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Platelet Antibody Screen (Indirect)

Test code(s) 94303

Question 1. What antibodies are detected by this test?

This test is an enzyme-linked immunosorbent assay (ELISA) designed to screen for anti-IgG/A/M antibodies to 1) HLA class I and platelet glycoprotein (GP) IV antigens and 2) polymorphic epitopes on the platelet glycoproteins IIb/IIIa, Ib/IX, and Ia/IIa.

Question 2. For what clinical situations is this test useful?

This test can be useful to identify platelet refractoriness due to the development of the specific antibodies to platelet-specific or HLA class I antigens as a result of pregnancy or transfusion. In addition, this test may be useful in the diagnosis of neonatal alloimmune thrombocytopenia (NAIT) and help differentiate post-transfusion purpura from other causes of thrombocytopenia.  Typically, platelet antibody testing is performed after a clinical suspicion of development of a platelet alloantibody.

The results of this assay should not be used as the sole basis for a clinical decision. Reaction patterns should not be used as the sole basis to establish the platelet antibody identity. Results obtained using this assay should be used in conjunction with clinical findings or other serological tests.

Question 3. What is the significance of an indeterminate result for GPIa/IIa?

An indeterminate result indicates that the sample was reactive toward human platelet antigen (HPA) 5a and 5b within GPIa/IIa. This pattern of reactivity is not consistent with an alloantibody response and may indicate the presence of auto-antibodies, non-specific binding, or other unknown causes. Correlation with clinical and other laboratory findings is necessary for interpretation.

Question 4. What is the significance of a result just above the cut-off for positivity?

The designation of “weak” positive is based on the observation that the color development is just above the cut-off for positivity. This finding reflects either low titer antibodies or possibly non-specific binding. Correlation with clinical and other laboratory findings is necessary for interpretation. Exogenous infusion of immunoglobulin (IG)-containing medications may also result in weak reactions.  

Question 5. What is the significance of a pan-reactive pattern?

A pan-reactive pattern indicates that multiple platelet and/or HLA antibodies are detectable. This may indicate the presence of auto-antibodies, non-specific binding, or other unknown causes. Exogenous infusion of immunoglobulin (IG)-containing medications may yield a pan-reactive pattern. Results from patients on IVIG therapy should be evaluated carefully in light of this limitation.

Question 6. Can I use this test to detect autoantibodies to platelet antigens?

No. This test has not been evaluated for the detection of autoantibodies to platelet antigens, as is seen in immune (idiopathic) thrombocytopenic purpura.

Question 7. If the result is negative, does that mean that platelet antibodies are not present?

Not necessarily. A negative result may mean that low-titer, low-avidity antibodies are present or possibly that low-incidence HLA class I antibodies are present. In rare instances, other anti-platelet and HLA antibody detection methods may better detect these antibodies.

 

References

  1. Rozman P. Platelet antigens. The role of human platelet alloantigens (HPA) in the blood transfusion and transplantation. Transpl Immunol.  2002;10:165-181.
  2.  Metcalfe P, Watkins NA, Ouwehand WH, et al. Nomenclature of human platelet antigens. Vox Sang. 2003;85-240.
  3. McFarland JG. Detection and identification of platelet antibodies in clinical disorders. Transfus Apher Sci. 2003;28:297-305.
  4. Bussel JB, Primiani A. Fetal and neonatal alloimmune thrombocytopenia: progress and ongoing debates. Blood Rev. 2008;22:33-52.
  5. Metcalfe P. Ensuring quality in platelet immunology. Vox Sang. 2007;93:287-288.
  6. George JN, Woolf SH, Raskob GE, et al. Idiopathic thrombocytopenic purpura: a practice guideline developed by explicit methods for the American Society of Hematology. Blood. 1996;88:3–40.
  7. Goddard EA. Intravenous immunoglobulin. Current Allergy & Clinical Immunology. 2008:21:26-31.
  8. McFarland JG. Posttransfusion purpura. In: Popovsky MA, ed. Transfusion Reactions. 4th edition. Bethesda, MD: AABB Press; 2012:263-287.
  9. Smith GA, Ranasinghe E, Ouwehand WH. The importance of using multiple techniques for detection of platelet antibodies. Vox Sang. 2007;93:306-308.
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
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Effective 05/31/2017 to present