Choosing a Fecal Occult Blood Test
Abstract & Commentary
By Mary Elina Ferris, MD, Clinical Associate Professor, University of Southern California; Dr. Ferris reports no financial relationship to this field of study.
Synopsis: Correlation of 6 different immunochemical qualitative fecal occult blood tests with colonoscopy findings showed wide variation in diagnostic performance.
Source: Hundt S, et al. Comparative evaluation of immunochemical fecal occult blood tests for colorectal adenoma detection. Ann Intern Med 2009;150:162-169.
Twenty gastroenterology practices in Germany used stool samples collected before screening colonoscopies to evaluate 6 different fecal occult blood tests (FOBTs) that used immunochemical techniques (I-FOBTs) rather than traditional guaiac-based testing. Laboratory technicians and colonoscopists were blind to each other's results.
Patients were 55 years old and older, referred for screening colonoscopies from general practices. Of the 1785 who agreed to participate, 1319 were left after exclusions for pre-existing disease, having had a prior colonoscopy within the last 5 years, and inadequate colon preparation; mean age was 63 years; and an equal number of men and women were included. Samples of both guaiac-based and I-FOBTs were collected at home by patients 1 week prior to colonoscopy. Adenomas were found in 405, or 31%, and of those 10% were considered advanced.
Significant differences were found among the 6 types of I-FOBTs used, with sensitivity ranging for any adenoma ranging from 11.4% to 58.0%, and for advanced adenomas from 25.4% to 71.5%. Specificity ranged from 58.8% to 96.7%. Positive predictive values for the different I-FOBTs with respect to any adenoma ranged from 40.2% to 60.5%, whereas the range of negative predictive values was more narrow, from 71.1% to 76.0%.
Commentary
Early detection of colorectal adenomas and cancers by colonoscopy saves lives, but is not routinely accessible to many populations. FOBTs thus remain an important screening tool for identifying risk and need for colonoscopy, giving mortality reductions between 15%-30% when used annually or biennially in asymptomatic persons. New types of FOBTs may give us opportunities for better accuracy, but also raise challenges to determine how they should be utilized.
Immunochemical techniques are the latest advance in colorectal cancer screening. I-FOBTs use specific antibodies for human blood, which overcomes the limitation of traditional guaiac-based testing, which can generate false positives and false negatives from other substances found in the stool. Guaiac-based FOBTs detect the pseudoperoxidase activity of heme or hemoglobin, and is not specific for human blood. Another difference is the sampling method for I-FOBTs, which is collected on a paper in the toilet bowl and gathers a more standardized amount of stool for testing than guaiac card testing.
Whether I-FOBTs are superior to guaiac testing is currently the subject of many articles. An extensive review of published studies on the accuracy of both types of tests concluded that no clear evidence clarified which performed better,1 but another randomized study of 20,620 persons older than age 50 showed better participation rates for I-FOBTs (13% more), which led to better detection.2 Specificity in this study of I-FOBTs was lower than guaiac, resulting in 3 times more negative colonoscopies, but guaiac testing missed more tumors, so in the final analysis the number-to-scope to find one cancer was comparable with both tests.
This conflicting evidence leaves us cautious before we rush to change our current FOBT methods. Results can vary depending on which populations are studied and whether the I-FOBT is compared directly with guaiac testing. I-FOBTs do not require dietary restrictions and may be better tolerated, so if costs are comparable it would seem the best choice for compliance reasons. However, more costs are incurred with unnecessary colonoscopies, which also influences the decision. The answer may come with the next generation of I-FOBTs, which will report laboratory-based quantitative results, giving tumor detection in 1 study of 67% specificity and 91% sensitivity.3 This would allow clinical decisions based on actual hemoglobin values in the results. Cost analysis will continue to be important in these choices, so we have to wait for more information to guide us in our choice of FOBT screening strategies.
References
1. Burch JA, et al. Diagnostic accuracy of faecal occult blood tests used in screening for colorectal cancer: A systematic review. J Med Screen 2007;14:132-137.
2. Van Rossum LG, et al. Random comparison of guaiac and immunochemical fecal occult blood tests for colorectal cancer in a screening population. Gastroenterology 2008;135:82-90.
3. Levi Z, et al. A quantitative immunochemical fecal occult blood test for colorectal neoplasia. Ann Intern Med 2007;146:244-255.
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