4Ts Score Calculator for HIT

The 4Ts score is a clinical prediction rule used to estimate the pretest probability of immune heparin-induced thrombocytopenia, commonly called HIT. HIT is an antibody-mediated adverse reaction to heparin that activates platelets and creates a strong risk of venous or arterial thrombosis. Although the platelet count falls, bleeding is not its defining danger. Prompt recognition matters because continued exposure to heparin can increase thrombotic risk, while unnecessary treatment for suspected HIT also carries costs and bleeding risks. The score covers four domains whose names begin with the letter T: thrombocytopenia, timing of the platelet-count fall, thrombosis or other sequelae, and other causes of thrombocytopenia. Each domain receives zero, one, or two points. Thrombocytopenia scoring considers the percentage fall from the patient's peak after heparin exposure and the platelet nadir, rather than relying only on whether the count is below a laboratory reference range. Timing considers the typical immune response around days five through ten and whether recent heparin exposure could explain a rapid fall. The thrombosis domain includes confirmed new thrombosis and characteristic skin or systemic reactions. The final domain asks how convincingly another condition explains the platelet decline. Add the four selections for a total from zero through eight. Scores from zero through three indicate low pretest probability, four through five indicate intermediate probability, and six through eight indicate high probability. The score is most useful for ruling out HIT when a carefully determined score is low. Intermediate and high scores do not prove HIT. They identify patients who generally need prompt management according to institutional guidance and appropriately selected laboratory testing, often beginning with an immunoassay and sometimes a functional assay. Accurate scoring requires a reliable medication and platelet timeline. Review all heparin sources, including flushes, coated catheters, procedures, dialysis, and recent hospital stays. Confirm suspected thrombosis objectively when possible and actively assess competing explanations such as sepsis, surgery, disseminated intravascular coagulation, medications, mechanical devices, transfusion, or marrow disease. Missing history can shift a domain by one or more points and materially change the probability category. This calculator is intended for trained healthcare use and education. It cannot determine whether heparin should be stopped, select an alternative anticoagulant, interpret laboratory assays, or diagnose HIT. Follow current professional and local protocols, especially for intermediate or high probability. Treatment decisions also depend on bleeding risk, kidney and liver function, planned procedures, and whether thrombosis is present. If HIT is clinically suspected, obtain urgent specialist or protocol-directed guidance rather than waiting for this tool alone.