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Anti-Beta-2-Glycoprotein I Antibody

The antiphospholipid antibody syndrome (APS) may be defined as the occurrence of arterial or venous thrombosis or recurrent miscarriage in association with laboratory evidence of persistent antiphospholipid antibody. The antibody may manifest itself as either a lupus anticoagulant detected by clotting tests or an anticardiolipin antibody (ACA) detected by immunoassay. The syndrome may be associated with an autoimmune disorder, especially SLE (secondary), or may occur independently of other autoimmune disorders (primary). A large proportion of antiphospholipid antibodies are actually directed against complexes of phospholipid with protein, most notably beta-2-glycoprotein I. The ACA immunoassay detects both these clinically significant antibodies and antibodies directed purely against cardiolipin and not beta-2-glycoprotein I. The latter antibodies are transient, and are often associated with infection but not thrombosis. A positive anti-b₂GPI assay is evidence in favor of APS, once persistence is demonstrated by repeating the assay after eight weeks.

Positivity for this test is more closely associated with clinical manifestations of APS, including thrombosis, than the conventional ACA test. In one study of patients with primary and secondary APS (J Lab Clin Med 1997; 129: 499-506), anti-beta-2-glycoprotein I showed greater specificity (98% versus 54%) for the syndrome than the ACA assay, although it was less sensitive (54% versus 87%). Furthermore, a subset of patients with APS may test positive for anti-beta-2-glycoprotein I and negative for ACA.

In view of the marked heterogeneity of antiphospholipid antibodies, a comprehensive approach to laboratory diagnosis of the syndrome is recommended. An ELISA assay for anti-beta-2 GPI is helpful in any one of the following circumstances:

IgG ACA titer < 40 GPL units
Positive IgM ACA only
Negative ACA, but clinical suspicion of APS

Reference range is 0-20 G units.

Sample requirement is one SST tube of blood. The sample submitted for the anticardiolipin assay can be used for this assay.