one of the main,noninvasive test performed to find any suspect for colorectal masses is the guaiac test (a fecal occult blood detecting test). the problem with this test is that it is affected by many variables that may produce false positives and false negatives. the presence of non human hemoglobin, rehydration may produce false positives meanwhile hemoglobin degradation or vitamin C may produce false negatives. however the rate of these false results depend on the bleeding site. to prevent these false results,new methods are developed and their efficiency is getting analysed. the following article talks about one of these. it mainly concerns the usage of these methods in detecting colorectal cancers
also, i'd like to recommend you a review article present in the New England Journal of Medicine (NEJM 1999; 341: 38-46) that analysis the main fecal occult blood detecting test. i found it very interesting and i hope you find it the same
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More Support for Fecal Immunochemical Testing
Immunochemical testing is emerging as superior to guaiac-based testing for colorectal cancer screening.
Many studies have indicated that immunochemical testing is superior to guaiac-based testing for detecting fecal occult blood. Adding to this evidence base is a French study, in which 10,673 average-risk adults completed both standard guaiac-based testing (nonrehydrated Hemoccult II) and quantitative immunochemical testing (Immudia/RPHA). Of the 886 study participants who had positive results on one or both tests, 644 underwent colonoscopy. Of these, 294 had adenomas or cancer.
The immunochemical test outperformed the guaiac-based test at every evaluated hemoglobin threshold. At the usual threshold of 20 ng/mL, the immunochemical test identified 50% more cancers and 256% more high-risk adenomas than did the guaiac-based test. However, the increased sensitivity resulted in lower specificity, such that 47 false-positive results would be required to detect 1 extra case of invasive colorectal cancer, and 2.2 false-positive results would be required to detect 1 extra advanced adenoma. With a threshold of 50 ng/mL, the immunochemical test detected 2.3 times as many advanced neoplasias as the guaiac-based test did, with no loss of specificity. At a threshold of 75 ng/mL, the overall rate of positive results on the immunochemical test was similar to that of the guaiac-based test (2.4%), with a 90% gain in sensitivity and an actual improvement in specificity (a 33% reduction in false positives).
Comment: The time has come to replace guaiac-based testing for colorectal cancer with fecal immunochemical testing wherever possible. One powerful advantage of the quantitative testing method is that the performance characteristics can be adjusted to meet the goals of specific screening programs by altering the threshold level for a positive test.
— Douglas K. Rex, MD
Published in Journal Watch Gastroenterology May 25, 2007
Citation(s):
Guittet L et al. Comparison of a guaiac based and an immunochemical faecal occult blood test in screening for colorectal cancer in a general average risk population. Gut 2007 Feb; 56:210-4.