A method is disclosed of assaying circulating microparticles contained in a sample of whole blood or blood plasma from a patient to determine the patient's ability to generate thrombin or Factor Xa as blood-clotting factors, wherein the circulating microparticles are microparticles of platelets, endothelial cells, monocytes, and smooth muscle cells, which carry on their surfaces both negatively charged phospholipids as well as tissue factor. The results of the assay may be used to determine the ability of the patient to generate thrombin or Factor Xa as blood clotting factor based upon the circulating microparticles.