Does Acetylsalicylic Acid or Warfarin Affect the Accuracy of Fecal Occult Blood Tests?

Sumeet Gandhi; Neeraj Narula; Samir Gandhi; John K Marshall; Michael E Farkouh


J Gastroenterol Hepatol. 2013;28(6):931-936. 

In This Article

Abstract and Introduction


Background: Current guidelines for screening of colorectal cancer do not offer specific recommendations for cessation of antithrombotic agents prior to fecal occult blood test (FOBT).

Aim: To asess the accuracy of FOBT in patients taking acetylsalicylic acid (ASA) or warfarin.

Methods: A literature search was conducted for studies that investigated the accuracy of FOBT in patients taking ASA and warfarin. The primary outcome was the pooled relative risk (RR) for true positive FOBT for detecting significant colonic neoplasia in patients taking ASA or warfarin compared with controls. The secondary outcome was a pooled RR for true positive in guaiac FOBT (g-FOBT) compared with immunochemical FOBT (i-FOBT).

Results: Five observational studies included 759 patients taking ASA and 1652 control subjects. In patients taking ASA, pooled RR for true positive FOBT was 0.82 (95% confidence interval [CI] 0.73–0.93, P=0.0009), pooled RR for true positive g-FOBT was 0.69 (95% CI 0.60–0.79, P<0.0001), whereas pooled RR for true positive i-FOBT was 1.013 (95% CI 0.81–1.30, P =0.8182). Five observational studies included 806 patients taking warfarin and 10338 control subjects. In patients taking warfarin, pooled RR for true positive FOBT was 1.559 (95% CI 1.349–1.801, P<0.0001).

Conclusion: The results of our meta-analysis demonstrate that in patients taking ASA, there is a decrease in the positive predictive value (PPV) of g-FOBT but no significant difference in the PPV of i-FOBT compared with control subjects for detecting significant neoplasia. In patients taking warfarin, the PPV of FOBT was increased for detection of colorectal cancer compared with control subjects.


Colorectal cancer (CRC) is the second leading cause of malignancy-related morbidity and mortality in Canada responsible for approximately 12% of cancer-related deaths.[1,2] Population-based screening with fecal occult blood test (FOBT) followed by colonoscopy is cost-effective and confers an overall mortality benefit.[3] Screening FOBT allows for an increase in detection of early-stage cancer and facilitates the removal of high-risk precursor adenomas.[4–7]

Recommendations for FOBT screening are based upon randomized control trial evidence with the use of guaiac FOBT (g-FOBT). A positive test results from the oxidation of stool guaiac by hydrogen peroxide catalyzed by the peroxidase activity of hemoglobin. A disadvantage of this test is that bleeding from any site in the gastrointestinal tract, such as the stomach, can yield a false-positive result.[8] Newer fecal assays such as fecal immunochemical testing and immunochemical FOBT (i-FOBT) use antibodies against human globin, a protein component of blood. Compared with g-FOBT, these newer testing modalities have greater specificity in detecting bleeding from the distal gut, colon, and rectum, and have shown superior positive predictive value (PPV) in population-based CRC screening programs.[9–12]

The use of antiplatelet and anticoagulant therapies for secondary cardiovascular prevention has increased. Such therapies may increase the sensitivity of FOBT by facilitating bleeding from dysplastic colonic lesions leading to earlier detection of CRC and high-risk adenomas. Conversely, these drugs may lead to lower the PPV of FOBT by promoting bleeding from other gastrointestinal lesions, may increase the number of unnecessary colonoscopies, and may reduce the cost-effectiveness of population-based screening.

In 2000, Sharma etal. published a survey of American physician practices related to cessation of antiplatelet and anticoagulant therapy prior to FOBT. This study revealed that only 10.1% of internal medicine residents, 15.9% of primary care physicians, and 32% of gastroenterologists terminated anticoagulant therapy prior to FOBT.[13–15] Current guidelines for population-based screening of CRC do not offer specific recommendations for cessation of antithrombotic agents prior to FOBT.[1] We performed a meta-analysis of studies assessing the effect acetylsalicylic acid (ASA) and warfarin have on the PPV of FOBT for the detection of CRC or high-risk adenomas found on colonoscopy. As well, we compared the PPV of g-FOBT with i-FOBT in patients taking ASA and warfarin.