Haematology, coagulation and transfusion

Heparin-induced thrombocytopenia

Reviewed Nov 2026·2 references
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Use the 4Ts score, stop all heparin and start a non-heparin anticoagulant when probability is intermediate or high, and confirm with PF4 and functional assays.

Definition
HIT is an immune (anti-PF4/heparin IgG) prothrombotic disorder: platelet fall > 50% typically 5-10 days after heparin start (or within 24 h if exposed in the previous 30-100 days), with venous or arterial thrombosis in up to 50%.

At the bedside

  1. 1

    Calculate 4Ts

    Low probability (≤ 3): HIT unlikely, do not test, continue heparin. Intermediate (4-5) or high (6-8): act.

  2. 2

    Stop all heparin

    Now

    Including flushes, heparin-coated catheters and LMWH.

  3. 3

    Start non-heparin anticoagulant

    Now

    Argatroban (critically ill, renal failure, may need procedures), bivalirudin, fondaparinux or a DOAC (stable patients) - at treatment doses even without thrombosis.

  4. 4

    Test

    PF4/heparin immunoassay; if positive, functional assay (serotonin release or HIPA) to confirm.

  5. 5

    Look for thrombosis

    Bilateral lower-limb compression ultrasound in all; upper limb if CVC.

  6. 6

    Avoid

    Platelet transfusion (unless bleeding/high-risk procedure); warfarin until platelets recovered to ≥ 150 × 10⁹/L (warfarin necrosis, limb gangrene); give vitamin K if HIT diagnosed on warfarin.

  7. 7

    Duration

    Without thrombosis (isolated HIT): until platelet recovery and up to 3 months; with thrombosis: 3 months.

HIT in critically ill

Argatroban

IV
Dose
0.5-1.2 mcg/kg/min in organ failure (2 mcg/kg/min if no organ dysfunction)
Preparation
250 mg in 250 mL NS = 1 mg/mL
Target
aPTT 1.5-3 × baseline (check 2 h after each change)
Worked example
70 kg at 1 mcg/kg/min = 70 mcg/min = 4.2 mg/h = 4.2 mL/h
Open infusion calculator

HIT, stable patient

Fondaparinux

SC
Dose
7.5 mg daily (50-100 kg)

Avoid if CrCl < 30 mL/min.

4Ts score

Category2 points1 point0 points
ThrombocytopeniaFall > 50% and nadir ≥ 20 × 10⁹/LFall 30-50% or nadir 10-19Fall < 30% or nadir < 10
Timing of fallDay 5-10, or ≤ day 1 with heparin in past 30 daysDay > 10 or unclear; or ≤ day 1 with heparin 30-100 days ago≤ day 4 without recent heparin
ThrombosisNew proven thrombosis, skin necrosis, anaphylactoid reaction to IV heparin bolusProgressive/recurrent/suspected thrombosis; non-necrotising skin lesionsNone
Other causeNone apparentPossibleDefinite

0-3 low, 4-5 intermediate, 6-8 high probability.

Red flag · call your senior now
Limb ischaemia, new thrombosis or skin necrosis with falling platelets on heparin.
Pitfall
Ordering HIT antibodies for every thrombocytopenic ICU patient: a low 4Ts score has high negative predictive value; positive ELISA alone is common after surgery/CPB.
Pitfall
Stopping heparin without starting an alternative anticoagulant: thrombosis risk is high.
Pearl
Sepsis-related thrombocytopenia in ICU is far more common than HIT, but post-cardiac surgery a second platelet fall after day 5 suggests HIT.

Calculators for this topic

Infusion rate from mcg/kg/min

Pump rate in mL/h for drugs dosed in mcg/kg/min (noradrenaline, adrenaline, dobutamine, milrinone, nitroglycerin by weight).

References

  1. 1

    Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv 2018;2:3360-92.

  2. 2

    Lo GK, Juhl D, Warkentin TE, et al. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006;4:759-65.

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