Finally, Trehel-Tursis and colleagues1 provide some noteworthy findings around laboratory testing. Of the 60 patients that had negative ELISA results, 10 tested positive in the PAT—an assay that examines heparin-dependent platelet aggregation when patient plasma is tested against (citrate-anticoagulated) platelet-rich plasma obtained from normal platelet donors. This profile of ELISA-negative—but PAT-positive samples—is not consistent with the “iceberg” model of HIT, which states that platelet-activating HIT antibodies (detected in a functional assay) represent a subset of patients who test positive for anti-PF4/heparin antibodies by ELISA.7 However, it would appear that the iceberg principle is correct, and the PAT results are wrong. This statement is supported by the authors’ observation that for the seven ELISA/PAT discrepant samples that were further tested in the other functional assay (the SRA), all seven yielded negative results. This strongly points to the PAT as having given false-positive test results, a problem that was observed in an earlier study of patients in the ICU.10 Presumably, elevated fibrinogen levels or other acute-phase reactants common in plasma from critically ill patients cause heparin-dependent platelet aggregation in the PAT system for reasons other than HIT antibodies, a phenomenon that is minimized in the SRA, which uses washed platelets. Notably, of the three other ELISA/PAT discrepant patients who, unfortunately, did not undergo SRA testing, the authors classified two as “likely” HIT based upon 4Ts scores of 5 and 6 points and one as HIT “unlikely” based upon a score of only 3 points. However, it seems more probable that neither of the two patients classified as “likely” HIT actually had this diagnosis, given the poor specificity of a positive PAT result in an ICU population and given the authors’ uniformly SRA-negative findings in the remaining patients. Although PAT testing has fallen out of favor in the United States, clinicians who continue to use this assay need to be aware of the risk of a false-positive test result, and they should be skeptical about a diagnosis of HIT when the ELISA result is negative and the PAT result is positive.