Fecal Blood Testing for Colorectal Neoplasia
Abstract & Commentary
By Malcolm Robinson, MD, FACP, FACG, Emeritus Clinical Professor of Medicine, University of Oklahoma College of Medicine, Oklahoma City. Dr. Robinson reports no financial relationship to this field of study.
Synopsis: Immunochemical testing of stool for human blood appears to be a more sensitive and specific test for advanced colon neoplasia than current guaiac-based tests.
Source: Zohar Levi, et al. A Quantitative Immunochemical Fecal Occult Blood Test for Colorectal Neoplasia. Ann Inten Med. 2007:146;244-255.
An ideal screening test for colorectal neoplasia should be noninvasive, inexpensive, and accurate. High sensitivity should be coupled with specificity to best identify early colon cancer and advanced adenomas (those adenomas with dysplasia and/or villous features). Since colonoscopy is invasive and expensive, a positive noninvasive screening test should minimize the need for colonoscopy in cases unlikely to have significant colon neoplasia. Guaiac-based fecal occult blood tests have been the office mainstay for many years, but these tests do not specifically identify human hemoglobin. Guaiac testing is relatively insensitive to the presence of advanced colon neoplasia although the employment of even this seemingly inadequate test seems to reduce mortality from colon cancer in screened patient groups. Nevertheless, a better test for occult GI bleeding seems to be highly desirable. The present study assessed a laboratory-based immunochemical test that is specific for human hemoglobin, requires no dietary restrictions, and that can be reliably quantitative.
One thousand consecutive ambulatory patients scheduled for colonoscopy had quantitative immunochemical testing of three separate stool specimens for hemoglobin content. Some patients were having routine screening colonoscopies, and others had various worrisome symptoms or other high risk profiles for colonic neoplasia. Hospitalized patients, those with inflammatory bowel disease, and patients having hematuria or active menstruation at the time of stool sampling were excluded. Some patients were taking NSAIDs, and some had been anticoagulated. Forty-nine patients with incomplete colon exams were excluded. Polyps were enumerated, assessed histologically, and classified by location. Mean age was 63.2, about 10% of patients had been found to have positive guaiac-based tests, and about 35% had a history of previous colon neoplasia. Ninety-one patients had significant neoplasia found at colonoscopy including 17 cancers and advanced adenomas in 74 patients. A hemoglobin threshold of 75 ng/mL of buffer seemed to have particularly high sensitivity and specificity for neoplasia, 94.1% and 87.5% respectively. The authors pointed out that stool sample size depended on fecal consistency, and they noted that the overall population studied was at increased risk of colon neoplasia. For this reason, the test performance in average-risk populations might be quite different.
Despite the medically recognized value of colorectal cancer screening, the rates of actual screening in the total population remain low (far lower than rates of breast and cervical cancer screening). Immunochemical stool testing for human blood content has considerable appeal as a potentially much more sensitive and specific test for important colon neoplastic lesions than guaiac-based tests as currently used. However, immunochemical tests are relatively expensive ($18-$30) and have not been widely adopted in the United States. In the present study, 16 of 17 cancers found at colonosocpy were Dukes A or B. This is typical of the relatively early cancers found during colonoscopic screening programs.
In an accompanying editorial,1 some of the particular advantages of a quantitative stool hemoglobin test were explored. Specificity and sensitivity at various levels could be utilized for different types of screened patients, eg, those with pre-test high or low likelihoods of colon neoplasia. For example, patients currently thought to be at high risk might still be able to avoid colonoscopy if cancer sensitivity level of the test was extremely high (averting colonoscopy if the test were negative). Although the concept of immunochemical fecal occult blood testing seems appealing in many ways, this reviewer doubts that such testing will be widely utilized anytime soon. First, the less expensive guaiac type testing has been found to be clinically effective despite its drawbacks. Physicians are very accustomed to employing this test and to its interpretation. Unless there is strong guideline-based pressure to change, and unless the cost of immunochemical testing drops, guaiac-based testing may continue to be selected by most physicians as their preferred approach.
1. Imperiale, Thomas F. Editorial. Ann Intern Med. 2007;146:309-311.