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SAN DIEGO, Sept. 12 -- Routine screening by questionnaire for caffeine intake before pharmacologic cardiac stress testing fails to identify most patients who have detectable serum levels of the stimulant, investigators said here.

Pretest screening identified only four of 37 patients who had positive serum caffeine tests, Lesan Banko, M.D., of New York Methodist Hospital in Brooklyn, reported at the American Society of Nuclear Cardiology meeting. A more detailed pretest questionnaire was no better than a routine screen for identifying caffeine-positive patients.

Unexpectedly, however, positive caffeine did not confound stress test results, leaving Dr. Banko and colleagues to ponder the evidence backing the long-held belief that caffeine interferes with pharmacologic stress testing by competitively blocking coronary vasodilatory effects.

"More studies are needed to determine whether serum caffeine should be measured in patients undergoing pharmacologic stress tests," said Dr. Banko. "Patients who were positive for serum caffeine were just as likely to have a positive stress tests as those who tested negative, and we're not sure what to make of that at this point."

Adenosine and dipyridamole are the agents most commonly used in pharmacologic myocardial perfusion stress testing. Methylxanthines, such as caffeine, competitive block the vasodilatory effects of adenosine and dipyridamole.

Routine pretest screening for caffeine usually focuses on caffeine-containing beverages, such as coffee, tea, and cola, as well as chocolate, said Dr. Banko. However, caffeine also is found in a number of other products, including herbs and medications, both prescription and over the counter.

"We wanted to see whether more intensive screening for caffeine would identify patients who might have been missed by a routine screen," said Dr. Banko.

Investigators prospectively evaluated 194 consecutive patients scheduled for dipyridamole myocardial perfusion stress testing. The patients were randomized to a routine or an intensive caffeine screening questionnaire, the latter of which listed all known food, herbal, and drug products that contain caffeine.

All patients' serum caffeine was measured, and patients who tested positive were switched to dobutamine for the stress test.

Four patients in each group had positive screens for caffeine. In contrast, 37 of the 191 patients (19.4%) of patients tested positive for serum caffeine. Additionally, four of the eight patients who screened positive had undetectable serum caffeine levels.

Dr. Banko reported that 54% of patients who tested positive for serum caffeine had positive stress tests compared with 44% of patients who tested negative, a nonsignificant difference. Among patients who tested positive for serum caffeine, the mean serum caffeine value did not differ significantly between those who had positive or negative stress tests (1.60 versus 1.82 mg/L).

Although the findings showed that more detailed screening fails to identify many patients who have caffeine in their circulation, the results don't make a case for routine serum testing, said Dr. Banko.

"We need to know more about the implications of a positive serum test before any recommendations can be made," he said.

Dr. Banko had no disclosures.

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