Prospective evaluation of a new platelet glycoprotein (GP)-specific assay (PakAuto) in the diagnosis of autoimmune thrombocytopenia (AITP)

Abstract

Assays measuring platelet-associated immunoglobulin G (PAIgG), while highly sensitive, lack specificity in diagnosing autoimmune thrombocytopenia (AITP). We prospectively evaluated a new commercially available glycoprotein (GP)-specific assay, the PakAuto (GTI, Brookfield, WI), for its clinical usefulness in distinguishing immune from nonimmune thrombocytopenia (TP), in 216 patients with autoimmune TP (both primary ''idiopathic'' and ''secondary'') and 46 patients with TP due to other causes. This assay is designed to detect both platelet-associated (direct assay) and plasma (indirect assay) antiplatelet antibodies specific for GPs IIb/IIIa, Ib/IX, and Ia/IIa. The mean platelet counts of the immune (79 ± 7 · 10 9 /L) and nonimmune groups (78 ± 7 · 10 9 /L), were similar (P = 0.95). The direct assay was positive in 114/216 patients with AITP (53%), and 13/46 with nonimmune TP (28%). Among the AITP group, the majority (61%) of patients with positive test results had autoantibodies reactive against all three GP targets. The sensitivity, specificity, positive, and negative predictive values for the direct PakAuto were 53%, 72%, 90%, and 24%, respectively, comparable to previously published experience of GP-specific assays. However, in some cases of TP due to nonimmune cause, the PakAuto was highly specific. Only 3 of 22 patients with gestational and 1 of 8 with familial/congenital TP had a positive direct assay, indicating that the test may be particularly useful for excluding an immune etiology for TP in certain patient subgroups. Am.

Platelet‐associated antibodies, cellular immunity and FCGR3a genotype influence the response to rituximab in immune thrombocytopenia

“…The sensitivity/specificity of the Pak Auto method has been reported (Davoren et al, 2005). Using a well-characterized cohort of ITP patients, we found that the sensitivity of the direct method was 91% and of the indirect (plasma phase) was 40%.…”

“…twice the value obtained for the mean of the negative control for the corresponding GP complex were regarded as positive results. Positive reactions were graded from 1+ to 4+ based on multiples of the control value (Davoren et al, 2005).…”

The American Society of Hematology 2011 evidence-based practice guideline for immune thrombocytopenia

“…If during the course of treatment or monitoring atypical features develop-for example, abnormalities in the white blood cell count, lymphadenopathy, multiple cytopenias-then the diagnosis of ITP should be reassessed. As in childhood ITP, we found insufficient evidence to recommend or suggest the routine use of antiplatelet, [61][62][63][64][65][66][67][68][69][70][71] antiphospholipid [72][73][74][75] and antinuclear antibodies, 13,76 thrombopoietin levels, 64,65,77 or platelet parameters obtained on automated analyzers 64,65,[78][79][80][81][82][83][84] in the evaluation of patients with suspected ITP.…”

Diagnóstico y tratamiento de la púrpura trombocitopénica inmunológica

“…La guía práctica para PTI de la Asociación Americana de Hematología (ASH) no recomienda el uso rutinario de las pruebas para detección de anticuerpos por su pobre sensibilidad y especificidad y porque el resultado no va a modificar el manejo del paciente (24,25) y tampoco sirven como predictores del tipo de evolución clínica (26).…”