In vitro tests are used to assist in the diagnosis of both allergic and immunologic diseases. Unfortunately, there is no single test that is pathognomonic for most allergic diseases. The most commonly ordered in vitro test by allergists is allergen specific IgE (sIgE), which is used to help diagnose IgE mediated hypersensitivity to foods, aeroallergens and venoms. Multiple assays exist, although none of these assays have been adopted as the industry standard. Epicutaneous skin test is also a fundamental test in the diagnosis of IgE mediated hypersensitivity. In addition, total IgE, basophil activation test (BAT), and serum tryptase may also be useful in elucidating allergic diseases. Immunologists rely on laboratory testing to diagnose primary immunodeficiency diseases. These tests include serum quantitative immunoglobulins, lymphocyte immunophenotyping by flow cytometry and immune cell functional testing. Furthermore, genetic testing is invaluable in the diagnosis of many primary Immunodeficiencies.