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HIT should be suspected in any patient on heparin in whom the platelet count falls to <100 x 109/L or drops by >30—40% or develops a new thromboembolic event 5—10d after ongoing heparin therapy. ►► Heparin should be discontinued immediately and confirmatory investigations undertaken.

Diagnostic test

ELISA using PF4 to detect antibodies to heparin-low molecular weight protein complex; may miss 5—10% of cases with antibodies to other proteins and up to 50% false positives after CABG.

Management

• Discontinue heparin (platelet count normally recovers in 2—5d).

• Substitute alternative anticoagulation where necessary and to prevent further thromboembolic events:

• Recombinant hirudin

— Thrombin inhibitor; anticoagulant effect lasts ~40min.

— Slow IV bolus followed by IVI.

— Dose determined by body weight and renal function (see product literature).

— Monitor 4h after IV bolus dose using APTT or ecarin clotting time (ECT); target range 1.5—2.5 x mean normal APTT; reduce target to

1.5 if concomitant warfarin therapy and discontinue hirudin when INR >2.0.

- Adverse effects bleeding (esp. with warfarin), anaemia, haematoma, fever and abnormal LFTs.

• Argatroban

- Thrombin inhibitor; ti^~45min.

- Initiate IV infusion at dose of 2|jg/kg/min.

- Check APTT at 2h and adjust dose for APTT 1.5-3 x baseline (max 100s).

- 5 dose by 75% if hepatic insufficiency.

- Side effect—bleeding.

• Danaparoid

- A heparinoid with low level cross-reactivity with HIT antibodies.

- IV bolus dose by weight (see product literature) followed by decre-mental infusion schedule and maintenance infusion.

- Monitor by factor Xa inhibition assay 4h after dose (target range 0.5-0.8U/ml).

- Side effect—bleeding.

►► Low molecular weight heparins frequently cross-react with HIT antibodies and are not recommended.

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