Abstract: This disclosure provides an assay and method for evaluating platelet function by measuring clot retraction in a grooved assay well using light transmittance in a low-volume, micro-plate formatted assay. The method takes advantage of the ability of platelets to draw the fibrin clot toward one side of the microplate well through an optical light path with readings recorded by a microplate reader. The method is rapid, tractable, has high precision, and yields time-series data that is quantitative. This allows clinicians and transfusion medicine practitioners to perform high throughput platelet function testing in patient samples and in blood/platelet products. Clot retraction serves as a functional biomarker to determine platelet function by performing the assay in a vessel that is scored to form a groove in which clot retraction occurs. Multiple samples can be tested simultaneously, and optionally an algorithm can be used to extract relevant parameters from the data.