Abstract: Methods of calculating the platelet count of an individual, by measuring the amount of a released platelet granule protein of interest in a sample of whole blood or of platelet-rich plasma from the individual, are described. The platelet granule protein of interest is either thrombospondin or .beta.-thromboglobulin. The amount of released platelet granule protein of interest in the whole blood sample or platelet-rich plasma sample is measured using an enzyme-linked immunosorbent assay; radioimmunoassay; sandwich assay; a quantitative immunochromatographic assay; or non-solid phase nephelometry. The platelet count is directly related to the amount of released platelet granule protein of interest in the sample, and can be determined from the amount of platelet granule protein of interest that is released from a known number of platelets.