Currently there are two types of allergy testing, blood testing and skin testing. Recent research has shown that both types of testing have led to creation of successful injection regimens. However, the skin testing has long been thought to be superior to blood-based testing.

Intradermal skin testing is considered an imperfect but helpful method for identifying substances that cause allergies. It tends to be a last resort after many other therapies have been tried. It is more useful for determining inhaled and contact allergens, as opposed to food allergens.

In this test, the horse is usually sedated and the skin on one side of the neck is clipped. Several dozen potential allergens are injected into the skin in a large grid pattern. A key tracks which antigen has been injected in what location. A positive response is the increased size of the wheal. Some allergic reactions are immediate, while some are delayed. For this reason, horses are monitored over a 24 hour period and the reactions charted over that time.

Reactions may be subtle and hard to read for a vet who does not perform the test frequently. For this reason, this diagnostic is usually performed by a board certified veterinary dermatologist who is trained in the performance and interpretation of this test.

The skin test is typically read at 15 minutes, 30 minutes, four to six hours, and 24 hours. Some horses have an immediate reaction only, others have a delayed reaction only, and some have both immediate and delayed reactions. The skin test is thought to be more sensitive than a blood test.