Quantification of the galactose-deficient immunoglobulin A1, a form of immunoglobulin A1 deficient in the 3-6 O-glycans usually present in the hinge-region segments of the heavy chains of immunoglobulin A1. Galactose-deficient IgA1 (Gd-IgA1) glycoforms are significantly more abundant in individuals with IgA nephropathy than in healthy individuals.