A thrombocytopenic patient whose platelet count fall began between days 5 and 10 of heparin treatment (inclusive) should be considered to have HIT unless proved otherwise (first day of heparin use is considered "day 0").

Rule 2

A rapid fall in the platelet count that began soon after starting heparin therapy is unlikely to represent HIT unless the patient has received heparin in the recent past, usually within the past 30, and latest, 100 days.

Rule 3

A platelet count fall of more than 50% from the postoperative peak between days 5 and 14 after surgery associated with heparin treatment can indicate HIT even if the platelet count remains higher than 150 x 109/L.

Rule 4

Petechiae and other signs of spontaneous bleeding are not clinical features of HIT, even in patients with very severe thrombocytopenia.

Rule 5

HIT is associated with a high frequency of thrombosis despite discontinuation of heparin therapy with or without substitution by coumarin: the initial rate of thrombosis is about 5-10% per day over the first 1-2 days; the 30-day cumulative risk is about 50%.

Rule 6

Localization of thrombosis in patients with HIT is strongly influenced by independent acute and chronic clinical factors, such as the postoperative state, arteriosclerosis, or the location of intravascular catheters in central veins or arteries.

Rule 7

In patients receiving heparin, the more unusual or severe a subsequent thrombotic event, the more likely the thrombosis is caused by HIT.

Rule 8

Venous limb gangrene is characterized by (1) in vivo thrombin generation associated with acute HIT; (2) active DVT in the limb(s) affected by venous gangrene; and (3) a supratherapeutic INR during coumarin anticoagulation. This syndrome can be prevented by (1) delaying initiation of coumarin anticoagulation during acute HIT until there has been substantial recovery of the platelet count (to at least 150 x 109/L) while receiving an alternative parenteral anticoagulant (e.g., lepirudin, argatroban, danaparoid), and only if the thrombosis has clinically improved; (2) initiating coumarin in low, maintenance doses (e.g., 2-5 mg warfarin); (3) ensuring that both parenteral and oral anticoagulant overlap for at least 5 days, with at least the last 2 days in the target therapeutic range; and (4) if applicable, physicians should reverse coumarin anticoagulation with intravenous vitamin K in a patient recognized with acute HIT after coumarin therapy has been commenced.

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