Assay of ristocetin co-factor using fixed platelets and a platelet counting technique

A new assay of ristocetin co-factor has been developed which is based on platelet counting using a Coulter Counter. A modified method of washing and fixing platelets has also been devised to provide a suitable platelet preparation which can be used in this assay. These fixed platelets have consistently proved to be satisfactory and have given high levels of maximum percentage aggregation. They have been stored for periods up to 6 months without significant deterioration. The method of platelet counting is simple in operation and sensitive to relatively low levels of platelet aggregation. It is precise and seems to offer distinct advantages over existing methods.