Backgrounds: Heterogeneous clinical features of antibody deficiency (AD) may cause diagnostic delays. Calculated globulin (CG) (total protein minus albumin) has been proposed as a screening test to prevent morbidity due to diagnostic delays in AD. Our aim is to validate CG as a screening test in AD in Turkish adult patients by comparing its role with gamma globulin analysis in protein electrophoresis. Methods: Fifty serum samples were randomly collected for each level of CG from 1.5 to 2.5 mg/dl and tested for serum IgG, IgA, IgM levels and protein electrophoresis. Cut-off values predicting low IgG levels were calculated for electrophoretically determined gamma globulin and CG. Additionally, the data of 47 patients followed up in our clinic with a diagnosis of primary antibody deficiency (PAD) were retrospectively analyzed. Results: A total of 550 adult patients were included in the study. The CG value predicting patients with IgG <600 mg/dl as a screening test was determined as <2.0 with 83.8% sensitivity and 74.9% specificity. The gamma globulin value which predicted patients with the same IgG value of 89.0% sensitivity and 89.4% specificity was determined as <0.7. In the retrospective analysis, 37 of 47 patients (78.7%) with PAD had a CG value of <2.0 at the time of the diagnosis and all 13 patients (%100) whose gamma globulin values were measured at the time of the diagnosis had a gamma globulin value of <0.7. Conclusion: The determined CG cut-off value of <2.0 can be used as a screening test in Turkish adult patients.