Prospective evaluation of the immunobead assay for the diagnosis of adult chronic immune thrombocytopenic purpura (ITP)

Abstract

Summary. Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disease characterized by antibody-induced platelet destruction. To better define the role of antigen-specific assays in adult chronic ITP, we prospectively measured plateletassociated autoantibody against either glycoprotein (GP) IIb/ IIIa or GPIb/IX in 282 patients with chronic ITP and 289 patients with thrombocytopenia of other causes. We divided chronic ITP into four subgroups: presplenectomy, mild (platelet count >30 000 mL À1 requiring no therapy), presplenectomy, severe (platelet count <30 000 mL À1 requiring therapy but not splenectomy), postsplenectomy, remission (postsplenectomy partial or complete remission without further therapy) and postsplenectomy refractory (required therapy after splenectomy failure). Positive results: total ITP group, 55.4%; presplenectomy, mild, 31.1%; presplenectomy, severe, 42.6%; postsplenectomy, remission, 50.0%; and postsplenectomy, refractory, 87.8%. In addition, the degree of positivity increased with the severity of the patient's disease. The assay had a minimum specificity of 84.4% if clinical factors, consistent with immune thrombocytopenia, were not considered in patients with thrombocytopenia associated with other diseases. However, if clinical factors consistent with immune thrombocytopenia were considered and only patients with questionable immune thrombocytopenia and patients 'lost to follow-up' were included in the false-positive group the specificity was 93.1%. We conclude that the presence of immune thrombocytopenia is highly probable if the immunobead assay is positive and that antigenspecific assays are diagnostically useful in adult chronic ITP.

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

“…Why, as observed in this and other studies [17][18][19], almost half of all patients with a clinical diagnosis of AITP have negative platelet autoantibody test results is an intriguing question. A number of possible explanations can be hypothesized: 1) variable eluate yields in extremely thrombocytopenic patients-although we detected platelet-bound autoantibody at platelet counts as low as 1 Â 10 9 /L in some patients, the minimum number of platelets required for a valid positive direct assay is unknown and may vary from one patient to another depending on the efficiency with which a particular antibody is eluted and the number of autoantibody molecules present on the recovered platelets; 2) very low levels of antibody in some patients with less severe or successfully treated disease; 3) binding of the MoAb to the same epitope on the platelet GP as the target of the patient's autoantibody (steric hindrance), thereby causing a false-negative result; 4) the presence of platelet autoantibodies specific for epitopes on GPs other than GPIa/IIa, IIb/IIIa or Ib/IX; 5) platelet destruction is cell-mediated rather than antibody-based; or 6) the clinical diagnosis may be incorrect.…”

“…In two recently reported studies which evaluated the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay, in the diagnosis of primary AITP, sensitivities for the direct assay ranged from 49-66%, specificities from 78-92%, and positive predictive values from 78-95% [17,18]. In another large study, evaluating the immunobead assay, the sensitivity was 55% and the minimum specificity 84% in the diagnosis of adult chronic AITP [19].…”

“…The immunobead assay [19] is similar to the MAIPA assay, except that sensitized platelets are first solubilized and then added to beads coated with MoAb specific for the GP of interest. An advantage of the PakAuto assay is that it is quick to perform and requires only minimal technical expertise.…”

“…Phase III assays are capable of detecting antibodies bound to individual GP complexes, and are also referred to as ''antigen'' or ''GP''-specific assays [15][16][17][18][19]. In studies to date, Phase III assays have shown superior specificity but are less sensitive than the PAIgG assays [17][18][19].…”

ITP in the 21st Century

“…22 Historically, assays for antiplatelet antibodies have not proven to be pathognomonic tests for a diagnosis of ITP, and their routine use has not been recommended by practice guidelines. In particular, routine measurement of platelet-associated IgG is rarely indicated due to the low specificity of an abnormal result and the variability of commercials tests of antiplatelet antibodies.…”