Patients are usually told not to take aspirin before having a stool test to screen for colon cancer, but that recommendation seems to be unnecessary, a new study shows.

The regular use of aspirin - or similar painkillers called nonsteroidal anti-inflammatory drugs or NSAIDs - inhibits blood clotting. This is a benefit for people with a propensity to form clots that can cause strokes or heart attacks, but it does mean that some degree of internal bleeding is more likely. Therefore, blood is more likely to show up in stool, it has been thought.

However, aspirin and NSAIDs do not increase the risk for a false-positive fecal occult blood test - that is, a finding of blood in stool but no colon cancer - according to a report in the American Journal of Medicine.

“If patients are able to safely stop their aspirin prior to FOBT (fecal occult blood test) without risk of adverse events and remember to stop it, then that’s fine,” Dr. Charles J. Kahi from Indiana University Medical Center, Indianapolis, advised.

“But if they cannot do it safely ... or they forget to do so, they can still proceed with collecting the specimens for FOBT while on aspirin without compromising the validity of the results,” he told Reuters Health.

Kahi and Dr. Thomas F. Imperiale looked into the question of whether regular use of aspirin or NSAIDs increased the likelihood of a false-positive FOBT result in a study involving 193 veterans. They had all had a positive stool test result, and so underwent a colonoscopy to check for colon cancer.

For nearly 80 percent of the patients, nothing seen on the colonoscopy could explain the positive FOBT result, the authors report.

On the other hand, colon lesions were found in one-fifth of the regular aspirin or NSAID users and in a similar fraction of those using neither type of drug.

After adjusting for age, family history of colorectal cancer, and other factors, the researchers saw no association between regular aspirin or NSAID use and the lack of colonoscopy findings that would explain a positive stool test.

“Our results should not be misinterpreted as showing that positive FOBTs obtained while on aspirin are somehow less valid than positive FOBTs obtained while off aspirin,” Kahi cautioned. “Any positive FOBT has to be worked up appropriately until a satisfactory explanation is found.”

SOURCE: American Journal of Medicine, December 1, 2004.