Diagnosing canine food allergies is no fun at all. The condition’s most common symptoms of itching and chronic/recurrent skin and ear problems (with or without concurrent GI signs) are hardly unique to food allergies, so a complete work-up is the first order of business. And then finally, when we get to the point where we’re highly suspicious that a food allergy is to blame for a dog’s misery, we have to institute a food trial that lasts from eight to twelve weeks, during which time a dog eats absolutely nothing except an elimination diet and water (no treats, no flavored heartworm preventives … nothing).
An elimination diet is a food that contains a protein source to which a dog has never been exposed, or one in which the proteins are broken down (hydrolyzed) into such tiny fragments that the body does not mount an allergic reaction against them. Elimination diets also typically contain either rice, which is rarely allergenic, or a novel source of carbohydrates (e.g., sweet potato).
It’s easy enough for a veterinarian to explain what’s involved in a food trial and sell owners an appropriate elimination diet or provide you with a recipe for a home-cooked version, but actually carrying the food trial out successfully at home is really hard. Invariably, I get calls from owners who say, "I just found out my father-in-law has been sneaking the dog steak," or, "My toddler keeps dropping her crackers on the floor, is that a problem?" The answer is, "Yes, it’s a big problem."
When the rules of the food trial are not strictly followed, it becomes very difficult to determine if a dog’s continuing symptoms are a result of the “extras” he’s been receiving or because he suffers from something other than a food allergy.
In human medicine, doctors sometimes employ a "patch test" to diagnose food allergies. A recent study looked into whether or not patch tests could be used in a similar manner in dogs and also evaluated the effectiveness of blood tests in diagnosing canine food allergies. I won’t saddle you with the results of the study’s statistical analyses (in truth, I don’t want to bone up again on the differences between sensitivity, specificity, and negative and positive predictability), but the paper’s authors concluded that “patch testing (and to a lesser degree serum testing) can be helpful in choosing ingredients for an elimination diet in a dog with suspected AFR [adverse food reaction]” but shouldn’t be used to diagnose the condition itself.
In other words, patch testing is better at telling you what a dog is NOT allergic to than if he is allergic in the first place, and if so, which foods are to blame.
Oh well. It looks like neither patch nor blood testing will replace the food trial any time soon. On the other hand, I can see a role for patch testing once a food allergy has been diagnosed. Many owners understandably do not want to go through the rigorous process of reintroducing ingredients one by one and monitoring for a return of symptoms to determine exactly what their dogs are allergic to, but they also do not like the idea of having to feed only the elimination diet used in the food trial for the rest of the pet’s life.
Patch testing could determine what ingredients are most likely to be safe, which would help guide decision making as to which new foods to try first.