Table 2 4 T Scoring system: A pretest score for evaluating probability of HIT

The timing of onset of platelet count decrease

• Clear onset of platelet count decrease (5–10 days)

• Platelet count decrease ≤1 day, if last heparin exposure is within 30 days

• Suspected 5–10 days but was not documented or missing

• Onset of platelet decrease after 10 days or ≤ 1 day, with the last heparin exposure at 30–100 days ago

• < 4 days with no recent heparin exposure

Thrombocytopenia

• Platelet count fall is > 50%and

• Platelet nadir ≥20 × 10⁹ /L

• Platelet count fall is 30–50%or/and

• Platelet nadir = 10–19 × 10⁹ /L

• Platelet count fall is < 30%

Or/and

• Platelet nadir < 10 × 10⁹ /L

Thrombosis or other sequelae,

• New thrombosisor/and

• Skin lesions or necrosis at the heparin injection sitesor/and

• Acute systemic reaction (post-intravenous heparin injection)

• Progressive thrombosisand/or

• Recurrent thrombosis

Or/and

• Non-necrotic (erythematous) skin lesions

Or/and

• Silent, suspected

• None

Other causes of thrombocytopenia

None

Possible

Defined