The GAD ELISA is a qualitative ELISA test for the detection of circulating autoantibodies to glutamic acid decarboxylase (GAD) antigen. A purified GAD antigen is immobilized onto microwells. GAD specific IgG antibodies present in the patient's serum sample are allowed to react with the antigen. The excess /unbound serum proteins are washed-off from the microwells. An enzyme (alkaline phosphatase) labeled goat-antibody, specific to human IgG is added to the GAD antibody complex. After washing off excess unreacted enzyme conjugate from the microwells, a substrate (PNPP) is added and the color generated is measured spectrophotometrically. The intensity of the color developed gives directly the concentration of GAD autoantibodies in the test serum sample. GAD positive and negative controls serve as an internal quality control to ensure valid results.