If an immuno-response is seen in the form of a rash, hives, urticaria or (worse) anaphylaxis it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen. Further testing can be done to identify the particular allergen.
The "scratch test" as it's called, is still very commonly used as an allergen test. A similar test involving injecting the allergen is also used, but is not quite as common due to increased likelihood of infection and general ineffectiveness by comparison.
Skin end point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response. To prevent a severe allergic reaction, the test is started with a very dilute solution. After 10 minutes, the injection site is measured to look for growth of wheal, a small swelling of the skin. Two millimeters of growth in 10 minutes is considered positive. If 2mm of growth is noted, then a second injection at a higher concentration is given to confirm the response. The end point is the concentration of antigen that causes an increase in the size of the wheal followed by confirmatory whealing. If the wheal grows larger than 13mm, then no further injection are given since this is considered a major reaction.
US Patent Issued to CSM Instruments on Sept. 11 for "Method for Analyzing a Scratch Test" (French, Swiss Inventors)
Sep 13, 2012; ALEXANDRIA, Va., Sept. 13 -- United States Patent no. 8,261,600, issued on Sept. 11, was assigned to CSM Instruments S. A....