Laboratory confirmation of heparin-induced thrombocytopenia (HIT) is of crucial importance and remains challenging and relies on platelet functional assays highlighting the presence of heparin-dependent platelet-activating antibodies in patient serum or plasma. Platelet functional assays using washed platelets include the 14C-serotonin release assay (SRA), usually described as the gold standard, and the heparin-induced platelet activation assay (HIPA). Since its first comparison with SRA there has been no additional published study regarding HIPA diagnostic performances compared to SRA. Aim of our retrospective study was to compare the concordance between HIPA and SRA in HIT suspected-patients with positive anti-PF4/heparin antibodies between October 2010 and October 2015. Fifty-five HIT-suspected patients who beneficiated from both HIPA and SRA were included. Positive and negative percent agreements were 83.8% (95% CI 68.0–93.8%) and 66.7% (95% CI 41.0–86.7%), respectively. Overall percent agreement was 78.2% (95% CI 65.0–92.2%). Agreement was higher in patients who underwent cardiopulmonary bypass with extracorporeal circulation circuit for cardiac surgery. We also confirm that the use of a minimum of 2 platelet donors to establish positive HIT diagnosis and 4 platelet donors to exclude HIT diagnosis allows obtaining a good agreement with SRA. Although HIPA and SRA were performed with different platelet donors and in different laboratories, HIPA had a good positive agreement with SRA for HIT diagnosis, showing that HIPA is a useful functional assay that does not require radioactivity and could be developed worldwide to improve HIT diagnosis.

Background:Early recognition of heparin‐induced thrombocytopenia (HIT) is crucial because of increased risk of life threatening venous and/or arterial thrombosis. In case of positive immunoassay, HIT diagnosis has to be confirmed by functional assays detecting heparin‐dependent platelet‐activating antibodies (anti‐PF4/H). Heparin‐induced platelet activation test (HIPA) and serotonin release assay (SRA) are two functional assays that use washed platelets. SRA is considered the gold standard, but HIPA offers several advantages over SRA: it does not require radioactivity or specialized equipment and is less time consuming. HIPA closely correlates to SRA but 20 years after its first description, there is no additional published data regarding its diagnostic performances.Aims:To evaluate HIPA performances compared to SRA in a retrospective cohort of patients suspected for HIT. Secondary objectives were to evaluate negative or positive cut‐off criteria for HIPA and to identify clinical and/or biological factors associated with positive HIPA.Methods:We retrospectively analyzed the medical records of 81 consecutive patients who were suspected for HIT in our center between October 2010 and October 2015. Clinical suspicion of HIT was based on otherwise unexplained fall of platelet count (>40%) 5 to 21 days after the initiation of heparin and/or the occurrence of any thrombotic event during heparin treatment. Patients with positive anti‐PF4/H IgG immunoassay (Zymutest HIA IgG, Hyphen BioMed, France) who were tested by SRA and HIPA were included in this study. Anti‐PF4/H IgG was positive when optical density was greater than 0.5 OD units. Patients with indeterminate SRA or HIPA results or who were tested with less than two and more than four platelet donors for the HIPA were excluded. SRA was considered positive when serotonin release was greater than 20% at 0.1 U/mL and/ or 0.5 IU/mL unfractionated heparin (UFH) and decreased by at least 50% at high UFH concentration (100 IU/mL). HIPA was considered positive when the suspension became transparent due to platelet aggregation with UFH 0.2 IU/ml and/or 0.5 IU/ml but not with UFH 48 IU/ml and when positive results were obtained with at least two platelet donors. HIPA was considered negative if negative results were obtained with four platelets donors. Positive percent agreement (PPA), negative percent agreement (NPA), overall percent agreement (OPA) and Cohen's kappa (κ) and their 95% confidence intervals (CI) were calculated.Results:HIPA and SRA results were compared in 47 patients fulfilling eligibility criteria. PPA and NPA were 83.8% (95% CI 71.9–95.7%) and 75.0% (95% CI 53.8–96.2%), respectively. The OPA was 81.1% (95% CI 78.4–83.8%; κ = 0.58, CI 0.33–0.80). Defining HIPA positive when positive results were obtained with two donors was associated with a higher NPA (80.0% versus 75.0%) with a minimal decrease of the PPA (81.3% versus 83.8%) compared to positive result with only one patient. No association was found between surgery, extracorporeal circulation, anti‐PF4/H IgG titer, platelet count nadir or type of heparin and HIPA results.Summary/Conclusion:HIPA showed a good performance against SRA when testing suspected HIT patients. A good concordance was found between the two assays. HIPA performance can be enhanced considering two positive results to establish HIT diagnosis and four negative results to rule out HIT diagnosis. HIPA appears to be an optional sensitive and specific functional assay for HIT diagnosis easy to handle without radioactivity and safety issues.