As such, this assay should be limited to patients with higher pretest probabilities (for HIT) found in those with intermediate (positive predictive value 10% to 20%) or high (positive predictive value 40% to 80%) clinical scores. That usually occurs when the fall in platelet count ex- ceeds 50% or a thrombotic event occurs between post- operative days 5 and 14. Patients with low antibody titers (optical density less than 0.4) would be candidates for heparin anticoagulation without further intervention given the high sensitivity (90% to 98%) of this clinical assay [43, 51]. Postoperatively, close monitoring of platelet counts are recommended and screening for HIT anti- bodies considered if clinical scores indicate further sero- logic testing is warranted. Patients with mild elevations in anti heparin-PF4 titers (optical density 0.4 to 1.0) should have further serologic Ann Thorac Surg 2018;105:650 62 ANTICOAGULATION DURING CARDIOPULMONARY BYPASS REPORT testing using a functional assay especially if their clinical scores are in the intermediate or high ranges [37]. The high sensitivity (90% to 98%) and speci city (80% to 97%) of the functional assays (SRA, HIPA) make them the gold standard for diagnosing HIT [61]. Negative SRA and HIPA frequently occurs in patients with elevated anti- body titers using the clinical antigen assays (ELISA) owing to PF4-heparin antibodies that do not activate platelets or cause thrombocytopenia or thrombosis [33]. Patients with high clinical scores and high antibody titers (optical density greater than 1.0) are candidates for hep- arin alternatives when urgent operations preclude the use a functional assay (SRA, HIPA). True hypersensitivity to heparin is rare, yet can occur in patients allergic to heparin or the sources of heparin (eg, porcine or bovine sources of heparin) [35, 36, 62 64]. 