Platelet Count Fall from Peak

Platelet fall < 30% from peak+0Platelet fall 30–49% from peak+1Platelet fall ≥ 50% from peak (significant thrombocytopenia)+3

Platelet Count Nadir (Lowest Value)

Nadir < 20 × 10⁹/L (very low — increases likelihood of alternative cause)+0Nadir 20–100 × 10⁹/L (consistent with HIT)+2Nadir > 100 × 10⁹/L (milder thrombocytopenia — less typical for HIT)+1

Timing of Platelet Fall

Fall begins on days 5–10 after starting heparin (typical HIT window)+3Fall begins ≤ 1 day after starting heparin AND prior heparin within 5–100 days (rapid-onset HIT)+2Fall begins > 10 days after starting heparin OR no clear heparin exposure+0

New Thrombosis (Venous or Arterial)

No new thrombosis+0New confirmed venous or arterial thrombosis after heparin initiation+3

Skin Necrosis at Subcutaneous Heparin Injection Sites

Absent+0Present — skin necrosis at heparin injection sites+3

Anaphylactoid Reaction Following IV Heparin Bolus

Absent+0Present — acute systemic reaction after IV heparin bolus+2

Alternative Explanation for Thrombocytopenia

No alternative explanation identified+0One possible alternative cause identified (partially explains thrombocytopenia)-1Two or more plausible alternative causes (adequately explains thrombocytopenia)-3

HEP Score (HIT Expert Probability) for Heparin-Induced Thrombocytopenia

HEP Score (HIT Expert Probability) for Heparin-Induced Thrombocytopenia