Pre-Test Probability Scoring for HIT

The 4T Score

TOTAL SCORE

  • < 3 = low probability of HIT
  • 4-5 = intermediate probability of HIT
  • > 6 = high probability of HIT
Category 2 points 1 point 0 points
Thrombocytopenia Platelet count fall > 50% AND platelet nadir > 20 × 109 L−1 Platelet count fall 30-50% AND platelet nadir 10-19 × 109 L−1 Platelet count fall < 30% OR platelet nadir < 10 × 109 L−1
Timing of platelet count fall Clear onset between days 5 and 10 OR within 1 day of re-exposure after prior  exposure within 30 days Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) OR  onset after day 10 OR  within 1 day of re-exposure after prior  exposure 30–100 days ago Platelet count fall < 4 days AND no heparin exposure within the last 100 days
Thrombosis or other sequelae New thrombosis (confirmed) OR skin necrosis at heparin injection sites OR acute systemic reaction after intravenous heparin bolus Progressive or recurrent thrombosis OR non-necrotizing (erythematous) skin lesions OR suspected thrombosis (not proven) None
Other causes for thrombocytopenia None apparent Possible Definite

Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T’s) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4: 759–65.

Score for HIT in Cardiopulmonary Bypass

TOTAL SCORE

  • < 2 = low probability of HIT
  • > 2 = high probability of HIT
Variables Clinical Scenario Points
Platelet count time course Pattern A ((Platelet count begins to recover after CPB, but then begins to fall again > 4 days after CPB) 2
Pattern B (Thrombocytopenia occurs immediately after CPB and persists for > 4 days without recovery) 1
Time from CPB to index date > 5 days 2
< 5 days 0
CBP duration < 118 minutes 1
> 118 minutes 0

Lillo-Le Louët A, Boutouyrie P, Alhenc-Gelas M, Le Beller C, Gautier I, Aiach M, Lasne D. Diagnostic score for heparin-induced thrombocytopenia after cardiopulmonary bypass. J Thromb Haemost 2004; 2: 1882–8.

HIT Expert Probability (HEP) Score

NOTE:  This scoring has not been prospectively validated and is not yet recommended for clinical use.  Nevertheless, it offers another option for guiding clinical decision-making.

In the initial validation study (referenced above) the following screening cut-offs for total HEP Score were modeled for sensitivity and specificity:

  • > 2 = positive for HIT
  • < 2 = negative for HIT
    (Sensitivity 1.00 [0.56 – 1.00], Specificity 0.60 [0.45 - 0.75])
  • > 5 = positive for HIT
  • < 5 = negative for HIT
    (Sensitivity 0.86 [0.42 – 0.99], Specificity 0.88 [0.74-0.96])
Clinical feature Presentation Score
Magnitude of fall in platelet count (measured from peak to nadir since heparin exposure) < 30% -1
30-50% +1
> 50% +3
Timing of fall in platelet count For patients in whom typical onset HIT is suspected:
Fall begins < 4 days after heparin exposure -2
Fall begins 4 days after heparin exposure +2
Fall begins 5-10 days after heparin exposure +3
Fall begins 11-14 days after heparin exposure +2
Fall begins > 14 days after heparin exposure -1
For patients with previous heparin exposure in the last 100 days in whom rapid onset HIT is suspected:
Fall begins < 48 hours after heparin exposure +2
Fall begins > 48 hours after heparin exposure -1
Nadir platelet count < 20 × 109 L−1 -2
> 20 × 109 L−1 +2
Thrombosis
(select no more than one)
For patients in whom typical onset HIT is suspected:
New VTE or ATE > 4 days after heparin exposure +3
Progression of pre-existing VTE/ATE while receiving heparin +2
For patients with previous heparin exposure in the last 100 days in whom rapid onset HIT is suspected:
New VTE or ATE after heparin exposure +3
Progression of pre-existing VTE /ATE while receiving heparin +2
Skin necrosis Skin necrosis at subcutaneous heparin injection sites +3
Acute systemic reaction Acute systemic reaction after intravenous heparin bolus +2
Bleeding Presence of bleeding, petechiae, or extensive bruising -1
Other causes of thrombocytopenia (select all that apply) Presence of a chronic thrombocytopenic disorder -1
Newly initiated non-heparin med known to cause thrombocytopenia -2
Severe infection -2
Severe DIC (fibrinogen < 100 mg/dL and D-dimer > 5 mcg/ml)Indwelling intra-arterial device (IABP, VAD, ECMO) -2
Cardiopulmonary bypass within previous 96 hrs -1
No other apparent cause +3
VTE=venous thromboembolism; ATE=arterial thromboembolism; DIC=disseminated intravascular coagulation

Cuker A, Arepally G, Crowther MA, Rice L, Datko F, Hook K, Propert KJ, Kuter DJ, Ortel TL, Konkle BA, Cines DB. The HIT Expert Probability (HEP): Score: a novel pre-test probability model for heparin-induced thrombocytopenia based on broad expert opinion. J Thromb Haemost 2010; 8:2642-50.