CVID should be suspected in children or adults who have a history of recurrent bacterial infections involving ears, sinuses, bronchi and lungs. The characteristic laboratory features include low levels of serum immunoglobulins, including IgG, often IgA and sometimes IgM. Another part of the diagnosis of CVID is to determine if there is a lack of functional antibody. This is done by measuring serum levels of antibody, against vaccine antigens such as tetanus or diphtheria, measles, mumps, or rubella, hemophilus or pneumococcal polysaccharide. Patients with CVID have very low or absent antibody levels to most of these vaccines.
Immunization with killed vaccines is used to measure antibody function, and this functional testing is crucial prior to beginning treatment. These tests also help the physician decide if the patient will benefit from immunoglobulin replacement therapy and can be key in obtaining insurance authorization for this therapy. The number of B- and T-lymphocytes may also be determined and their function tested in tissue cultures.