Diagnosis of heparin-induced thrombocytopenia

Evaluation of Suspected HIT (“4T’s”)

• Clinical: plt <100k or ↓ 50% from baseline; or venous (DVT/PE) or arterial (limb ischemia, CVA, MI) thrombosis (4:1 ratio); skin necrosis; ? ↑ heparin resistance
• Pathologic: ⊕ HIT Ab using PF4-heparin ELISA (≥90% Se, IgG-specific ELISA Sp 94%), may confirm w/ functional plt assay (serotonin-release) (>95% Se/Sp)
• Clinical context important: HIT Ab (esp. IgM ELISA) may be ⊕ in 10–20% of Pts on UFH/LMWH (Am J Hem 1996;52:90), up to 50% of cardiac bypass Pts (Circ 1997;95:1242)
• Pretest prob w/ “4 T’s” criteria (Blood 2012;120:4160): ≤3 points → 99% NPV, investigate other causes; 4–5 points 22% PPV & 6–8 points 64% PPV, ✓ lab test & replace UFH