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4 Ts

Created: 22/2/2007
Updated: 28/2/2007

Suspicion of HIT (The 4 Ts):

Four features have been used to estimate the pre-test probability of HIT. These features are:

 Timing of platelet count fall
 oTher causes for thrombocytopaenia are not evident.

Treatment of thrombosis complicating HIT

A rapidly acting anticoagulant is required to treat thrombosis complicating HIT. This substitution of heparin with an alternative anticoagulant is required even when HIT is suspected because of thrombocytopaenia alone.

Paradoxes in treatment: The paradoxes in treating HIT can be summarised as follows:

1. Coumarins (e.g. warfarin) are contraindicated in acute HIT because they increase the risk of microvascular thrombosis, causing venous limb gangrene and skin necrosis.

2. LMWH, due to the high risk of in vivo cross-reactivity with UFH, is contraindicated to treat HIT.

3. Platelet transfusions are a relative contraindication in HIT because spontaneous bleeding is uncommon and due to a theoretical risk of further thrombosis.

 Click here for more information regarding the 4Ts


[1] Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4: 759-765

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