This month I will review the reliability of “allergy” testing in dogs. If you are going to recommend allergy testing to clients, you should understand their limitations. The topics I have planned for the coming weeks are:
- Intradermal testing reliability
- Serological testing of normal dogs
- Serological testing repeatability
- Serological testing reproducibility
Intradermal testing (IDT), sometimes referred to by the redundant term intradermal skin test, involves injecting numerous allergen extracts into the dermis. Commonly, between 50 and 70 individual injections are performed. A positive control (usually histamine) and a negative control (saline) are also injected. After some period of time, usually 15 to 20 minutes, the sites are graded for wheal formation. This is usually done with a subjective 0 to 4 ordinal scale, although some also measure the average wheal diameter.
The test results can be influenced by a number of factors, including medications, skin pliability, and patient stress. The optimal testing concentration of aqueous allergens is also of paramount importance and varies with different allergens. The ideal concentration causes few wheal reactions in normal dogs, but does so in dogs truly allergic to the allergen. Despite the importance of the testing concentration, guidelines are often not followed by those performing IDT.
Even when performed under ideal conditions, IDT causes false positive reactions to occur. These may represent irritant reactions as described by Koebrich et al to house dust mites (HDM). Seventeen clinically normal beagles were tested with IDT for sensitivity to HDM. Seven of 17 reacted to HDM on IDT at the standard testing concentration. Mueller et al found that clinically normal dogs and atopic dogs reacted to storage mites at the same rate (about 1/3 of each group).
I participated in a study that looked at the fundamental questions of IDT repeatability and reproducibility in dogs. To examine repeatability, duplicate intradermal injections were evaluated in a blinded study and the agreement of the two readings by the same observer examined. To examine reproducibility, the agreement of three independent observers, each scoring the same dogs, were examined. The results were instructive, but not surprising to someone who has done a lot of intradermal testing. The negative (0) and strongly positive (4+) reactions were generally repeatable and reproducible, but those values in the mid-range were less reliable than desired of a health measurement scale that has been considered the “gold” standard.