INTERNATIONAL STUDY TO COMPARE ANTIGEN-SPECIFIC METHODS USED FOR THE MEASUREMENT OF ANTIPLATELET AUTOANTIBODIES

Platelet-associated and plasma autoantibodies against platelet glycopr oteins (GP) have been demonstrated in patients with autoimmune thrombo cytopenia (AITP) using various methods. Eight laboratories in seven co untries participated in this international study to evaluate the inter laboratory agreement using glycoprotein-specific immunoassays for thes e autoantibodies. The participating laboratories received blind sample s of frozen washed platelets and plasma from 22 normal donors and 22 A ITP patients. Platelet-associated and plasma autoantibodies against GP IIb-IIIa and GPIb-IX were measured by MAIPA, immunobead assay or modif ied antigen capture assay. Of the control samples, 96.0% and 97.2% of all results for platelet-associated and plasma autoantibodies to GPIIb -IIIa/GPIb-IX, respectively, were negative. The mean variation coeffic ient of the control samples of platelet-associated and plasma autoanti bodies was 89.5% (range 11.1-272.9%) and 46.5% (range 21.0-78.0%), res pectively. In 20/22 patient samples, platelet-associated autoantibodie s to either glycoprotein were noted by at least two laboratories. The mean degree of agreement in these samples was 74.0%. There was a signi ficant correlation in the individual antibody measurements between all laboratories (Kendall coefficient of concordance 0.60 and 0.38, P<0.0 01; Spearman rank order test, range of correlation coefficient 52.3-94 .0% and 42.2-85.0%, P<0.05, for anti-GPIIb-IIIa and anti-GPIb-IX, resp ectively). In contrast, plasma autoantibodies to either glycoprotein w ere noted by at least two laboratories in only 13/22 patient samples. Moreover, the degree of agreement was poor (50.1%) and a significant c orrelation was noted between only six pairs of laboratories. We conclu de that methods used in this study yield good interlaboratory agreemen t in measuring platelet-associated autoantibodies against GPIlb-IIIa a nd GPIb-IX. In contrast, poor agreement was found in detecting plasma autoantibodies to the same glycoproteins.