Diagnosis of X-linked Agammaglobulinemia Low immunoglobulin levels and absent B cells Genetic testing Diagnosis of X-linked agammaglobulinemia is by detecting low (at least 2 standard deviations below the mean) levels of immunoglobulins (IgG, IgA, IgM) and absent B cells (< 1% of all lymphocytes are CD19 + cells, detected by flow cytometry).