Performing 4T pretest probability scoring for the possibility of heparin-induced thrombocytopenia (HIT) is the most appropriate next step in management. This patient's platelet count decreased following a recent exposure to heparin, raising concern for the possibility of HIT. The 4T score is used to estimate the pretest probability of HIT based on clinical factors, including degree of thrombocytopenia, timing of the decrease in platelet count, presence of any potential sequelae of HIT (such as thrombosis), and whether another potential cause for thrombocytopenia exists. Possible point values range from 0 to 8; scores of 0 to 3 indicate low probability, 4 or 5 indicate intermediate probability, and 6 to 8 represent high pretest probability. The 4T score has been validated in a study of more than 3000 patients with a negative predictive value of 99.8% in those with low probability scores. Although specific decisions must be made for individual patients based on their clinical status, many physicians reserve further evaluation for patients with intermediate or high pretest probability scores, with close clinical observation in those with low probability scores. This patient has a 4T score of 1 based on a possible alternative explanation for his thrombocytopenia (blood transfusion), indicating a low pretest probability of HIT; therefore, observation without additional testing for HIT would be reasonable.

Confirmatory testing for antiplatelet antibodies may be done by direct antibody testing or by functional assays that test the ability of serum from patients with HIT to activate test platelets. Direct antibody testing is typically performed by enzyme-linked immunosorbent assay (ELISA) that detects antibodies directed toward heparin-platelet factor 4 complexes. The two commonly used functional assays are the serotonin release assay (SRA) and the heparin-induced platelet aggregation (HIPA) assay. SRA is considered the gold standard study for HIT with a sensitivity and specificity of more than 95%; HIPA is also very specific but has a lower sensitivity than SRA. However, no clear indication exists for confirmatory testing for HIT considering this patient's low pretest probability.