Reference — Hematology
Heparin-Induced Thrombocytopenia (HIT) Reference
The drug that prevents clots can cause them. HIT is an immune reaction to heparin that drops the platelet count yet drives thrombosis, not bleeding. The reflex is simple and absolute: stop all heparin and switch to a non-heparin anticoagulant.
Educational use only. HIT is a high-risk diagnosis; anticoagulant selection and dosing are provider-directed. This reference is an educational aid. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
The Mechanism & Paradox
Antibodies form against the heparin–platelet factor 4 (PF4) complex. These antibodies then activate platelets, causing them to clump and form clots — which both consumes platelets (the low count) and produces widespread thrombosis. So despite a low platelet count, the danger is clotting, not bleeding. Onset is typically 5–10 days after heparin starts, or within hours if the patient had recent prior exposure.
The 4Ts Score
| T | What it captures |
|---|---|
| Thrombocytopenia | Magnitude of the platelet fall — a drop of > 50% scores highest |
| Timing | Fall onset 5–10 days after starting heparin (or faster with recent prior exposure) |
| Thrombosis | New clot (DVT/PE, arterial, limb/skin necrosis) or other sequelae |
| oTher cause | Absence of another explanation for the low platelets |
The 4Ts estimate pretest probability; a low score makes HIT unlikely. Confirmatory testing is the anti-PF4/heparin antibody (ELISA) screen, with the serotonin release assay (SRA) as the gold-standard confirmatory test.
Management — Do & Don’t
DO: stop ALL heparin immediately — IV infusions, subcutaneous prophylaxis (including LMWH/enoxaparin, which cross-reacts), line flushes, and heparin-coated catheters. DO start a non-heparin anticoagulant (a direct thrombin inhibitor like argatroban or bivalirudin, or fondaparinux) — the clotting risk persists even after heparin is stopped.
DON’T give platelet transfusions (adds fuel to thrombosis — reserved for life-threatening bleeding only). DON’T start warfarin until platelets recover and the patient is anticoagulated — warfarin alone early can cause venous limb gangrene/skin necrosis. Document the HIT, flag the chart, and band the patient for lifelong heparin avoidance.
NCLEX Pearls
- ✦HIT causes CLOTTING, not bleeding — antibodies to heparin-PF4 activate platelets.
- ✦Classic timing: platelet fall (often >50%) about 5–10 days after heparin starts.
- ✦First action: STOP all heparin — IV, subQ, LMWH, and line flushes.
- ✦Switch to a non-heparin anticoagulant: argatroban, bivalirudin, or fondaparinux.
- ✦Do NOT give platelets and do NOT start warfarin alone early (limb gangrene risk).
- ✦Document HIT and teach lifelong heparin avoidance (medical alert).
Related Resources
Standards & sources
Fact-checked Jun 21, 2026This page is written to align with AABB (transfusion standards) · American Society of Hematology (ASH). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →