What is the clinical significance when samples test positive for the anti-cardiolipin antibodies and are negative for anti-b2GPI?
The aCL assay can detect antibodies of differing specificities. This includes antibodies specific for the cardiolipin molecule itself, and antibodies that are directed against either a cofactor molecule such as b2GPI, or a special binding site created by the interaction of a cofactor with CL. Antibodies directed against CL may be associated with infectious disease, or may be specific for a different cofactor, such as prothrombin. The clinical significance of these antibodies must be assessed in conjunction with the patient’s symptoms, clinical history, and other laboratory findings. Follow-up testing of these patients is recommended in 3-6 months to confirm reactivity. Only b2GPI cofactor dependent antibodies react in the anti-b2GPI assay; these antibodies show a higher correlation with thrombosis and are more specific for the antiphospholipid syndrome.
What is the advantage of an assay for anti-phospholipid antibody cofactors over an assay for the antibody itself?
Binding of b2GPI to the microwell surface in an ELISA assay may produce a neoepitope similar to that when combined with a phospholipid; assay results with this system show a good correlation with the APS. The serologic detection of anti-b2GPI antibodies provides enhanced clinical sensitivity for thrombosis.
The REAADS anti-Cardiolipin test kit and the b2GPI kit are reagent-complete kits. The anti-cardiolipin kit features specific determination of IgG, IgM, and IgA aCL antibodies. The kits are convenient, cost-effective ELISA procedures which give objective, accurate, and reproducible results with short incubations at room temperature.
Patient serum is diluted with sample diluent and incubated in microwells coated with human b2GPI. Antibodies to b2GPI present in the sample will bind to the coated wells. After washing, enzyme conjugated anti-human IgG, IgM, or IgA immunoglobulin is added, the wells are washed again, substrate added, and color development measured in a spectrophotometer at 450 nm. With a simple calculation, semi-quantitative results in G, M, or A units are available in less than 1 hour. The normal range is less than 20 units for each isotype (IgG, IgM, or IgA).
b2GPI, also called apolipoprotein H, is an antiphospholipid protein cofactor with natural anticoagulant properties and an affinity for negatively-charged phospholipids. Antibodies directed against b2GPI have been shown to be specific markers for thrombosis in individuals with systemic lupus erythematosus (SLE) and lupus-like disorders (APS). Most autoimmune anti-phospholipid antibodies require the serum cofactor b2GPI for optimal binding. It has been shown that many anti-phospholipid antibodies may react to a neoepitope formed on the b2GPI molecule by the interaction between the phospholipid and b2GPI. The physiological role of b2GPI may additionally be to participate in apoptosis.