DESCRIPTION: Antiplatelet agents such as clopidogrel and aspirin are used in the treatment of heart attacks and strokes and are among the most commonly used drugs in the U.S. Problems associated with these drugs include bleeding risk (which complicates invasive procedures) and resistance (which can be overcome by alternative antiplatelet drugs). These common problems can be managed by platelet function monitoring. Current methods for monitoring platelet function, however, require expensive equipment, pricey consumables, and provide complicated readouts. These global approaches to platelet function are not widely used because their cost far exceeds reimbursement for the tests. We have developed a rapid, inexpensive platelet function immunoassay for use either in seamless conjunction with standard high throughput analyzers operating in clinical laboratories or as stand-alone devices. This immunoassay is based on our mapping of the unique signaling pathway leading to Drp1 phosphorylation. We find that phospho-Drp1 is exquisitely sensitive to all commercially available antiplatelet agents tested, including aspirin and clogpidogrel. The phospho-Drp1 signal is stable over time and can be used in conjunction with a total Drp1 value to provide a ratio that tolerates differences in blood draw volumes and platelet counts. The test is rapid and relies only on inexpensive reagents. This practical approach to testing platelet function in the context of antiplatelet therapy will fill an unmet clinical need in the doctor’s office and in hospitals to help doctors manage their patients who receive these commonly used drugs for life-threatening indications.