Assessment of the 4Ts pretest clinical scoring system as a predictor of heparin-induced thrombocytopenia

To evaluate the utility of the 4Ts clinical scoring system as a pretest probability method for the detection of heparin-induced thrombocytopenia (HIT).

Prospective observational study.

Medical and surgical inpatients at a tertiary care medical center.

Eighty consecutive patients with suspicion of HIT who had a polyspecific enzyme-linked immunosorbent assay (ELISA) performed between December 1, 2008, and April 1, 2009, for detection of platelet factor 4 (PF4)-heparin antibodies.

The predictive value of the 4Ts scoring system as determined by using a standard laboratory marker of HIT--the ELISA--and the interrater reliability of the scoring system were assessed. Sixty-seven (84%) of the 80 patients had low clinical probability of HIT based on the calculated 4Ts score. The ELISA result was negative for PF4-heparin antibodies in 74 patients (93%). Based on the results of the ELISA, the negative predictive value of the 4Ts score was 91%. Each 4Ts score was calculated by two independent investigators and adjudicated by a third investigator when necessary. The interrater reliability of the scoring system was fair (Cohen κ coefficient 0.362, 95% confidence interval [CI] 0.222-0.502; weighted κ coefficient 0.554 (95% CI 0.441-0.667). Determination of the timing of HIT was associated with the largest number of discrepancies (16) between evaluators, followed by other causes of thrombocytopenia (15), degree of decline in platelet count (14), and the presence of thrombosis or other sequelae (2).

A low 4Ts score supports a low probability of HIT based on the results of the polyspecific ELISA. Overall, the interrater reliability of the scoring system was fair. Components of the 4Ts scoring system need to be further clarified or modified in order to improve interrater reliability and thereby increase the clinical utility of this pretest probability model.