Section 864.7695 - Platelet factor 4 radioimmunoassay(a)Identification. A platelet factor 4 radioimmunoassay is a device used to measure the level of platelet factor 4, a protein released during platelet activation by radioimmunoassay. This device measures platelet activiation, which may indicate a coagulation disorder, such as myocardial infarction or coronary artery disease.(b)Classification. Class II (performance standards).45 FR 60625, Sept. 12, 1980; 46 FR 14890, Mar. 3, 1981