4T Score for HIT
Select findings for each domain (Thrombocytopenia, Timing, Thrombosis, oTher causes) to calculate the 4T score and stratify HIT probability.
Thrombocytopenia
Timing of platelet fall
Thrombosis or sequelae
Other causes for thrombocytopenia
Total 4T score
5
Maximum 8 points
Risk category
Intermediate probability of HIT
How to Use This Calculator
Assess thrombocytopenia severity
Review platelet count trends and nadir relative to baseline. Choose the option that best matches the observed fall.
Determine timing and complications
Identify when the platelet fall began relative to heparin exposure and whether thrombosis, skin necrosis, or acute systemic reactions occurred.
Evaluate alternative explanations
Consider other causes of thrombocytopenia (sepsis, medications). Select the category that reflects their likelihood before reviewing the result.
Formula
4T score = Thrombocytopenia + Timing + Thrombosis/other sequelae + oTher causes
Each domain scores 0, 1, or 2 points. Total score range 0–8.
Interpretation: 0–3 low; 4–5 intermediate; 6–8 high probability of HIT.
Full Description
The 4T score provides a standardized approach to estimate the pre-test probability of heparin-induced thrombocytopenia. It combines platelet kinetics, timing relative to heparin, thrombotic events, and competing diagnoses. Low scores effectively exclude HIT, avoiding unnecessary testing or anticoagulant changes. Intermediate and high scores warrant laboratory evaluation, typically beginning with immunoassays followed by functional tests when positive.
Frequently Asked Questions
Can a low 4T score rule out HIT?
Yes. A score ≤3 has <1% probability of HIT, allowing clinicians to continue heparin without further testing in most cases.
What testing is recommended for intermediate/high scores?
Order a PF4/heparin ELISA or rapid immunoassay. Positive results should be confirmed with functional assays (e.g., serotonin release assay).
How do direct oral anticoagulants affect scoring?
Current anticoagulant therapy does not change the 4T score; focus instead on platelet kinetics and heparin exposure history.
What if platelet counts are unreliable?
Confirm pseudothrombocytopenia or laboratory artefacts before scoring. Repeat testing or use different anticoagulant tubes if clumping is suspected.