Evaluation of the Pre-Test Scoring System (4Ts) for the Evaluation and Management of Heparin Induced Thrombocytopenia

Abstract 2551

The initial diagnosis of Heparin Induced Thrombocytopenia (HIT) is made on clinical grounds, because the assays with the highest sensitivity (Heparin-PF4 Ab ELISA) and specificity (Serotonin Release Assay) may not be readily available and may have a slow turn-around time. The clinical utility of the pretest scoring system, the 4Ts, was developed and validated by Lo, GK et al in the Journal of Thrombosis and Haemostasis in 2006. The pretest scoring system looks at the degree of thrombocytopenia, timing of platelet fall, thrombosis or other sequelae, and the possibility of other etiologies for thrombocytopenia. Based on the 4T score, patients can be divided into high, intermediate, and low probability of having HIT. The American Society of Hematology developed a clinical practice guideline in 2009 incorporating the 4T scoring system in the evaluation and management of patients with suspected HIT. It is recommended that patients with intermediate to high probability for HIT start direct thrombin inhibitor (DTI) therapy without delay. We conducted a retrospective study on 100 consecutive patients who were tested for the HIT assay during their hospitalization at Hahnemann University Hospital in 2009. In the 100 patients analyzed, the distribution of the 4T score of low, intermediate, and high pretest probability were seen in 73, 23 and 4 patients, respectively. A positive HIT ELISA (optical density > 1.0) was detected in 0/73 (0%) of the low probability group, 5/23 (22%) of the intermediate probability group and 2/4 (50%) of the high probability group. The average turn-around time for the HIT ELISA was 4 to 5 days. Fourteen (19%) out of the 73 patients with a low pre-test probability for HIT were treated with a direct thrombin inhibitor (DTI). Ten out of the 14 (71%) patients in the low probability group treated with a DTI had a major complication of bleeding requiring blood transfusion support. Overall, twenty five patients received a DTI. Argatroban was the DTI used in 24 of 25 patients. Fourteen patients started on argatroban had a contraindication for the use of lepirudin. In this retrospective study, a low 4T score correlated 100% with a negative HIT antibody assay. The 4T score also predicted the patients likely to test positive for HIT into the intermediate to high probability group. There was significant patient morbidity in the group with low probability when treated with a direct thrombin inhibitor. Based on previous data and this retrospective study, empiric direct thrombin inhibitor therapy should not be initiated in patients with low probability of having HIT. Moreover, many of these patients were started on the more expensive direct thrombin inhibitor, argatroban. For our institution, if no contraindication exists, we recommend lepirudin for the treatment of HIT because it is easier to monitor and less costly. Furthermore, we recommend incorporating the 4T scoring system into institutional core measures when assessing a patient with suspected HIT, selecting only patients with intermediate to high probability for further therapeutic intervention, which may translate into reduced morbidity and lower health care costs.