Inpatient / Hematology and Oncology

Heparin-Induced Thrombocytopenia (HIT)

Last Updated: 12/30/2022

# Heparin-Induced Thrombocytopenia (HIT)

-- Calculate 4T's Score - if >3, stop heparin and warfarin and consult heme
-- Chart Check: timing of platelet drop, % platelet drop, timing of heparin product administration, previous heparin product administration
-- HPI Intake: ***
-- Can't Miss: *** stroke, acute limb ischemia, mesenteric ischemia,
-- Orders: *** coags, fibrinogen, d-dimer, smear for schistocytes
-- Initial Treatment to Consider: *** hold heparin, decide if need AC for other reasons or if c/f clotting

-- History: dates of prior heparin use, timing of platelet fall, prior clotting
-- Clinical: bleeding, abdominal pain, chest pain, AMS
-- Exam: skin necrosis, edema, AMS, focal deficit c/f stroke, evidence of limb ischemia, mesenteric ischemia
-- Data:  % platelet fall, platelet nadir, 4T's score, fibrinogen
-- Etiology/DDx: Type 1 HIT, ITP, DIC/TTP/HUS, infection/sepsis, drug-induced

-- trend CBC for platelets and coags
-- f/u fibrinogen, d-dimer, smear for schistocytes
-- f/u anti-PF4 ELISA and confirmatory serotonin release assay (SRA)
-- if positive, consider screening with bilateral ultrasound with dopplers

-- hold heparin products while undergoing workup until plt >150 or 3-6 months if evidence of clotting
-- begin *** DOAC if non-urgent, fondaparinux (irreversible, if GFR >30), argatroban (renal failure, surgical patients; monitor Xa), bivalirudin (if PCI, liver failure)

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If You Remember Nothing Else

Clinically significant Type 2 HIT is rarer than other inpatient causes of thrombocytopenia but can be very serious if there is arterial clotting. Calculate the 4T's and stop heparin product if >3. Consult heme for the best testing, and AC choice when suspicion is especially high.

Clinical Pearls

  • Type 1 HIT is due to heparin aggregation with platelets - it is more common and transient (and is usually clinically insignificant); you do not need to stop heparin in these patients; Type 2 is the immune-mediated process that can lead to thrombosis
  • Type 2 HIT is caused by IgG anti-heparin complex that binds to and activates platelets, leading to a hypercoagulable state
  • 4T's Score is based on timing and severity of thrombocytopenia, evidence of thrombosis, and whether there are other possible causes of the thrombocytopenia
  • If 4T's Score is 0-3, the negative predictive value is 99%; similarly the anti-PF4 tests have high NPV with low PPV
  • Incidence of HIT is highest in patients undergoing major (cardiac) surgeries, those who get UFH have 10x the risk compared to those who receive LWMH, and in those getting product for >5 days
  • If the received heparin in the last 30-90 days, often still PF4 antibodies in circulation so platelets can drop abruptly when re-exposed to heparin product; otherwise once cleared, it takes about 5 days for newer antibodies to be formed (usually clear around 50-60 days)
  • Because the nadir is usually >20k, rarely have clinical evidence of thrombocytopenia (petechiae, purpura, bleeding with brushing teeth, etc) but you may see oozing if the patient develops DIC
  • Arterial thrombi are less common than DVT in HIT, but are more clinically devastating - Venous thrombosis of the large vessels of the lower limbs and pulmonary embolism are the most frequent complications, followed by peripheral arterial thrombosis and then stroke; myocardial infarction is uncommon (Am J Med, 1996)
  • 50% of patients will have thrombotic event within 30 days if HIT is not treated, with a 20% mortality

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