College of American Pathologists
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What laboratories should do

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June 2000

So what should laboratories do about the preanalytical variability of partial-draw sodium citrate coagulation tubes? According to Becton Dickinson, laboratories should stop using the partial-draw tubes immediately for monitoring activated partial thromboplastin time in patients receiving unfractionated heparin and for conducting platelet counts.

But, as BD vice president for medical affairs Jeffry Lawrence, MD, acknowledged, heparin monitoring represents probably less than five percent of all the tests performed with coagulation tubes. Partial-draw tubes are entirely suitable for other forms of testing, including determining prothrombin time or coagulation testing of patients who have long APTTs unrelated to unfractionated heparin, such as those with hemophilia or von Willebrand’s disease.

"For the time being, while laboratories are transitioning to a different product, they should have no problem using the partial-draw tubes to generate prothrombin times or APTTs for other patient populations," he says.

However, laboratories will need to regroup when BD pulls the partial-draw tubes off its assembly line on Sept. 30. The best answer for most laboratories is to return to the full-draw sodium citrate tube. "This, after all, is the tube that’s been used to generate all the clinical recommendations, and it is considered to be the gold standard around the world for coagulation testing," Dr. Lawrence says. The full-draw tube’s traditional 13 ¥ 17 mm configuration is compatible with holders, centrifuges, and coagulation analyzers.

Laboratorians and physicians who want to avoid preanalytical variability and still draw lower volumes of blood have other alternatives. Pediatric tubes that draw either 2.7 mL or 1.8 mL of blood have a different configuration than that of the partial-draw tubes and, consequently, do not cause platelet activation. Because these tubes are smaller than standard coagulation tubes, however, they may not be compatible with laboratory instrumentation.

The CTAD tube has been available in Europe for a few years and is being released now in the U.S. It is a full-size, 13 ¥ 75 mm tube that is under-evacuated to draw 2.7 mL of blood. Because of its platelet-inhibiting cocktail of theophylline, adenosine, and dipyridamole, the tube does not interfere with unfractionated heparin monitoring or platelet counting, Dr. Lawrence says. This additive-specifically dipyridamole-is light-sensitive, so there are special handling requirements to consider.

When a laboratory switches to one of these alternatives, it should establish a new reference range, just as it would when adding another instrument or a different reagent. "The laboratory would need to determine the reference range for a healthy population and follow the CAP recommendations for establishing its heparin therapeutic range, using the Brill Edwards or some other method described by CAP," Dr. Lawrence says.

-Karen Sandrick