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Tests to Measure Safety of Anti-Clotting Drugs of Limited Value

Can't predict which patients are at risk for excessive bleeding during surgery, study finds

TUESDAY, Feb. 23 (HealthDay News) -- Tests that try to single out who will have bleeding problems when they get a clot-preventing drug such as Plavix before surgery aren't ready for regular use, a new Dutch study concludes.

Three of the six tests, which measure the function of platelets, the blood cells that clump together to form clots, did provide some useful information, according to the report in the Feb. 24 issue of the Journal of the American Medical Association.

"However, the predictability of these three tests was only modest," the report added. "None of the tests provided accurate prognostic information to identify patients at higher risk of bleeding."

And so, the study "does not support the use of platelet function testing to guide clinical practice in a low-risk population of patients," the Dutch researchers wrote.

The conclusion did not surprise U.S. doctors who have studied the issue. "For my purposes, these tests are research tools," said Dr. Deepak Bhatt, chief of cardiology at the VA Boston Healthcare System, and a member of a group of U.S. cardiologists who said pretty much the same thing in a 2008 statement.

Yet, the Dutch study "is well-done and a valuable contribution to the field," Bhatt said. "It shows just how much different tests add to clinical judgment."

The study included 1,069 people given clopidogrel (Plavix) before having stents implanted after artery-opening angioplasty. They were followed for a year to record the incidence of death, nonfatal heart attacks or strokes, and new blockages of the treated arteries. Three of the tests had some predictive value for those events, but three didn't. None predicted the major side effect of Plavix treatment, which is excessive bleeding.

The study is a starting point for understanding a complex clinical situation, Bhatt said. "It sets us up for the next step. You have a patient at moderate risk; what do you do with that test information? That is a question that needs to be answered."

Several large studies are underway to provide an answer, Bhatt said. They are looking at large numbers of people who get clot-preventing drugs. "Right now, you can conclude that some tests are correlated with an increased risk of adverse events, but what to do with that information is unclear," Bhatt said. "Those ongoing trials may provide an answer."

Trail results will not be available for "a minimum of one to two years," he noted. "My feeling is that until then, there is not enough data to test routinely."

While the Dutch study provided "an important observation," it included a relatively narrow portion of people eligible for clot-dissolving drug therapy, added Dr. Magnus Ohman, director of the Program for Advanced Coronary Disease at Duke University in Durham, N.C.

"This tested patients after stenting, a relatively isolated group of patients," Ohman said. "All patients who are treated medically with clot-preventing medication tend to be older, have more bleeding, have more problems with metabolism. The study fails to give an overall picture of how high-risk patients fare."

Even though the study population was relatively large, it is small in relation to the total population of people who get clot-preventing drugs, Ohman said. "If we studied more patients and higher-risk patients, we might get a better handle on this. We might find other cutoff points of greater value."

Some of the tests "clearly give information we doctors need," Ohman said. Right now, Duke cardiologists use only the most complex test in the study, light transmittance aggregometry, which requires sophisticated laboratory equipment and takes hours for results, he said.

"We are in the process of evaluating the bedside tests because they will have an important role to play in the future," Ohman said.

More information

Platelet test basics are described by the University of Maryland Medical Center.

SOURCES: Deepak Bhatt, M.D., chief, cardiology, VA Boston Healthcare System; Magnus Ohman, M.D., professor, medicine, and director, Program for Advanced Coronary Disease, Duke University, Durham, N.C.; Feb. 24, 2010, Journal of the American Medical Association

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