There are several challenging aspects to treating patients with HIT. These patients are not clinically homogeneous; they represent a complex mix of varying initial indications for heparin, location, and severity of HIT-associated thrombosis, and, not infrequently, dysfunction of one or more vital organs. This presents difficulties both for performing clinical studies as well as in the application of treatment recommendations for individual patients. Furthermore, there are important differences among countries in the approval or availability status of certain recommended treatment approaches. Thus, the treatment recommendations we make cannot be indiscriminately applied to all patients with suspected or confirmed HIT.

A further practical problem is that the major treatment options for HIT include relatively new and, for some physicians, unfamiliar or even unapproved anticoagulant agents. This presents extra challenges to physicians and also to laboratories asked to monitor anticoagulant treatment effects, as the treatment "learning curve" may occur in emergency situations. Also, immediate results of reliable laboratory tests for HIT are usually unavailable. Difficult management decisions may be needed amid diagnostic uncertainty: A diagnosis of HIT that seems obvious in retrospect may not have been so clear during its early evolution.

As an iatrogenic illness that occurs unpredictably and unexpectedly, often in a setting of antithrombotic prophylaxis, medicolegal aspects must also be considered (Mclntyre and Warkentin, 2004; Ulsenheimer, 2004). Thus, once HIT is entertained as part of a differential diagnosis, we suggest that physicians document carefully the various diagnostic and treatment considerations as events unfold.

As a common, rare disease (we acknowledge Prof. R. Hull [Calgary, Canada] for this description of HIT) that physicians only occasionally manage and that only rarely enters into clinical studies, we need to acknowledge that no final answer for treatment is likely to emerge. Therefore, even in this fourth edition, this chapter should be viewed as a basis for further discussion and study of the treatment of HIT patients.

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