HIT


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  • 4T score

    • Thrombocytopenia
      • 2: plt fall > 50% AND nadir >= 20k
      • 1: plt fall 30-50% OR nadir 10-19k
      • 0: plt fall < 30% OR nadir < 10k
    • Timing of onset s/p heparin
      • 2: clearly 5-10 days
      • 2: <= 1 day but has prior heparin exposure within 30 days
      • 1: not clear
      • 1: after day 10
      • 1: <= 1 day but has prior heparin exposure within 30-100 days
      • 0: < 4 days without recent heparin exposure
    • Thrombosis
      • 2: new thrombosis
      • 2: skin necrosis
      • 2: acute systemic reaction (fever, chills, HTN, tachy, CP, SOB)
      • 1: progressive or recurrent thrombosis
      • 1: non-necrotizing skin lesions like erythema
      • 1: suspected thrombosis but not proven
      • 0: none
    • oTher causes
      • 2: none
      • 1: possible
      • 0: definite
    • scoring: 2 for each, 8 total maximum
      • 0-3 = low pretest probability, don't do ELISA
      • 4-5 = intermediate pretest probability, do ELISA
      • 6-8 = high pretest probability, do ELISA

  • Testing

    • ELISA
      • pt serum + PF4:heparin -> optic densitiy
      • positive: OD > 2
      • negative: OD < 0.4
    • functional assay (less used)
      • more specific
      • used if ELISA is indeterminate or discrepant
      • tests: serotonin release assay (SRA) and heparin induced plt aggregation (HIPA)
      • SRA: pt serum + PF4:heparin -> radio-labeled serotonin released by reagent plt activation
      • HIPA: pt serum + donor plt +/- heparin -> aggr only at low (stoichiometric) dose heparin if HIT

  • Types

    • HIT II (the one we're worried about)
      • antibodies to PF4:heparin complex, only IgG is pathogenic
      • 5-10 days after start of heparin (delayed onset very rare)
      • plt drop > 50% with nadir ~60k is typical, < 20k is rare
      • 5% has plt drop 30-50% with nadir > 150k
      • risk: unfractionated heparin > LMWH (10 fold less) > fondaparinux (negligible)
      • risk of thrombosis -> skin necrosis, organ ischemia
      • bleeding is rare, but can occur
      • Tx: stop heparin, switch to argatroban, danaparoid, fondaparinux
      • resolves within 7 days when heparin stops (if not, look for alternate cause)
    • HIT I (benign mimick)
      • mild, transient, 1-4 days of exposure, self resolves even with continued heparin
      • plt nadir 100k
      • non-immune mediated plt aggregation by heparin
    • other types
      • Subclinical HIT
        • persistent HIT antibodies after recovery from HIT
        • high risk for HIT if re-exposed to heparin
        • this is not same as clinically silent anti PF4 antibodies
      • spontaneous HIT
        • HIT w/o heparin exposure
        • recent infection, surgery or inflammatory event
        • thrombocytopenia, thrombosis, lack of inciting heparin exposure, positive HIT assay
      • heparin induced antibodies
        • Ab crossreacting with HIT assay
        • from heparin exposure (dialysis, CABG) or other conditions (such as lupus)
        • +/- HIT, so clinical significance uncertain
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