A false positive result – a positive test for occult blood in faeces with no significant disease subsequently found on colonoscopy – is quite a common finding in bowel screening programmes. One of the reasons given for this is that bleeding into the gut could occur other than when colorectal neoplasia is present, including in participants using oral anti-coagulants (OCs) and non-steroidal anti-inflammatory drugs, including aspirin (NSAIDs). Of course, these medicines are commonly used in the 50-74 year old population invited for bowel screening in Scotland.
Now that faecal immunochemical tests for haemoglobin (FIT) are the first-line investigation in the Scottish Bowel Screening Programme, a very relevant question is whether this newer test, which can detect much smaller quantities of blood than the traditional guaiac-based faecal occult blood test (gFOBT), gives more false positive results in participants taking OCs or NSAIDs.
A number of studies on gFOBT have implied that this interference is a possibility and some have suggested that participants should cease taking these medicines before undertaking collection of faeces for testing. Institution of this imposition would very likely lead to lowering the uptake of screening. On the other hand, it has been postulated that continuing these medicines might have real benefit through increasing the tendency of any neoplastic lesion present to bleed, in turn increasing the positive predictive value of FIT.
This important question has been addressed in a new, the first, systematic review and meta-analysis on the effect of OCs and NSAIDs on the diagnostic accuracy of FIT in colorectal cancer screening.1
The results show that the use of these medicines does not affect the positive predictive value of FIT for advanced neoplasia (colorectal cancer plus advanced adenoma). This might be because FIT are much more specific for colorectal bleeding than the traditional gFOBT, since they detect the globin moiety of haemoglobin rather than the haem component: bleeding from the upper gastrointestinal tract is not detected by FIT due to rapid degradation of any haemoglobin released there. It was concluded that, based on the current literature evidence, stopping taking OCs or aspirin and other NSAIDs before collecting samples for use in FIT-based screening programmes is not recommended. Keep on taking the medicine!
1 Nieuwenburg SAV, Vuik FER, Kruip MJHA, Kuipers EJ, Spaander MCW. Effect of anticoagulants and NSAIDs on accuracy of faecal immunochemical tests (FITs) in colorectal cancer screening: a systematic review and meta-analysis. Gut. 2018 Jun 5. pii: gutjnl-2018-316344. doi: 10.1136/gutjnl-2018-316344. [Epub ahead of print].