Physicians use blood testing and skin testing to diagnose food allergies, including nut allergies. They often start with a scratch test on the skin and then perform a blood test to confirm their findings with antibodies in the blood, as stated by KidsHealth.
To perform the scratch test, the nurse or doctor scratches a spot on the patient's skin with a small amount of liquid extract of a particular allergen, such as a peanut. Scratch tests generally take place on a patient's back or forearm. After 15 minutes, the doctor looks for raised, reddish places known as wheals. If they have formed, there is the potential for an allergy. This process takes place for as many allergens as the allergist wants to examine, notes KidsHealth.
If a skin test comes back with a positive result for an allergy, that only means the allergy is possible. Next, the doctor sends a small vial of blood off to the lab for testing. The technicians look at the blood for IgE antibodies to the suspected foods. If there are enough IgE antibodies present, the allergy is quite likely. When the skin and blood tests disagree, the allergist may try a food challenge. This involves giving the patient slowly increasing amounts of that food to eat while he watches the patient for symptoms, according to KidsHealth.