The food trial, done right

• Use a prescription diet. I usually reach for a vegetarian diet or a true limited-ingredient novel protein your dog has never eaten. Hydrolyzed diets, where the protein is chopped too small for the immune system to recognize, work too.
• It has to be prescription grade, or the trial doesn't count. Pet-store "limited ingredient" bags aren't reliable. In one study, all 21 that were tested had meat DNA in them that wasn't on the label.
• Home-cooked works only with a recipe from a veterinary nutritionist.
• The best diet is the one your dog will actually eat for 12 weeks. Taste matters more than the brand.

Feed it for 10 to 12 weeks, and don't judge it at week three. A lot of dogs look about the same at week three, and a lot of owners give up right there. Most of the action happens after the first month.

The strictest dermatologists allow nothing but the diet. My clinic uses a slightly gentler list, and I didn't make it up. It's the one I hand my clients, from the veterinary dermatologist I refer to, with my safety notes added.

On Purina HA and seeing soy and corn starch on the bag? That's fine. The soy is hydrolyzed, which is the whole idea of the diet.

My tip: pick one or two. Every extra is one more thing that could muddy the answer. So pick one or two from the list, keep them the same the whole trial, and write them down. Keep them small, well under a tenth of what he eats in a day. If the result is unclear, they're the first thing we drop. On a hydrolyzed diet, any whole food is a new protein, so ask first.

• Flavored heartworm, flea and tick chews
• Gelatin capsules, usually beef or pork
• Fish oil, and joint, probiotic and vitamin chews. Ask whether there's a plain version.
• Flavored toothpaste, which usually tastes like poultry or beef (and never human toothpaste)
• Pill pockets, cheese, hot dogs and deli meat
• Dental chews, rawhide, bully sticks and flavored antlers
• The other pets' bowls, the cat's above all
• Table food, licked plates, the kids' dropped crusts, and scavenging on walks (poop-eating counts)

These hide a pill without breaking the trial: peanut butter (no xylitol, soybean oil or corn syrup), cooked white or sweet potato, berries, plain canned pumpkin, banana, or oatmeal or rice. Pick one hider and use it the whole trial. It counts as one of your extras.

• Never stop heartworm or flea prevention during the trial. In Texas, that's not optional.
• Flavored heartworm and flea chews generally contain beef, pork or soy protein, and Heartgard Plus is a real-beef chewable. Most veterinary dermatologists will have you switch to a topical product for the trial (Revolution or Imoxi, for example). A topical isn't as good on fleas, but it removes all doubt.
• For most dogs I'm fine keeping them on their usual chew. For a really bad case, a dog I suspect is beef-allergic, or a trial that has to be as strict as it can be, I'd go topical only, the way the dermatologist would.
• Either way is OK. Decide with your vet before you start, and whatever you use, it has to be made for that species and weight.

One cheat resets the clock. A food-allergic dog can flare within a day or two of eating the problem protein, and the skin then needs time to calm back down. It happens in the best homes. If it does, don't hide it, and don't beat yourself up. Write it in the notebook and call your vet. It may only cost a week or two, not the whole trial.

It's fine, and normal, to treat infections and control the itch at the start so he isn't miserable. Don't stop any itch medicine on your own; your vet will plan when it comes off so you can read the result.

If he gets better, the trial isn't finished. Going back to the old food (the challenge) is what proves it. If he doesn't get better after a clean trial, that's still an answer. Food becomes very unlikely, and that points to atopy.