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We read with interest the report by Warkentin and colleagues,¹ in which the application of a new platelet count fall definition in heparin-induced thrombocytopenia (HIT) revealed twice as many patients to have HIT than originally reported. The improved new definition abandoned the criterion of a fall in platelet count below 150 x 10⁹/L (absolute thrombocytopenia) in postoperative patients, since postoperative thrombocytosis like other preexisting chronic thrombocytosis² can shift the platelet count nadir following a platelet count fall of 50% or higher to values greater than 150 x 10⁹/L (relative thrombocytopenia).

These findings reflect the fact that the platelet count nadir is dependent on platelet production and platelet consumption. Interestingly, there have been reports in which HIT-associated thrombosis (HITT) was postulated even in the absence of relative thrombocytopenia following cardiovascular surgery.^3-4

In these cases, relative thrombocytopenia was thought to be masked by a supposedly excessive postoperative platelet production. In . . . [Full Text of this Article]

Lorenz Risch, MD; Andreas R. Huber, MD

Correspondence: Dr Huber, Laboratory Medicine, Kantonsspital Aarau, Tellstrasse, CH-5001 Aarau, Switzerland (andreas.huber@ksa.ch).

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