What is an ideal diagnostic allergy test? Are skin tests ideal or are other tests better? Are skin tests more sensitive or merely less specific than other test methods? Are there significant differences in the performance of different tests depending upon the type of allergen evaluated? Ideally, a diagnostic allergy test provides two different forms of information. A test first provides information about whether a specific form of allergic disease was present (e.g., if a patient has allergic or nonallergic rhinitis). A test then provides information about which allergen or allergens were responsible for the allergic disease. Compared with this ideal, current tests leave much to be desired because they are tests for immunoglobulin E (IgE)–mediated sensitization, a condition that is not invariably associated with disease. In the majority of clinical situations, the clinician must make a disease diagnosis almost totally from the patient's history and physical examination. At times, patient histories are clear and consistent, but at other times, forming a firm diagnostic opinion is difficult because of unusual or vague symptoms or because of difficulty correlating symptoms with probable allergen exposure. Ancillary tests such as spirometry and examination of nasal cytology may be helpful in many situations but are rarely diagnostic. An advantage and disadvantage of skin tests is that they are the end result of a sequence of events from introduction of the allergen into the skin to allergen-IgE binding, mediator release, response of the end organ, and release of mediators. The advantage is that the skin test provides information on the end result of the entire sequence of events and presumably an estimate of the potential level of reactivity of an individual to the allergen following natural exposure. The disadvantage of skin tests is that it is possible that the level of activity of individual components of the sequence may vary substantially between different organ systems in an individual. This could account for the observation of a less than perfect correlation between skin test sensitivity and clinical sensitivity. In theory, this disadvantage largely could be overcome by directly challenging the organ involved in the allergic disease with allergen, but this is inconvenient when compared with skin tests. The advantages and disadvantages for skin tests are reversed when tests designed for the in vitro detection of allergen-specific IgE are considered. These tests only detect IgE antibodies and provide no estimate of the relative activity of these antibodies in vivo. The activity of the antibodies presumably depends on the avidity with which the antibodies bind allergen and the probability that the antibodies will be found in high density on effector cells. 15 It is relatively easy to compare the value of different tests when there is an accepted standard of accuracy, commonly called the standard. Unfortunately, in allergy it has been difficult to agree on a gold standard for allergic disease in part because of confusion in the literature on the difference between allergic disease and allergic sensitization. The clinical history obtained from the patient is a possible standard, but when critically examined clinical histories often do not coincide with IgE test results. 17,18,25,46 There is also the problem that many patients are not able to provide a clear and consistent history. Skin tests often are used as a gold standard, but they are not infallible. Positive skin tests are not always associated with allergic disease, and skin tests may be falsely negative even when high concentrations of allergen-specific IgE are present. 10,35,42 Challenge tests involving exposure of a specific organ, other than the skin, to allergen have specific problems such as determining a realistic dose of allergen and delivering the allergen in a realistic fashion. Given the problems inherent with selecting a standard to use for comparing the results of tests, it is easy to see why the literature is full of contradictions and inconsistencies. Though the ultimate criteria for judging a test is the amount of help the test provides in caring for patients, finding and evaluating this information can be difficult. An additional problem is that testing methods, especially in vitro testing, evolve more rapidly than it is possible to complete well-done comparative studies. In addition to diagnostic value, other criteria are important when comparing tests, such as the risk of major or minor adverse reactions from the test, the convenience of the test for the patient, and the cost of the test.