4Ts Score for Heparin-Induced Thrombocytopenia

Why Use

Consider using the 4Ts scoring system as an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant.

When to Use

Consider using the 4Ts scoring system to stratify patients’ risk for HIT in patients with thrombocytopenia who are currently or were recently on heparin derived agents. Patients falling into the low-risk category frequently do not need further testing for HIT. Consider further laboratory evaluation for HIT or switching to a non-heparin derived anti-coagulant in those patients in the intermediate or high-risk groups.

Formula

Pearls / Pitfalls

The 4Ts for the diagnosis of heparin-induced thrombocytopenia (HIT) is a tool developed to help clinicians rule out HIT in patients who develop thrombocytopenia in the clinical setting. Included patients being evaluated for thrombocytopenia or suspected HIT in two clinical settings: inpatients at Hamilton General Hospital (HGH) in Canada and various clinicians in a variety of healthcare settings in Germany and Austria. The study prospectively applied previously developed scoring systems using various clinical features of HIT. Scoring system consisted of four criteria, each of which was worth 0, 1, or 2 points. Study used ≤3 points to define low probability group (≤5%) for HIT, 4-5 points for intermediate and 6-8 points for high. Gold standard for diagnosis of HIT was either: A platelet serotonin release assay (SRA) and a PF4/polyanion-enzyme immunoassay (EIA) with ≥50% serotonin release and positive EIA, OR A positive heparin-induced platelet activation (HIPA) test in at least three of four donor platelets. Points to keep in mind: Scoring system criteria for HIT was slightly different between the two main sites. The study used two different tests to diagnose HIT, depending on the site. One of the four components of their scoring system, other causes for thrombocytopenia, is subjective. Results between the two sites in the intermediate and high probability groups were statistically different. A subsequent review and meta-analysis of the 4Ts scoring system for HIT found that patients in the low-risk group had a negative predictive value of 0.998, irrespective of type of clinician, prevalence of HIT, or patient population.

Management

Low Probability (score 0-3): HIT is unlikely but continue to monitor platelet counts. Consider hematology consultation if HIT remains on the differential despite the low score. Evaluate for other causes of thrombocytopenia. Intermediate Probability (score 4-5): HIT is possible. Consider immediate discontinuation of heparin products and initiation of alternative anticoagulation therapy if medically indicated. Continue to monitor platelet counts. Order HIT antibody (PF4 ELISA) and confirmatory testing (with the serotonin release assay) as indicated. Consider consultation with hematology. High Probability (score 6-8): HIT is likely. Immediate discontinuation of heparin products is recommended, and alternative anticoagulation should be started. Consultation with hematology is recommended. Order confirmatory lab work with ELISA and SRA. Continue to monitor platelet counts, and monitor for signs and symptoms of developing thrombosis.

More Information

≤3 points: low probability for HIT (≤5% in original study, <1% in meta-analysis). 4-5 points: intermediate probability (~14% probability of HIT). 6-8 points: high probability (~64% probability of HIT).