π©Έ 4Ts Score Calculator
Estimate the probability of heparin-induced thrombocytopenia (HIT).
4Ts total score
For informational purposes only, not medical advice. Consult a healthcare professional.
The 4Ts score answers one urgent question: how likely is it that this patient's falling platelets are caused by heparin? Heparin-induced thrombocytopenia (HIT) is an immune reaction where heparin triggers antibodies that activate platelets, paradoxically causing clots instead of preventing them. Missing it can be catastrophic; overcalling it exposes patients to riskier anticoagulants.
The score grades four features, each worth 0 to 2 points: the size of the platelet fall, its timing, any thrombosis, and whether another cause explains the drop. Totals of 0 to 3 mean low probability, 4 to 5 intermediate, and 6 to 8 high. This calculator scores it for you and maps the result to the standard next steps.
How to use this calculator
Answer the four questions from the patient's chart: the platelet trend, when heparin started relative to the fall, any clotting events, and competing explanations like sepsis or chemotherapy.
Grade the platelet fall. A drop of more than 50 percent with the lowest count at 20 or above scores 2; smaller falls or very low nadirs score less.
Grade the timing. The classic window is 5 to 10 days after heparin starts. A fall within a day of re-exposure (heparin in the past 30 days) also scores 2.
Grade thrombosis and other causes. New clots add points; a definite alternative explanation for the low platelets subtracts.
Read the probability band and the suggested next step, and confirm everything with the treating clinician before acting.
The four Ts in detail
Thrombocytopenia: HIT typically cuts platelets by more than half, but the nadir usually stays above 20. A fall under 30 percent, or a nadir below 10, points away from HIT. Timing: antibodies need about 5 days to form, which is why day 5 to 10 is the classic window. Rapid-onset HIT (within 24 hours) happens when antibodies from heparin exposure within the past 100 days are still circulating.
Thrombosis: new venous or arterial clots, skin lesions at injection sites, or an acute reaction (fever, chills, blood pressure drop) right after a heparin bolus all count. Other causes: this is the tiebreaker clinicians miss most. Sepsis, disseminated intravascular coagulation, intra-aortic balloon pumps, and recent chemotherapy are definite alternative explanations that push the score down.
Worked example
A patient started heparin 7 days ago. Platelets fell from 240 to 95, a 60 percent drop with a nadir above 20: 2 points. Onset on day 7 sits squarely in the classic window: 2 points. She developed a new deep vein thrombosis yesterday: 2 points. She has no sepsis and no other reason for the fall: 2 points. Total 8, high probability, roughly 64 percent in validation studies.
Contrast that with a septic ICU patient whose platelets drifted from 200 to 150 (25 percent fall: 0 points) on day 2 of heparin (0 points), with no clots (0 points) and definite sepsis (0 points). Total 0, low probability. Same drug, same falling platelets, completely different answer, which is exactly what the 4Ts are for.
What each band means
Low (0 to 3): about a 5 percent chance of HIT. Guidelines say skip the antibody test, because in a low-probability population a positive immunoassay is more likely to be a false alarm than a true hit. Keep investigating other causes.
Intermediate (4 to 5): about a 14 percent chance. Send the PF4-heparin immunoassay, and many clinicians start a non-heparin anticoagulant while waiting if the patient needs ongoing anticoagulation. High (6 to 8): about a 64 percent chance. Stop all heparin immediately, including flushes and heparin-coated catheters, start an alternative anticoagulant, and send confirmatory testing such as the serotonin-release assay.
Limits and common pitfalls
The 4Ts were validated in specific populations and perform best when scored carefully from the chart, not from memory. The most common error is scoring timing loosely: a platelet fall on day 2 or 3 of first-ever heparin exposure is almost never HIT. Another is forgetting heparin flushes and subcutaneous prophylaxis doses, which count as exposure.
Scores also shift with the patient population. In cardiac surgery and ICU patients, low platelets have many causes, so intermediate scores are common and judgment matters more. When in doubt, hematology input beats any calculator. For related clotting assessments, see our age-adjusted D-dimer calculator and the ANC calculator for low white cell counts.
What else drops platelets on heparin
HIT is the diagnosis you cannot miss, but it is far from the most common cause of falling platelets in a heparinized patient. Sepsis tops the list in ICUs, consuming platelets through disseminated intravascular coagulation. Postoperative patients drop platelets from hemodilution and consumption for a day or two after surgery, then rebound, a pattern that looks nothing like HIT's day 5 to 10 fall.
Medications are the next suspects: many antibiotics, diuretics, and antiplatelet drugs cause thrombocytopenia, and heparin often shares the blame unfairly. Hypersplenism, marrow suppression from chemotherapy, and massive transfusion dilute the count without any immune mechanism. The "other causes" T exists precisely to force this differential into the open.
Timing separates most mimics from HIT. Sepsis drops platelets early and erratically; dilutional drops happen within 48 hours of fluids or transfusion; drug reactions vary. HIT's signature is the delayed, steep, isolated fall around day 5 to 10 with the patient otherwise stable. When the pattern does not fit, trust the pattern over the drug list.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is the 4Ts score used for?
It estimates the pretest probability of heparin-induced thrombocytopenia (HIT) in a patient with falling platelets on heparin. The result guides whether to test for HIT antibodies and whether to switch to a non-heparin anticoagulant.
What are the 4 Ts?
Thrombocytopenia (size of the platelet fall), Timing of the fall relative to heparin exposure, Thrombosis or other clotting events, and oTher causes of thrombocytopenia. Each scores 0 to 2 points for a maximum of 8.
What is a high 4Ts score?
6 to 8 points, carrying roughly a 64 percent probability of HIT in validation studies. High scores call for stopping all heparin, starting an alternative anticoagulant, and sending confirmatory testing.
What is a low 4Ts score?
0 to 3 points, with about a 5 percent probability of HIT. Guidelines recommend against routine HIT antibody testing in this group, since false positives outnumber true positives.
What is an intermediate 4Ts score?
4 to 5 points, with about a 14 percent probability. The standard move is a PF4-heparin immunoassay, often with a non-heparin anticoagulant started while results are pending.
When does HIT typically occur?
Usually 5 to 10 days after starting heparin, the time needed to form the culprit antibodies. Rapid-onset HIT within 24 hours occurs with heparin re-exposure when antibodies from the past 100 days persist.
Can you diagnose HIT from the 4Ts score alone?
No. The 4Ts estimate probability; diagnosis needs laboratory confirmation (immunoassay plus a functional assay like serotonin release) interpreted alongside the clinical picture by the treating team.
What should be stopped if HIT is suspected?
All heparin: infusions, subcutaneous prophylaxis, flushes, and heparin-coated catheters. Warfarin alone is avoided in acute HIT because it can worsen clotting; a direct thrombin inhibitor or fondaparinux is preferred.
Why does the nadir matter in the 4Ts?
HIT usually drops platelets by more than 50 percent but the lowest count typically stays at or above 20. A nadir below 10 suggests a different cause, such as marrow failure or overwhelming sepsis.
Who should interpret a 4Ts score?
The treating clinician, ideally with hematology input for intermediate and high scores. This calculator is a decision aid for learning and bedside estimation, not a substitute for clinical judgment.