The nervous system conducts electricity, but becomes more resistant when presented with a substance that the body recognizes as a potential threat. This is the basis for identifying substances that may cause an inappropriate reaction when they come into contact with the immune system. Foods, additives and medications that do not affect the normal reading are passed as non-problematic. Any substances that impede the ability of the nervous system to conduct the current normally, giving a low reading, are identified as possible allergens.

One study at Southampton University concluded that this kind of testing could not diagnose allergies to common allergens such as the house dust mite, and that it was inappropriate for diagnosing any immediate hypersensitivity. More studies are needed to make any firm conclusions regarding the safety and efficacy of electrodermal testing.

Electrodermal testing utilizes an apparatus called a Wheatstone bridge, which measures electrical resistance by passing a small electric current between two points of the body.

The apparatus must be specifically calibrated for each individual.

During testing, the patient holds one electrode in one hand while the doctor carries out the necessary calibration using a second electrode to complete the circuit, usually on the patient's foot.

The normal level of current flowing through the body is then measured on a galvanometer.

Each substance to be tested is placed into the circuit in turn and a reading of the resulting electrical flow is taken; each reading takes about 15 seconds to assess.