How to Calculate the HIT 4T's Score
The 4T's score is calculated by assigning 0,1, or 2 points to each of four clinical criteria (Thrombocytopenia, Timing, Thrombosis, and oTher causes), with total scores of ≤3 indicating low probability, 4-5 indicating intermediate probability, and ≥6 indicating high probability of HIT. 1
The Four Components
1st T: Thrombocytopenia (Degree of Platelet Count Drop)
- 2 points: Platelet count fall ≥50% from baseline AND nadir ≥20 × 10⁹/L
- 1 point: Platelet count fall 30-50% from baseline OR nadir 10-19 × 10⁹/L
- 0 points: Platelet count fall <30% from baseline OR nadir <10 × 10⁹/L
Key point: HIT typically causes moderate thrombocytopenia (often 30-70 × 10⁹/L) and rarely drops below 20 × 10⁹/L. A ≥50% drop from the highest pre-suspicion count is highly predictive, even without absolute thrombocytopenia. 1
2nd T: Timing of Platelet Count Fall
- 2 points: Platelet fall at days 5-10 after heparin initiation, OR ≤1 day if heparin exposure within past 30 days
- 1 point: Platelet fall after day 10, OR timing unclear, OR ≤1 day if heparin exposure 31-100 days ago
- 0 points: Platelet fall <4 days without recent heparin exposure
Critical nuance: Early thrombocytopenia (within first 2 days) can occur from heparin's direct aggregating effect, but may also represent rapid-onset HIT if the patient received heparin within the previous 3 months. 1
3rd T: Thrombosis or Other Sequelae
- 2 points: New confirmed thrombosis (venous or arterial), skin necrosis at heparin injection sites, or acute systemic reaction after IV heparin bolus
- 1 point: Progressive or recurrent thrombosis, erythematous skin lesions, or suspected thrombosis not yet confirmed
- 0 points: None of the above
Important consideration: Some thromboses may be asymptomatic. Some practitioners recommend systematic Doppler ultrasound of lower limbs to detect occult thrombosis. 1
4th T: oTher Causes of Thrombocytopenia
- 2 points: No other apparent cause for thrombocytopenia
- 1 point: Possible other cause present
- 0 points: Definite other cause present
Most challenging domain: This requires careful evaluation of competing causes including sepsis, liver disease, chemotherapy, other medications (antibiotics, diuretics, GPIIb-IIIa inhibitors), perioperative hemodilution, consumption in extracorporeal circuits, post-transfusion purpura, antiphospholipid syndrome, TTP, or DIC. Non-heparin causes typically produce more severe thrombocytopenia with bleeding rather than thrombosis. 1
Score Interpretation
- Low probability (≤3 points): HIT can be excluded; continue heparin without HIT testing 2
- Intermediate probability (4-5 points): Proceed with HIT laboratory testing; discontinue heparin and start non-heparin anticoagulant 2
- High probability (≥6 points): Immediately discontinue heparin and start therapeutic-dose non-heparin anticoagulant without waiting for lab results 1, 2
Special Populations
Cardiac surgery with cardiopulmonary bypass: The 4T's score is more difficult to apply in this setting. A "biphasic" platelet count evolution profile (initial drop followed by recovery then second drop) is equivalent to a high-probability score. 1
Critical care patients: The score performs well but interobserver agreement can be variable, particularly for the "oTher causes" domain. Missing information about pre-ICU heparin exposure can lead to underscoring. 3, 4
Performance Characteristics
The negative predictive value of a low 4T's score is excellent at 0.998 (95% CI: 0.970-1.000), making it highly reliable for ruling out HIT. 5 However, positive predictive values are more modest: 0.14 for intermediate scores and 0.64 for high scores, necessitating confirmatory laboratory testing. 5
Common pitfall: Ensure complete information about heparin exposure history, particularly in the 30-100 days prior to ICU admission, as missing this information can result in falsely low scores in patients with true HIT. 3