Abstract: A diagnostic method or process is disclosed for determining one or more parameters which are characteristic of the adhesiveness of blood platelets of a human patient. A blood sample is collected from a venipuncture of a patient through a filter which comprises a load of glass beads or spheres placed in a medically acceptable plastic, polyvinyl or like, tube. The time is measured that it takes for a predetermined volume of the sample to be collected through the filter. For control, a blood sample is also collected without passing the sample through a filter. The number of platelets per unit volume in each sample is determined by conventional counting techniques. From the measured platelet counts and the measured time period, the rate (in platelet count per second) is calculated at which the filter apparatus retains the blood platelets of the patient. The percentage of the retained platelets as compared to the unfiltered blood sample is also calculated.