Synopsis
Estimate HIT probability from platelet timing and thrombosis, stop every heparin source, use rational PF4 testing, and begin safe non-heparin anticoagulation before limb- or life-threatening thrombosis.
- Immune HIT is caused by IgG against platelet-factor-4–heparin complexes, activating platelets, monocytes and coagulation and creating a high risk of venous and arterial thrombosis.
- The platelet count typically falls by more than 50% beginning five to ten days after heparin, and the nadir can remain above 150 × 10^9/L.
- Rapid-onset HIT can occur within 24 hours when heparin exposure in the preceding weeks has left circulating antibodies; delayed presentations can occur after heparin stops.
- Use the 4Ts score—thrombocytopenia, timing, thrombosis and other causes—before ordering PF4 testing; a well-calculated low score makes immune HIT unlikely.
- For intermediate or high probability, stop every heparin source and begin a non-heparin anticoagulant while testing, adjusted for bleeding, renal and liver function and procedures.
- A sensitive PF4 immunoassay detects many non-pathogenic antibodies, so optical strength and a functional platelet-activation assay refine positive results.
- Do not use a positive PF4 test without a compatible clinical probability to label HIT; false diagnosis creates bleeding risk, costly alternatives and a lifelong inaccurate allergy record.
- Warfarin should not be started during acute thrombocytopenic HIT because rapid protein C depletion can cause venous limb gangrene and skin necrosis.
- Platelet transfusion is generally avoided unless serious bleeding or an essential procedure creates a compelling indication, because added platelets may fuel thrombosis.
- After platelet recovery, anticoagulation duration depends on whether thrombosis occurred, and the confirmed reaction needs precise documentation for future surgery, dialysis and pregnancy.
Key red flags
Pain, pallor, pulselessness or neurological loss with a compatible count fall needs immediate vascular and haematology intervention.
Investigation priorities
Define percentage fall and timing from heparin exposure.
Management branches
Platelets fall or thrombosis appears during or after heparin exposure.
- Calculate the exact percentage fall, exposure timing, thrombosis features and strength of competing causes using the 4Ts score.
- For a low score, seek the alternative cause and avoid indiscriminate PF4 testing unless new evidence changes probability.
- For intermediate or high probability, stop every heparin source, send immunoassay and functional confirmation and begin a suitable non-heparin anticoagulant.
Key medicines
ArgatrobanBegin a specialist intravenous infusion at the licensed weight-based rate, using a lower starting dose in critical illness or liver dysfunction and titrating to the local APTT target.Hepatic clearance causes accumulation in liver disease; it raises INR and complicates warfarin transition, while bleeding and APTT confounders require expert titration.
FondaparinuxFor stable selected adults, use the licensed weight-based once-daily subcutaneous therapeutic dose off label for HIT under local guidance, adjusted or avoided in renal impairment.Renal accumulation, long half-life, low body weight, bleeding and urgent procedures limit use; it is not readily reversible and requires specialist selection.