Unfractionated heparin (UFH) exposure before cardiac surgery is ubiquitous. Patients who undergo urgent coronary surgery have almost invariably received intravenous UFH, and patients who undergo elective surgery receive it during preoperative angiography. Patients who undergo cardiac surgery also frequently develop early postoperative thrombocytopenia. This combination often yields a 4 Ts score (Thrombocytopenia, Timing, Thrombosis, oTher cause) of 4, an intermediate pretest probability for heparin-induced thrombocytopenia (HIT). Per the 2018 American Society of Hematology (ASH) guidelines,¹ standard practice at intermediate or high pretest probability is to discontinue heparin, begin a nonheparin anticoagulant (often a direct thrombin inhibitor), send a platelet factor 4 (PF4) immunoassay, and await results. Two problems arise after cardiac surgery. Most such patients do not have HIT. In addition, a direct thrombin inhibitor started shortly after surgery carries a risk of bleeding in a patient with fresh suture lines, frequently impaired renal clearance, and no available reversal agent.² Many patients meet bedside criteria for intermediate or high 4 Ts probability, even though the actual incidence of HIT is only 0.5% to 3%.^3-6 This article contextualizes the American Association for Thoracic Surgery (AATS) HIT guidance⁷ for the cardiac surgical team.

 

After cardiac surgery, platelet counts typically decrease 40% to 50% from baseline, reaching a nadir on postoperative days 2 to 3 and generally recovering between days 5 and 7.³ Anti-PF4/heparin antibodies are present in roughly one half of patients after cardiopulmonary bypass (CPB), although most are IgM or low-titer IgG and do not activate platelets.^3,E1 Thrombocytopenia alongside a low-positive PF4 immunoassay is therefore common and often not pathologic. Reflexively ordering a heparin antibody test on postoperative days 2 or 3 for any degree of thrombocytopenia is of limited value and carries the potential for harm through unnecessary anticoagulation after a weakly positive result or false reassurance from a negative test when a true biphasic pattern later appears. When true HIT does occur, the consequences are severe. In a propensity-matched analysis, operative mortality was 21.8% versus 5.3% and thromboembolism 29.1% versus 2.9%, with mortality reaching 34.4% when thrombosis was present and 16.7% when absent.

 

The 2018 ASH guidelines¹ are rigorous and meticulously developed. The writing panel applied established evidence-appraisal methodology to the general hospitalized patient and treated cardiovascular surgery as a designated special situation, consistent with its charter, with input from experts in HIT and cardiac surgery, including the originator of the 4 Ts score, who flagged the limits of a general medical framework for post-CPB physiology 2 decades ago.⁹ The underlying observation is older still. HIT with thrombosis was reported after CPB in the early 1990s, and anti-PF4/heparin seroconversion after CPB was well characterized by the end of that decade.^E1 The 2026 AATS Expert Consensus Document (ECD) on HIT in cardiac surgery⁷ is best understood not as a correction of the ASH document but as a scope expansion: a complementary, cardiac surgery–specific framework built by a multidisciplinary panel that paired cardiac surgeons, anesthesiologists, intensivists, perfusionists, and pharmacists with hematology leadership, including the ASH 2018 lead author (Cuker) and the originator of the 4 Ts score (Warkentin).