This patient most likely has heparin-induced thrombocytopenia (HIT) with thrombosis after being exposed to heparin during coronary artery bypass graft surgery, so the most appropriate next step is to initiate argatroban, a direct thrombin inhibitor used for anticoagulation in HIT. Patients who receive heparin during cardiothoracic surgery or after orthopedic surgery are more likely to develop HIT than are patients who receive heparin for dialysis or deep venous thrombosis prophylaxis. HIT develops 5 to 10 days after exposure to heparin, with a decrease in platelet counts of 50% or more and, in a subset of patients, paradoxical arterial or venous thrombotic events despite the presence of thrombocytopenia. The “4T score” has been devised to help clinicians decide the pretest probability for diagnosing HIT based on clinical factors, including degree of thrombocytopenia, timing of the decrease in platelet count, presence of 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 probability. The patient's 4T score is 8 (2 points each for timing of platelet count decrease, presence of thrombosis, timing of thrombocytopenia, and no other cause of the thrombocytopenia), indicating a high pretest probability of HIT. In patients with a high pretest probability, immediate cessation of any heparin-containing products is indicated, with initiation of a nonheparin anticoagulant; the only anticoagulant approved for the treatment of HIT is argatroban.

Confirmatory testing for presumed HIT is performed by HIT antibody testing. Immunoassays detect the presence of a HIT antibody (such as those directed toward platelet factor 4) in a patient's serum. Functional assays measure the ability of a HIT antibody from a patient's serum to activate test platelets (such as by measuring the release of serotonin). Because of the high risk of thrombosis associated with HIT and the possible delay associated with obtaining these studies, anticoagulation with a heparin alternative should be started before performing confirmatory testing.

Heparin cessation and treatment with a nonheparin anticoagulant are mandatory when a high pretest probability of HIT exists, because 30% to 50% of patients experience thromboses with heparin withdrawal alone. This patient already has evidence of thrombosis and must start treatment immediately to prevent extension and embolization.

Warfarin initiation alone is inadequate, because it may take 3 to 5 days to achieve a therapeutic anticoagulation effect. Additionally, starting warfarin before the platelet count has normalized in patients with HIT or starting warfarin without a bridging anticoagulant has been associated with development of warfarin skin necrosis and clot progression.