People with IgG deficiency also often find that pneumonia and the flu vaccines donât keep them from getting these infections. How is an IgG deficiency diagnosed? A blood test that measures immunoglobulin levels can diagnose IgG deficiency. Itâs possible to have a normal level of total IgG, so the testing of the IgG subclasses is important. Tests can also be done on saliva and cerebrospinal fluid. But, a blood test is the most common. How is an IgG deficiency treated?