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Heparin-induced thrombocytopenia (HIT) is a rare, iatrogenic condition, characterized by its potential severity and diagnostic difficulties. The diagnosis is based on a set of arguments allowing the calculation of a pre-test score pointing to HIT. There are rapid diagnostic tests for suspected HIT. Among these, the STic Expert® HIT has a good sensitivity for detecting HIT. However, it must be performed within 2 hours after sampling. The aim of this study was to evaluate a delayed STic Expert® HIT test at 8 hours and in frozen plasma. Thirty-six patients were prospectively included for HIT testing between April 1, 2018, and July 1, 2022, at the University Hospital Rouen. For any request for HIT testing, an analysis using STic Expert® HIT was performed within 2 hours and 8 hours post-sampling. Any positive result was confirmed by a functional test, platelet aggregation with heparin, release of 14C-serotonin assay (SRA), and immunological assay using a search for anti-platelet factor 4 IgG antibodies. Twenty-three patients had a STic Expert® HIT. Sixteen presented platelet aggregations in the presence of heparin and had a positive anti-PF4 test, and 17 had a positive SRA. Six patients had no HIT. For the test performed within 2 hours of collection, the Se = 100%, Sp = 68.42%, PPV = 73.91%, and NPV = 100%. The X2 = 18.21 with p < 0.001. For the test performed at 8 hours post sampling, the Se = 100%, Sp = 68.42%, PPV = 73.91%, and NPV = 100%. The X2 = 18.21 with p < 0.001. In conclusion, we have demonstrated that the STic Expert® can be used to perform an HIT diagnostic test 8 hours after sampling and on thawed plasma. However, this study needs to be confirmed on a larger number of samples.