Assessing prognosis by quantifying FcγRIIa on fixed platelets.

FcγRIIa amplifies platelet activation and higher platelet FcγRIIa identifies patients at greater risk of subsequent cardiovascular events. We report the accuracy and precision of a modified test to quantify FcγRIIa on previously fixed platelets (pFCG test). An antibody clone (5G1) was developed after exposure of mice to formaldehyde treated FcγRIIa. Accuracy and precision of the modified test was evaluated with biologic specimens (platelets) and engineered synthetic cells conjugated with FcγRIIa (Slingshot Biosciences). The modified pFCG test on fixed platelets (using 5G1) consistently identified modestly more (∼300 molecules) of FcγRIIa on platelets compared with the pFCG test on nonfixed platelets (using clone FL18.26). With biologic specimens, the intra-assay coefficient of variation (CV) was 2.1 ± 0.1% (standard error of the mean, n = 750). The interassay CV was assessed intraday (4.5 ± 1%) and interday (up to 5 days after fixation, 6.5 ± 0.4%, n = 50). The pFCG test performed on Slingshot Synthetic cells conjugated with FcγRIIa demonstrated accuracy, linearity (R = 0.984) and similar interassay CV both intraday (2% ± 0.6%) and interday (20 nonconsecutive days, 9.9% ± 2.1%). In summary, modification of the pFCG test to be performed on fixed platelets allows accurate quantification of pFCG with high precision.