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Thrombin Time for Heparin

Hi George:

I am lead heme tech at a medium sized hospital.  My boss asked me why we sent out a thrombin time on a patient on heparin.  There’s all sorts of ways to answer it. I wonder what you suggest. Do I call the ordering physician and ask him why he’d want to run this? Or am I just forgetting the clinical usefulness of running a thrombin time while the patient is on heparin? Your ideas will be greatly appreciated.

Kathy Wagner
Hi, Kathy,

While laboratories in North America prefer the partial thromboplastin time (PTT), many labs in Europe and a few in the US use the thrombin clotting time (TCT or TT) to monitor standard unfractionated heparin. It may be your ordering physician has used labs in the past that favored the thrombin time for this purpose. Your task is to contact the physician and convince him that you use the PTT for this purpose.

In the US many labs offer the TT, but its usual purpose is to detect, but not quantitative unfractionated heparin. This is convenient when there is an unexpectedly prolonged PTT and may be part of a mixing study protocol. This TT method employs a low reagent thrombin concentration and is sensitive to unfractionated heparin. The TT employed in European labs uses a higher reagent thrombin concentration that provides a linear response to unfractionated plasma heparin.

By the way, physicians are using more and more low molecular weight heparin, which cannot be measured using either the TT or PTT. For LMWH we use the chromogenic anti-Xa heparin assay. The recent heparin contamination problem is likely to result in even more LMWH use. Geo.

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