Absorption tests for detection of anti‐glucose‐modified β2glycoprotein I (G‐β2GPI) specificity. Sera of patients with anti‐phospholipid antibody syndrome (APS), positive for both anti‐native β2GPI and anti‐G‐β2GPI, and of healthy subjects (controls), unabsorbed or absorbed with native β2GPI, were analysed by enzyme‐linked immunosorbent assay (ELISA) for the detection of: (a) anti‐G‐β2GPI IgG, mean absorbance of unabsorbed sera [optical density (OD) 2·264] was set to 100%. Absorbed patients’ sera versus unabsorbed patients’ sera: P < 0·001; absorbed controls’ sera versus unabsorbed controls’ sera: P > 0·05. (b) anti‐β2GPI IgG, mean absorbance of unabsorbed sera (OD 1·116) was set to 100%. Absorbed patients’ sera versus unabsorbed patients’ sera: P < 0·001; absorbed controls’ sera versus unabsorbed controls’ sera: P > 0·05.