The 4T score is the most widely used bedside tool for estimating the pretest probability of heparin-induced thrombocytopenia (HIT) — a serious, immune-mediated adverse reaction to heparin that causes thrombocytopenia and a sharply increased risk of new thrombosis. By scoring four clinical criteria, it turns a bedside assessment into a reproducible 0–8 number that helps decide whether laboratory antibody testing and a switch away from heparin are warranted. This guide explains how the score is built, what each probability tier typically means, and where its limits are.
The four criteria that make up the 4T score
The 4T score was developed by Lo, Juhl, Warkentin, and colleagues (Journal of Thrombosis and Haemostasis, 2006) as a four-criterion clinical prediction rule, each scored 0, 1, or 2 points: the degree of Thrombocytopenia (how far the platelet count fell, and the nadir reached), the Timing of the platelet count fall relative to starting heparin (or to any recent prior heparin exposure), Thrombosis or other sequelae (new clots, skin necrosis, or an acute systemic reaction after an IV heparin bolus), and oTher causes of thrombocytopenia (whether an alternative explanation for the platelet fall has been excluded).
The four scores are summed to a total of 0–8. Because the timing and other-causes criteria depend on clinical judgement and chart review rather than a single lab value, the 4T score works best when completed thoughtfully by a clinician familiar with the patient's full course, not as a mechanical checklist.
What the probability tiers mean
A total score of 0–3 indicates a low pretest probability of HIT — laboratory antibody testing is generally not needed, and continuing heparin (while investigating other causes of thrombocytopenia) is usually reasonable. A score of 4–5 indicates intermediate probability, and 6–8 indicates high probability; both typically prompt HIT antibody testing (an immunoassay, sometimes followed by a functional assay) and consideration of switching to a non-heparin anticoagulant while results are pending.
These bands, and their strong negative predictive value at the low end, have been confirmed in a large systematic review and meta-analysis (Cuker et al., Blood, 2012) across diverse clinical settings, which is why the 4T score is embedded in most HIT diagnostic algorithms today.
Limits and when to seek expert review
The 4T score is a pretest probability estimate, not a diagnostic test. A high score raises suspicion but does not confirm HIT, and a low score makes HIT unlikely but does not rule it out with absolute certainty in every clinical context. The score also depends on accurate, complete information about the platelet count trend and heparin exposure history — incomplete records can bias the timing and thrombocytopenia-degree criteria.
This calculator is a clinical decision-support and education tool. It does not diagnose HIT, does not order or interpret laboratory tests, and does not decide anticoagulation management. Any decision to hold heparin, start a non-heparin anticoagulant, or order confirmatory testing should be made by the treating clinical team based on the full clinical picture.