Two Diagnostics

Test for HIT antibodiesh

Investigate for lower-limb deep-vein thrombosis (e.g., duplex ultrasound)1

aThese principles apply when HIT is strongly-suspected or confirmed. bSometimes HIT begins after all heparin has been stopped ("delayed-onset HIT"). cDanaparoid (Chapter 13), lepirudin (Chapter 14), and argatroban (Chapter 15) are three alternative, non-heparin anticoagulants that are approved for treatment of HIT, although approval status and drug availability varies in different jurisdictions.

dTherapeutic-dose regimens include aPTT-adjusted iv dosing schedules for lepirudin and argatroban; for danaparoid, there is evidence that a therapeutic-dose regimen (e.g., initial iv bolus, then 400 U/h iv x 4 h, followed by 300 U/h iv x 4 h, followed by 200 U/h iv (with subsequent dose adjustments made using anti-factor Xa levels, if available) is more effective than low-dose danaparoid (e.g., 750 U b.i.d. or t.i.d. by subcutaneous injection). eThere is evidence that therapeutic-dose anticoagulation of "isolated HIT," i.e., HIT recognized because of thrombocytopenia and in the absence of clinically-apparent thrombosis, reduces risk of subsequent thrombosis.

fCoumarin is a risk factor for microvascular thrombosis, e.g., venous limb gangrene; further, aPTT prolongation by coumarin can lead to underdosing of lepirudin or argatroban therapy.

gPetechiae and other signs of thrombocytopenic bleeding are not characteristic of HIT. hPF4-dependent enzyme-immunoassays have high sensitivity (>98%) for clinical HIT; however, their diagnostic specificity is lower than washed platelet activation assays. In experienced laboratories, the latter " functional" (platelet activation) assays also have high sensitivity for diagnosis of clinical HIT (see Chapter 10). iUp to 50% of patients with isolated HIT have deep-vein thrombosis.

Abbreviations: aPTT, activated partial thromboplastin time; b.i.d., twice daily, b.w., body weight; HIT, heparin-induced thrombocytopenia; iv, intravenous; PF4, platelet factor 4; t.i.d., three times daily.

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