Objective: To determine whether low-dose aspirin or warfarin induces fecal occult blood loss.
Patients and methods: A prospective, cross-over study, of 100 participants over 40 years of age in 1 of 3 groups, taking: (1) no aspirin or warfarin, (2) daily aspirin (either 81 or 325 mg), or (3) warfarin, but no aspirin. Stool samples were collected and analyzed for the presence of occult blood using HemoQuant and Hemoccult II. After collection of baseline samples, patients initially taking no aspirin (group 1) were asked to take regular aspirin (325 mg daily) for 2 months. Patients initially taking aspirin 81 mg daily (group 2) were switched to 325 mg daily for 2 months, and vice versa.
Results: Patients taking no aspirin had mean fecal blood of 0.68 +/- 0.05 mg hemoglobin/g stool, which increased to 1.41 +/- 0.36 mg/g after taking 325 mg of aspirin daily (P = 0.02). In contrast, patients in group 2, taking 81 mg and 325 mg of aspirin, had mean fecal blood of 0.82 +/- 0.08 mg/g (P = 0.57) and 1.04 +/- 0.23 mg/g (P = 0.13), respectively (comparisons with patients taking no aspirin). The mean blood loss in patients taking warfarin was 0.51 +/- 0.04 mg/g (P = 0.55), and fecal blood was not related to the degree of anticoagulation. There was no increase over normal in the rate of Hemoccult II-positive stool tests with aspirin or warfarin therapy.
Conclusion: Aspirin, but not warfarin, caused a small but clinically insignificant increase in occult fecal blood. The small blood loss in patients taking aspirin or warfarin is unlikely to interfere with fecal occult blood test. Therefore, positive fecal occult blood tests, in patients taking either low-dose aspirin or warfarin, should be managed in the same fashion as patients not taking these medications.