Home / Learn about HIT / When does HIT occur?

When does HIT occur?

Typical onset HIT

HIT usually presents between days 5 and 14 after the start of heparin therapy (1). Typically the platelet count begins to fall 5 -10 days after starting therapy with thrombocytopenic levels reached approximately after 7-14 days and /or a thrombotic event occurs between days 5 and 14 after heparin initiation. Timing of onset is a clinical feature of HIT that helps distinguish it from other forms of thrombocytopenia.

Rapid onset HIT

Approximately, two thirds of HIT patients have typical onset HIT, but in 25-30% of HIT patients the platelet count falls abruptly on starting a course of heparin (1). This occurs in patients who have been exposed to heparin within the previous 100 days and have residual circulating HIT antibodies.

Delayed onset HIT

In some 3 to 5% of patients, the onset of thrombocytopenia begins several days after heparin has stopped (1). In these patients HIT resembles an autoimmune disorder in which the antibodies recognise and bind to PF4 on the platelet surface even in the absence of heparin. They typically have  HIT antibody tests that are strongly positive caused by high levels of anti-PF4/heparin antibodies (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 K et al. Heparin-induced thrombocytopenia in intensive care patients. Crit Care Med 2007; 35(4):1165-76

Who is the patient?