Table 3.1-1. Modified 4T’s pretest HIT probability score

Only one option in each category may be selected

2 points

1 point

0 points

Thrombocytopenia: compare the highest and the lowest platelet counts to calculate % fall in platelet counts

– >50% fall in platelet counts and nadir ≥20,000/microL in patient who has not undergone surgery within prior 3 days

– >50% fall in platelet counts and surgery within prior 3 days, or

– Any other combination of platelet count fall and nadir not fulfilling the other criteria

– <30% fall in platelet counts, or

– Any fall in platelet counts with nadir <10,000/microL

Timing of platelet count fall or thrombosis

– Onset within 5-10 days of starting heparin, or

– Onset within 1 day of starting heparin if heparin exposure within past 5-30 days

– Unconfirmed onset within 5-10 days of starting heparin (eg, some platelet count results are lacking)

– Onset within 1 day of starting heparin if heparin exposure within past 31-100 days

– Onset after day 10

Onset of thrombocytopenia within ≤4 days with no heparin exposure (day 0 is the first day of most recent exposure to heparin) within past 100 days

Thrombosis or other clinical sequelae

– Documented new thrombosis (arterial or venous)

– Skin necrosis at site of injection

– Anaphylactic reaction after IV heparin administration

– Adrenal hemorrhage

– Recurrent VTE in patient treated with anticoagulants at therapeutic doses

– Documented thrombosis (still to be confirmed by imaging studies)

– Erythematous skin lesions at heparin injection sites

Suspected thrombosis

Other causes of thrombocytopenia not evident

No other evident causes for platelet count fall

Possible other causes are evident:

– Sepsis with no microbiologically confirmed etiologic factor

– Onset of thrombocytopenia associated with start of mechanical ventilation

Possible other causes:

– Within 72 h of surgical procedure

– Documented bacteremia/fungemia

– Chemotherapy or radiotherapy within prior 20 days

– DIC caused by conditions other than HIT

– Posttransfusion thrombocytopenia

– Platelet count <20,000/microL in patient who has received a drug known to cause immune thrombocytopeniaa

– Nonnecrotic skin lesions at LMWH injection sites (delayed hypersensitivity)

Pretest probability of HIT:

Score 6-8: High probability

Score 4-5: Intermediate probability

Score 0-3: Low probability

a Selected agents that may cause immune thrombocytopenia:

– Relatively frequently: GP IIb/IIIa antagonists (abciximab, eptifibatide, tirofiban), quinine, quinidine, sulfonamides, carbamazepine, vancomycin.

– Relatively infrequently: actinomycin, amitriptyline, piperacillin, nafcillin, cephalosporins (cefazolin, ceftazidime, ceftriaxone), celecoxib, ciprofloxacin, esomeprazole, fexofenadine, fentanyl, fusidic acid, furosemide, gold salts, levofloxacin, metronidazole, naproxen, oxaliplatin, phenytoin, propranolol, propoxyphene, ranitidine, rifampin, suramin, trimethoprim.

Adapted from J Thromb Haemost. 2010 Jul;8(7):1483-5.

DIC, disseminated intravascular coagulation; HIT, heparin-induced thrombocytopenia; IV, intravenous; LMWH, low-molecular-weight heparin; VTE, venous thromboembolism.