Sensitivity of the platelet immunofluorescence test (PIFT) and the MAIPA assay for the detection of platelet-reactive alloantibodies: a report on two U.K. National Platelet Workshop exercises

Progress and development of platelet antibody detection

Human platelet antibody (HPA) detection is necessary for the diagnosis and therapeutic decisions for refractoriness to platelet transfusions, post transfusion purpura and fetal and neonatal alloimmune thrombocytopenia. In the last four to five decades many new developments, both in knowledge and methods, have increased the quality of platelet serology. However, the quest for the optimal antibody detection method(s) encountered, sometimes unexpected, difficulties. In this review the various aspects concerning platelet antibody test methods and detection of platelet antibodies both for the diagnostic and screening setting are discussed.

Interlaboratory variation in the detection of HPA-specific alloantibodies and in molecular HPA typing

BACKGROUND AND OBJECTIVES

Platelet immunology quality assurance exercises have been organized by National Institute for Biological Standards and Control since 1991 and, as of 2006, 35 laboratories participate in the serology section. Molecular human platelet antigen (HPA) typing has been included in the exercises since 1998 and, as of 2006, 29 laboratories participate in this part of the exercise. This report details the performance of laboratories in these two areas.

MATERIALS AND METHODS

Every 6 months laboratories are sent up to four coded serum/plasma samples for testing in their in-house assays for HPA antibody detection/identification and four coded whole blood samples to be typed for HPA-1, -2, -3 -5 and (since 2003) -15 by their molecular typing assays ('genotyping').

RESULTS

Fifty-two sera containing HPA-specific alloantibodies and 13 sera that were inert, contained only human leucocyte antigen (HLA) class I or high-titre anti-A+B antibodies were distributed; 15 sera were issued in more than one exercise. The percentage of participating laboratories that were able to detect HPA-specific alloantibodies ranged from 15.0 to 100%; the percentage that were able to correctly determine the specificities also ranged from 15.0 to 100%. Over the 20 serology exercises the percentage of laboratories classified as poor performers ranged from 3.1 to 36.7%. A total of 12 780 individual HPA genotyping results were assessed. The overall error rate was 0.7% but there was considerable variation between HPA alleles. Over 11 exercises the percentage of laboratories classified as poor performers varied from 6.3 to 26.3%.

CONCLUSIONS

The ability to detect and to identify platelet-specific alloantibodies varied widely between laboratories and between various examples of antibodies issued. An increase in the number of laboratories screening for HPA-15 antibodies was seen, although detection and identification of these antibodies was problematic. The majority of examples of HPA-3a antibodies and some examples of HPA-1a and -5b were also difficult to detect and identify. In addition, this scheme has shown that despite the apparent reliability of molecular typing techniques, mistakes do occur, particularly with certain systems. Approximately one in five laboratories participating in the serology exercises and one in seven participating in the genotyping exercises were classified as poor performer at one point or more during the series of exercises.