Usefulness of Frozen Platelets in Platelet Antibody Testing by Flow Cytometry

Lee, Hyun Chul; Won, Dong Il

Abstract

BACKGROUND
Among immunofluorescence assays using flow cytometry for platelet antibodies, a direct method (platelet-associated IgG tesing, PAIgG) and an indirect method (antiplatelet antibody testing, APA) are used with fresh platelets. In this study, we investigated whether frozen platelets can be serve as an alternative to fresh platelets for these assays.
METHODS
Platelet rich plasma was separated and stored at -80degrees C. Prior to use, frozen platelets were thawed at room temperature for 30 min and then tested in the same manner as fresh platelets. Thirty eight samples for PAIgG and 93 samples for APA were included in this study to compare qualitative determinations and sample/control ratios of meanfluorescence intensity (MFI) for anti-IgG-PE staining between fresh and frozen platelets.
RESULTS
In comparison of qualitative determinations between 2 kinds of platelets, PAIgG was concordant in 27 positive and 9 negative cases, with a total 38 cases (94.7%, kappa=0.865, P<0.001). In 2 remaining cases, fresh platelet was suitable for their diagnosis. APA was concordant in 46 positive and 33 negative cases for a total 93 cases (84.9%, kappa=0.693, P<0.001). In 5 of 8 discordant cases, frozen platelets were suitable for their diagnosis. In comparison of MFI ratios, frozen platelets was significantly correlated with fresh platelets in both tests (PAIgG: fresh vs. frozen=6.5+/-11.0 vs. 4.0+/-5.5 (r=0.924, P<0.001); APA: 3.9+/-5.3 vs. 2.4+/-2.6 (r=0.656, P<0.001)), but the values for frozen platelets decreased more than those of fresh platelets.
CONCLUSION
For APA, although both fresh and frozen platelets have their own advantages, sole use of frozen platelet was more helpful for making a diagnosis than the sole use of fresh platelets. For PAIgG, frozen platelets can be suggested as alternatives to fresh platelets in inevitable cases or for convenience of both patients and assay procedures, it requires accepting a slight decrease in positivity rate (from 29/38 to 27/38).