Test Background HIT with or without thrombosis (HITT) is mainly associated with treatment with unfractionated heparin (UFH), but it can also occur with low molecular weight heparin (LMWH). HIT is caused by the formation of abnormal antibodies that activate platelets, and may be suspected even if heparin treatment has already been discontinued, typically developing 4-14 days after the administration of heparin. Despite the low platelet count, it is a thrombotic disorder, with very high rates of thrombosis in the arteries, with or without venous complications. Of note, the rate of DVT is roughly 4 times that of arterial thrombosis and, while thrombocytopenia is the most common ‘event’ in HIT, DVT is in fact the most common complication. A commonly used score to predict the likelihood of HIT is the ‘4 Ts’ score, where: ≥6-8 is highly suggestive of HIT; 4-5 is intermediate probability and Thrombocytopenia: 2 points = a fall in platelet count of >50 %, or a lowest count (nadir) of 20-100×109/litre; 1 point = a fall of 30-50 % or a nadir of 10-19×109/litre; 0 points = a fall of less than 30 % or a nadir of 9/litre. Timing: 2 points = a fall 5-10 days after the start of treatment, or if a patient has been exposed to heparin within the last 30 days and then has a drop in platelet count within a day of re-exposure; 1 point = a fall after day 10, or if the previous exposure was 30-100 days ago; 0 points if the fall is early and there has been no previous heparin exposure. Thrombosis: 2 points for new proven thrombosis, skin necrosis (see below) or systemic reaction; 1 point for progressive or recurrent thrombosis, silent thrombosis or red skin lesions; 0 points for no symptoms. AlTernative causes possible: 2 points if there is no other cause; 1 point if there is a possible alternative cause; 0 points if there is a definite alternative cause. Treatment involves platelet infusion (to arrest the thrombocytopenia) and administration of an alternate anti-Xa anticoagulant, such as lepirudin, to mitigate the risk of thrombotic sequelae. Please note: in cases where patients have received a transfusion in the previous 12 days, followed by a precipitous drop in platelet count, a diagnosis of post-transfusion purpura (PTP) should also be considered.
Clinical Indications Discuss with laboratory before requesting.
Falling platelet count while undergoing heparin therapy or within the last 3 months.
Sample Required 4.5 mL sodium citrate (pale blue top)Plus 10 mL serum (red top)
Turnaround Time 24 hours
Notes Special handling: avoid prolonged stasis during venepuncture. Sample must be received by lab within 4 hours of collection. Please note: samples will be rejected if underfilled or overfilled.