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Precise quantitation of PAIgG: a new radiometric microtechnique
K A Schwartz1, J A Gauger, J M Davis
1Department of Medicine, Michigan State University, East Lansing 48824.
Insights
A new radiometric assay accurately quantifies platelet-bound IgG (PAIgG) using minimal samples. This sensitive method aids in diagnosing immune thrombocytopenia and detecting antiplatelet antibodies.
Area of Science:
- Immunology
- Hematology
- Assay Development
Background:
- Platelet-bound IgG (PAIgG) is crucial in diagnosing immune thrombocytopenias.
- Accurate quantification of PAIgG is challenging, especially with limited patient samples.
Purpose of the Study:
- To develop a sensitive and quantitative radiometric assay for platelet-bound IgG.
- To enable diagnosis of immune thrombocytopenia and detection of antiplatelet antibodies.
Main Methods:
- Utilized 96-well plates with filter membranes.
- Employed a 125I-labeled monoclonal anti-human IgG secondary antibody.
- Quantitated antiplatelet IgG using purified 125I-labeled anti-PIA1 antibody.
Main Results:
- Assay requires minimal platelets (5 x 10^6) and plasma (10 µL).
- Detects ~200 molecules IgG/platelet with a linear range of 0-7,000 molecules/platelet.
- Identified increased PAIgG in ITP, malignancy, and drug-induced thrombocytopenia patients.
Conclusions:
- The developed radiometric assay is sensitive, quantitative, and reproducible.
- It enables precise PAIgG measurement from small sample volumes.
- The assay is valuable for diagnosing immune thrombocytopenias and detecting antiplatelet antibodies.
Abstract:
We report the development of a radiometric assay for platelet-bound IgG that is both sensitive and quantitative. The assay utilized 96-well millititer plates incorporating a 0.2 microns filter membrane in the bottom. A 125I-labeled monoclonal antihuman IgG, as a secondary antibody, detected the platelet-bound human IgG. Since 5 x 10(6) platelets were used for each assay, tests for platelet-bound IgG can be performed on persons with severe thrombocytopenia. For the detection of circulating antiplatelet alloantibodies, as little as 10 microliters of platelet-free plasma per assay is required. Antiplatelet IgG was quantitated by using anti-PIA1 antibody that was purified with affinity and elution and DEAE chromatography. This purified antiplatelet antibody was labeled with 125I and was used to determine the binding ratio of secondary antibody to primary antibody. Under our standard conditions, this ratio was found to be stable at approximately 0.35 over the sensitivity range of the assay. The assay can detect approximately 200 molecules of human IgG per platelet (0.1 ng of secondary antibody bound per 5 x 10(6) platelets). It has a linear range from 0 to 7,000 molecules per platelet. Quantitation of anti-PIA1 binding for platelets stored for up to 6 months under refrigeration showed no change in number of PIA1 binding sites. Clinical studies showed that 18 of 19 ITP patients had an increased number of IgG molecules per platelet as did patients with malignancy and drug-induced immune thrombocytopenia. Patients who had received multiple platelet transfusions had antiplatelet antibody in their plasma. Normal amounts of PAIgG were observed in platelets and plasma of patients with nonimmune thrombocytopenia. The advantages of this method are that it is: 1) a more precise quantitation of PAIgG via direct measurement of binding ratio with PIA1 antibody; 2) performed with small amounts of platelets and plasma; 3) both sensitive and specific; and 4) reliably reproducible with both fresh and stored platelets.
