The positive predictive value of the FOBT is 26.3%. This means that cancers and adenomas (including small adenomas) are detected 26% of the time that a positive FOBT is identified in average risk patients in a population. That value falls to 20% if you take out diminutiveadenomas. This does mean that a large proportion of the FOBTs are falsely positive. Having said that, there is strong evidence that colon cancer mortality is reduced by up to one third with a FOBT screening program. A positive test should be followed up by a colonoscopy if the patient has not had a screening colonoscopy in the immediate past (about two years). If an FOBT is positive and the most recent colonoscopy was two years prior, then a positive FOBT should trigger a further colonoscopy.
It is instructive to look at the outcomes of National Bowel Cancer Screening Program (NBCSP) in Queensland. Between 8/06 and 12/10, 552 400 kits were posted. The number returned was 183 000, of which 14 000 were positive. All patients with positive FOBT were offered screening at a QLD Health facility and 4800 accepted the offer. Amongst these 4800 individuals there were 210 colon cancers, and 3070 polyps of which about half were advanced (>1cm in diameter), i.e. a positive FOBT identified a significant polyp or cancer in 36% and any polyp or cancer in 68%. The false positive rate then for significant lesions was 64% and for any lesion, 32%. The figure of 26.3% mentioned above is a national figure and reflects the outcome in the screened population as a whole, including the majority who either did not return the test or did not attend for colonoscopy when indicated.