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After cardiac surgery

Diagnosing HIT is particularly difficult in patients who have undergone recent cardiac surgery. In such patients the risk of heparin antibody formation is high, ranging from 35-65% by days 7 to 10 (1). Thrombocytopenia is common and other medications may be administered that cause thrombocytopenia.

Due to the high risk of HIT in this population, platelet count monitoring for HIT is especially important if UFH or LMWH are used. A practical problem is that major haemodilution occurs both during cardiac surgery and several days after surgery. This causes a drop in the platelet count that typically reaches its nadir 2 days following surgery. However, the onset of HIT is rare in the first 4 days after cardiac surgery. The typical onset of HIT (5-10 days after cardiac surgery) coincides with the time period in which typically the platelet count is rising. 

 

It is recommended that for postoperative cardiac surgery patients a diagnosis of HIT should be suspected if the platelet count falls by ? 50% between postoperative days 5 to 14, and/or a thrombotic event occurs between days 5 -14  (day of cardiac surgery = day zero)(1).

HIT should also be suspected in postoperative patients in whom low platelet counts (typically below 100 x109/L)  persist for more than 7 days after cardiac surgery(2).

1. Warkentin TE, Greinacher A, Koster A, Lincoff AM. Treatment and prevention of heparin-induced thrombocytopenia: ACCP evidence based clinical practice guidelines (8th Edition). Chest 2008; 133:340-380.

2. Selleng S et al. Heparin-induced thrombocytopenia in patients requiring prolonged intensive care unit treatment after cardiopulmonary bypass. J Thromb Haemost 2008; 6(3):428-435

Laboratory testing for HIT