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Evaluating cases of immune platelet refractoriness, post-transfusion purpura, or neonatal alloimmune thrombocytopenic purpura
Platelet antibodies may be allo- or autoantibodies and may be directed to a wide range of antigenic "targets" carried on platelet cytoplasmic membranes.
Platelet alloantibodies are involved in several clinical situations such as alloimmune platelet refractoriness (APR), neonatal alloimmune thrombocytopenic purpura (NATP), and posttransfusion purpura (PTP). In these settings, the antibodies, usually HLA Class I in the case of APR, and platelet-specific antibodies eg, HPA-1a (PLA1) in the case of NATP or PTP, are found in the patient's plasma and are detected by tests performed on serum.
In contrast, conditions such as idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE), or sepsis are associated with the presence of excessive platelet associated immunoglobulin usually IgG. Testing for cell bound platelet antibody is indicated for the diagnosis of these autoimmune conditions. In some cases of ITP, platelet antibodies can also be found in the patient's serum but with less frequently than cell bound antibody.
An interpretive report will be provided.
Presence of reactivity to some glycoproteins has no clearly established clinical significance.
Results are based on clinical situation as well as test results.
Serum platelet antibody testing by solid-phase enzyme-linked immunoassay offers more than a positive/negative result. When the patient's serum is positive, the specific platelet glycoprotein will be identified as well as the probable specificity. The platelet glycoproteins reported are: IIb/IIIa, Ia/IIa, GPIb/IX. Specificities include the following: HPA-1a (PL[a1]), HPA-1b (PL[a2]), HPA-3a (BAK[a]/LEK[a]), HPA-3b (BAK[b]), HPA-5b (Br[a]), HPA-5a (Br[b]). Those specificities listed in parenthesis refer to old nomenclature. In addition, this assay screens for HLA Class I antibodies, but specificity is not determined.
No significant cautionary statements
Our method (solid-phase enzyme immunoassay: SPEIA) uses purified platelet membrane glycoproteins as "targets" rather than intact platelets and this provides a simple "panel" of target antigens.
We have performed extensive parallel studies of SPEIA and indirect immunofluorescence methods using monospecific, well-characterized reagent grade HLA Class I antibodies and antibodies to the most commonly encountered, clinically relevant platelet specific antigen (HPA-1a [PLA1]). These studies were performed using dilutions of the sera to determine relative sensitivity of the 2 methods. The results clearly indicate greater sensitivity of the SPEIA method for every 1 of 16 antibodies tested.
The HLA antibodies were also tested in parallel by complement dependent cytotoxicity (CDC), the universal standard method for testing for HLA antibodies. The SPEIA method was similarly more sensitive than CDC. No false-positive reactions were seen with the SPEIA method in testing these well-defined alloantibodies.
Although the detection of platelet autoantibodies is better accomplished by direct cell bound methods, such autoantibodies are sometimes detected in serum. We compared the SPEIA and indirect immunofluorescence tests in parallel using sera from patients thought to have autoimmune thrombocytopenia. The results indicated comparable sensitivity of the 2 serum tests and, as expected, greater sensitivity of the direct cell bound test run in parallel with the 2 serum tests. Interestingly, the reactivity patterns seen in the SPEIA suggested that where antibodies were detectable in idiopathic thrombocytopenic purpura patients, they were more frequently directed to glycoprotein IIb/IIIa as has been previously reported.
In summary, for the detection of platelet alloantibodies, the SPEIA method is more sensitive than the indirect immunofluorescence method. In addition, it permits discrimination of the reactivity patterns against the most commonly encountered platelet antigens. The SPEIA method is also a more rapid and automatable method which reduces test turnaround time from 8 to 4 hours.
1. Kiefel V, Santoso S, Weisheit M, et al: Monoclonal antibody-specific immobilization of platelet antigens (MAIPA): A new tool for the identification of platelet-reactive antibodies. Blood 1987;70:1722-1726
2. Moore SB, DeGoey SR: Serum platelet antibody testing: evaluation of solid-phase enzyme immunoassay and comparison with indirect immunofluorescence. Am J Clin Pathol 1998;109:190-195