Screening
Among patients receiving heparin in whom the risk of HIT is considered low (<0.1%), the American Society of Hematology (ASH) guideline panel suggests against platelet count monitoring to screen for HIT.[38] The potential benefit of screening is likely to be highest in settings where patients have a 1% or greater risk of HIT (e.g., patients receiving heparin or low molecular weight heparin [LMWH] following surgery or trauma) and a functional test for platelet‐activating antibodies is available to reduce the likelihood of false-positive results.[44]
Screening for HIT usually means obtaining serial platelet counts in patients who are receiving unfractionated heparin/LMWH, or, more rarely, fondaparinux for at least 4 days, or in patients who have presented with thrombosis in the context of exposure to these drugs within the past 100 days. Due to the large number of alternative causes of thrombocytopenia, routine platelet count monitoring has the potential to cause harm due to unnecessary withdrawal of heparin and institution of nonheparin anticoagulants in patients who do not have HIT.
HIT antibody assays are not appropriate for screening.
Platelet count monitoring
If screening is performed, platelet counts are typically checked every 2 or 3 days from day 4 to 14 (or until heparin is stopped, whichever comes first).[38] If a platelet count drop is noted during routine screening and the pretest probability of HIT is at least moderate (i.e., 4Ts score ≥4), confirmatory laboratory testing for HIT antibodies is recommended.
It is also worth noting that, if the patient undergoes surgery, the day that heparin is restarted after the procedure is considered day 0 of heparin exposure, even if the patient received heparin preoperatively. Surgery is a strong immunizing risk factor for HIT and can, therefore, potentially reset the clock for the development of HIT. Similarly, if heparin is given intraoperatively, the surgery date becomes day 0.
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