A method for assaying a heparin-induced thrombocytopenia in a patient's serum or plasma sample, the method comprising: incubating platelets with the patient's serum or plasma sample in the presence of either a low concentration of heparin (patient low heparin mix) or a high concentration of heparin (patient high heparin mix); incubating platelets in the presence (positive control mix) or absence (negative control mix) of a platelet activator; quantifying platelets and activated platelets in the patient low heparin mix, the patient high heparin mix, the positive control mix and the negative control mix; calculating percentage of activated platelets within the platelets for each of the aforementioned mixes and obtaining the following values: percentage of activated platelets in the patient low heparin mix [%R(Low)]; percentage of activated platelets in the patient high heparin mix [%R(High)]; percentage of activated platelets in the positive control mix [%R(Ct+)], and percentage of activated platelets in the negative control mix [%R(Ct-)]; calculating a heparin platelet activation (HEPLA) index by dividing the difference between %R(Low) and %R(High) by the difference between %R(Ct+) and %R(Ct-) and multiplying the obtained quotient with 100; measuring and calculating HEPLA indices of serum or plasma samples from donors not suffering from HIT; calculating cut-off values from the HEPLA indices of serum or plasma samples from donors not suffering from HIT, and determining whether a patient suffers of HIT or not by comparing the HEPLA index of the patient with the cut-off values. incubating platelets with the patient's serum or plasma sample in the presence of either a low concentration of heparin (patient low heparin mix) or a high concentration of heparin (patient high heparin mix); incubating platelets in the presence (positive control mix) or absence (negative control mix) of a platelet activator; quantifying platelets and activated platelets in the patient low heparin mix, the patient high heparin mix, the positive control mix and the negative control mix; calculating percentage of activated platelets within the platelets for each of the aforementioned mixes and obtaining the following values: percentage of activated platelets in the patient low heparin mix [%R(Low)]; percentage of activated platelets in the patient high heparin mix [%R(High)]; percentage of activated platelets in the positive control mix [%R(Ct+)], and percentage of activated platelets in the negative control mix [%R(Ct-)]; percentage of activated platelets in the patient low heparin mix [%R(Low)]; percentage of activated platelets in the patient high heparin mix [%R(High)]; percentage of activated platelets in the positive control mix [%R(Ct+)], and percentage of activated platelets in the negative control mix [%R(Ct-)]; calculating a heparin platelet activation (HEPLA) index by dividing the difference between %R(Low) and %R(High) by the difference between %R(Ct+) and %R(Ct-) and multiplying the obtained quotient with 100; measuring and calculating HEPLA indices of serum or plasma samples from donors not suffering from HIT; calculating cut-off values from the HEPLA indices of serum or plasma samples from donors not suffering from HIT, and determining whether a patient suffers of HIT or not by comparing the HEPLA index of the patient with the cut-off values.