The most common screening tests include:

• Blood count. A complete blood count (CBC) shows levels of red blood cells and white blood cells as well as platelets. A “differential count” itemizes the different types of white blood cells, including lymphocytes and neutrophils.
• Quantitative immunoglobulins. This standard laboratory test measures various immunoglobulin levels in the blood. In addition to total immunoglobulins, it shows levels of the different immunoglobulin types (IgG, IgM, and IgA).
• Antibody responses. Are immunoglobulins working properly? A blood test can show if the blood contains antibodies to the usual childhood immunizations, i.e., tetanus, measles, pertussis, or diphtheria. Sometimes a person may be given a booster shot, or a specific immunization such as a tetanus shot, to see if she or he responds by producing antibodies.
• Complement. A laboratory test using a sample of blood indicates how effectively the complement system is working.
• Skin tests. These tests, which are similar to TB skin tests, show how well T cells are functioning. Tiny amounts of several standard reaction-provoking antigens (including mumps and Candida) are injected into the skin. A person with a healthy immune system usually develops local swelling within 24 to 48 hours. However, these tests are not as accurate in very young infants.