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Taking low-dose aspirin does not interfere with screening for colorectal cancer when patients use newer immunochemical fecal occult blood tests (FOBTs) -- and actually may increase the accuracy of the tests, a large prospective study suggested.

In a study conducted among almost 2,000 patients (more than 230 of whom used low-dose aspirin), the sensitivity of the hemoglobin FOBT for detecting advanced neoplasms among the aspirin users was 70.8% (95% CI 48.9 to 87.4) compared with 35.9% (95% CI 28.9 to 43.4, P=0.001) among nonusers, according to Hermann Brenner, MD, and colleagues from the German Cancer Research Center in Heidelberg.

For another fecal test -- the hemoglobin-haptoglobin test -- the sensitivity among low-dose aspirin users was 58.3% (95% CI 36.6 to 77.9), and 32% (95% CI 25.3 to 39.4, P=0.01) among nonusers, Brenner and co-authors reported in the Dec. 8 Journal of the American Medical Association.

Screening for colorectal cancer using fecal occult blood tests is increasingly popular, as it has been shown to reduce the incidence and mortality associated with the disease.

But there have been concerns that the use of low-dose aspirin -- which is most common at the ages when colorectal cancer screening tends to be performed -- could interfere with the sensitivity and specificity of FOBT testing.

Of particular concern has been the possibility that the use of low-dose aspirin could increase bleeding from the upper gastrointestinal tract or from small, insignificant lesions in the colon.

Although this is less likely to occur with the newer immunochemical tests than with the conventional guaiac FOBT tests, little is known about the accuracy of these tests in patients taking low-dose aspirin and some investigators have suggested that the aspirin use might need to be stopped before screening is done.

To address this, Brenner's group analyzed data from 1,979 participants in an ongoing colorectal cancer screening study.

All participants provided stool samples for testing, and subsequently underwent colonoscopy, which served as the reference standard.

Among this cohort, 233 were currently using low-dose aspirin; most were men with a mean age of 65.

Using the cut points recommended by the manufacturer (2 μg/g of stool), the sensitivity of the two FOBT screening tests for detecting any neoplasm was "substantially higher among users than among nonusers (P=0.03 for both tests)," the researchers wrote.

Specificity for detection of advanced neoplasms was slightly lower for the hemoglobin test among users, at 85.7% (95% CI 80.2 to 90.1) compared with nonusers, for whom the specificity was 89.2% (95% CI 87.6 to 90.7, P=0.13).

For the hemoglobin-haptoglobin test, the specificity for users was 85.7% (95% CI 80.2 to 90.1) and 91.1% for nonusers (95% CI 89.5 to 92.4, P=0.01).

When the investigators repeated the calculations using different cut points -- ranging from 1 μg/g to 8 μg/g -- they found that for most values, sensitivity was "markedly higher," at about 30%, and specificity "slightly lower," at about 5% lower among the low-dose aspirin users.

The sensitivity for detecting advanced neoplasms was highest among men, with differences between aspirin users and nonusers reaching 46.4% for the hemoglobin test.

These findings support the notion that cessation of low-dose aspirin isn't needed when patients use the newer immunochemical FOBT colorectal cancer screening tests, according to Brenner and colleagues.

"On the contrary, our results may even raise the provocative suggestion of whether temporary use of low-dose aspirin might be considered to enhance performance of [immunochemical fecal occult blood tests]," the authors suggested.

Such a recommendation at this time would be premature, they added, but further research into this matter is warranted.

Limitations of the study included relatively small numbers of aspirin users and the self-report of use, so larger samples will be needed in future, preferably randomized, studies, the researchers noted.

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