstract

We would like to comment on the discussion of false positive results in "Immunochemical Detection of Fecal Occult Blood" following the paper by McDonald et al. ' The St. Vincent's Hospital Bowel Cancer Screening Unit has been using fecal occult blood testing with an immunological technique for two years and has previously reported some of its results. Z We found that of 200 control subjects, over the age of 40 years, 4% had positive results on immunological testing but had no colorectal carcinoma, polyps, or other pathology on colonoscopy. Evaluation of results obtained in our screening programme, over a period of 18 months, showed that false positivity rates for Hemoccult 11, without moistening of fecal specimens, to be 1.9%. With moistening the value increased to 3.9'7'0, which was the same figure obtained with our immunological test on six fecal specimens. However, the immunological test gave a much higher pick-up rate of carcinomas and polyps than the moistened Hemoccult I1 test.j The above figures refer to a true positive being only a patient demonstrated to have a carcinoma or adenoma of the large bowel. It is important when discussing false positive results to agree what constitutes a true positive result. Although a person may have had bleeding from hemorrhoids, ulcerative colitis, or other lesions of the bowel, from the point of view of bowel cancer screening these are effectively false positive results. We would further disagree with the comment of Williams and Hunter4 that the major non-neoplastic cause of a positive immunochemical test is bleeding from hemorrhoids. A person may have a positive immunochemical test and have hemorrhoids but it is uncommon, in our experience, that hemorrhoids can be assigned with any confidence as the cause of the positive test. Immunological detection of fecal occult blood. Aust NZ J Med 1984; 14: 105-10. 2. Frommer DJ, Kapparis A. Relationship between site of colorectal carcinomas and fecal immuno-reactive hemoglobin concentrations. Aust NZ J Med 1984; 14: 316. 3. Kapparis A, Brown M, Frommer DJ. Early results in the use of an immunological technique in occult blood screening for colorectal cancer. Aust NZ J Med 1985; IS: 160. 4. Williams JAR, Hunter R. Immunochemical detection of fecal occult blood. Aust NZ J Med 1985; 15: 81. REPLYThe purpose of the communication of Frommer et a/. appears to be to define what is meant by the term "positive" in relation to testing for occult blood in feces in the context of screening for colorectal neoplasia. Whilst we can appreciate the point that if the object of the screening test is only to detect colorectal neoplasia, then positive results in the absence of demonstrated neoplasia are rightly regarded as false positives. As the immunochemical tests are 100% specific for human blood, and as we have demonstrated that in almost all cases where a positive test has been found, a lesion is demonstrable which could have caused the positive test,' and in the majority of these cases the lesions are significant, it is reasonable to refer ...