A prospective study of protein‐specific assays used to investigate idiopathic thrombocytopenic purpura

Abstract

Summary. Idiopathic thrombocytopenic purpura (ITP) is a disorder in which platelets, sensitized by autoantibodies, are destroyed by the reticuloendothelial system. The diagnosis of ITP has been a clinical one because assays measuring platelet-associated IgG (PAIgG) have low specificity. The recently introduced assays that measure antibodies against specific platelet glycoproteins (GP) offer the possibility of improved specificity. In this report we describe two prospective studies. In the first study we compared two proteinspecific assays (AC and MAIPA) looking for the presence of autoantibodies against GP IIb/IIIa in 81 patient samples. These results were compared with an immunoradiometric assay for PAIgG. The second study investigated the enhanced sensitivity of measuring anti-GP Ib/IX autoantibodies in 76 patient samples. The protein-specific assays were able to differentiate immune from non-immune thrombocytopenia (specificity 91%, sensitivity 39%), whereas the PAIgG assay could not (specificity 19%, sensitivity 78%). The addition of the Ib/IX AC assay maintained a specificity of 92% while increasing the diagnostic sensitivity to 66%. In contrast to the PAIgG assay, there was no correlation between the platelet count and the likelihood or degree of positivity within the control samples using the glycoprotein assays. These studies confirm that glycoprotein assays can be used as diagnostic tests for ITP.

Detection and Identification of Platelet-Associated Alloantibodies by a Solid-Phase Modified Antigen Capture Elisa (MACE) Technique and Its Correlation to Platelet Refractoriness in Multi platelet Concentrate Transfused Patients

“…Recently, MACE and MAIPA were compared for the detection of GP IIb/IIIa-specific PA-IgG of 81 samples from thrombocytopenic patients. The GPspecific assays were found to be alike in sensitivity (39 %) and specificity (91 %) [22]. Although the sensitivity and specificity of the two methods (MAIPA and MACE) are almost similar, the procedure of MAIPA is quite tedious and time consuming.…”

Functional platelet defects in children with severe chronic ITP as tested with 2 novel assays applicable for low platelet counts

“…A possible explanation for the decreased PMA response might be an interference of autoantibodies present on patient platelets not detectable by indirect MAIPA or indirect PIFT because both assays have very low sensitivities (25% to 39% and 30%, respectively). [18][19][20] The presence of anti-GPIIbIIIa antibodies in these patients might explain why we predominantly see an effect with PMA stimulation and not with ristocetin, as these autoantibodies are most frequently found in ITP patients. 7,21 With the platelet reactivity assay, we found the severe patients to have significantly lower P-selectin expression after activation with ADP, but not after activation with CVX or TRAP.…”

“…Although direct variants of PIFT and MAIPA are more sensitive, these still require a high number of platelets and are therefore unsuitable for severe cases with a low number of platelets. 18,20 Therefore, direct confirmation of whether autoantibodies are to blame will have to wait until more reliable methods are developed to detect low levels of anti-platelet antibodies using a low number of platelets. Also, a larger prospective study determining platelet function, and estimating bleeding tendency with the new ITP bleeding assessment tool, will be required to determine if the assay reported here can predict severity.…”

Autoimmune and other cytopenias in primary immunodeficiencies: pathomechanisms, novel differential diagnoses, and treatment

“…Therefore, the first laboratory analyses that should be performed are for cell lysis parameters (eg, potassium, lactate dehydrogenase, aspartate transaminase, uric acid); for anemia, additional parameters include those for hemolysis (indirect bilirubin, absolute reticulocyte count, haptoglobin) and immunologic and metabolic parameters (eg, direct and indirect Coombs test; IgG, IgA, and IgM; fluorescence-activated cell sorter analyses for T-, B-, and NK-cell counts; serum ferritin concentration; vitamin B 12 ; and folate [soluble IL-2 receptor, IL-18, soluble Fas ligand]) (Table 1 and Sills 81 ). Antiplatelet antibodies are of no help in the differential diagnostic process because they are present in less than two-thirds of patients with immune thrombocytopenia and are not predictive, specific, or prognostically relevant, [82][83][84] whereas antierythrocyte (bound or soluble) and antigranulocyte antibodies have a rather high specificity for autoimmune hemolytic anemia and autoimmune neutropenia, respectively. Of note, the detection of antibodies against granulocyte surface antigens (not to be confused with antineutrophil cytoplasma antibodies) is a delicate analysis that depends on using specialized reference laboratories to perform tests and interpret results; it is not a test that can be performed under emergency conditions.…”