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Coagulation

Coagulation laboratory tests

Platelet antibodies


Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 10 February 2013, last major update November 2012
Copyright: (c) 2002-2013, PathologyOutlines.com, Inc.

General
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● Either autoimmune (idiopathic thrombocytopenic purpura), alloimmune (neonatal alloimmune thrombocytopenia, post-transfusion purpura, platelet transfusion refractoriness) or heparin-induced
● These tests must be ordered and interpreted cautiously, considering the clinical presentation (Blood 1997;89:1112)

Types of tests
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ELISA: test for specific antiplatelet antibodies; antigen of interest is bound to surface of microtiter plate, then add patient plasma and antibody will bind to antigen
Antigen capture immunoassay: specific antigens are bound to solid phase, then add patient serum and patient antibodies will bind to antigens
Platelet antigen typing by antigen capture immunoassays: patientís platelet antigens are immobilized by monoclonal antibodies onto a solid phase; then add antibodies of known specificity (Am J Clin Pathol 1990;93:552)
Flow cytometry: may be used
Lymphocytotoxicity assay: determine percent reactive antibody (HLA antibodies in patients who are refractory to platelet transfusions)
Polymerase chain reaction: can be used to identify patientís platelet antigens

Platelet antibody disorders
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Drug-induced thrombocytopenia:
● Detected by a difficult serotonin release assay (add patient plasma / serum plus drug and platelets with radiolabeled serotonin; drug antibodies, if present, stimulate platelets and radioactive serotonin is released)
● Also detected with flow cytometry
● Offending drugs include quinine and quidinine, sulfonamides, sulfonylureas, gold salts, salicylates
● Mechanism is either non-immune (marrow suppression or non-immune destruction) or immune (platelet counts due to immune causes may drop to < 10K, return to normal within 7 days of stopping offending drug)

Idiopathic thrombocytopenic purpura (ITP):
● Autoantibody against platelets, usually directed against GP IIb/IIIa, less commonly GP Ib/IX
● Diagnosis of exclusion; usually resolves in children but is chronic in adults
● Tests to order include peripheral blood smear, CBC, HIV, thyroid function tests, liver function tests and bone marrow biopsy
● Although not technically contraindicated, platelet transfusions tend to be futile until the offending antibody is removed via some kind of immunosuppression

Neonatal alloimmune thrombocytopenia (NAIT):
● Incidence of 1 per 1-5K live births
● Father and newborn have antigen that mother lacks, mother produces antibodies to this antigen (usually PI-A1 component of GP IIb/IIIa), which crosses the placenta and destroys fetal platelets
● Newborn platelet counts are < 100K at birth, return to normal within 2 weeks
● Newborn can sometimes develop intracranial hemorrhage due to extremely low platelet count
● If possible, give platelets negative for the antigen the mother lacks; otherwise, treatment of choice is washed maternal platelets

Platelet refractoriness:
● In thrombocytopenic patients with multiple platelet transfusions, due to formation of HLA-A, HLA-B or less commonly ABO antibodies that destroy transfused platelets
● Platelet crossmatch using immobilized platelets may be performed in referral centers

Post-transfusion purpura:
● Patient has antibody directed against transfused platelet antigen absent on patientís platelets
● For unknown reasons, these antibodies also destroy plateletís own antigens
● Typically antigen is PI-A1 component of GP IIb/IIIa or an HLA antigen the patient lacks
● Patients have sudden onset of severe thrombocytopenia 5-12 days after transfusion of platelet product, resolves 14 days after transfusion
● In thrombocytopenic phase, common anti-platelet antibodies (GPIIb-IIIa, GPIb-IX and GPIa-IIa) are present, in addition to antibodies to the specific antigen the donor platelets express that precipitated the event
● After resolution of the PTP episode, the general platelet antibodies will disappear, while the donor specific platelet antibodies will persist (Vox Sang 1999;76:120)

End of Coagulation > Coagulation laboratory tests > Platelet antibodies


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