Vet Brand vs Store Brand Hypoallergenic cat food

So I've been told by my vet that my cat has allergies - can be to anything - food, environmental, don't know what. Just that when he's on Prednisone he's better and when he's not, he's licking his fur off.  So he has to be permanently on Prednisone.  So of course they recommend the vet brand of hypoallergenic food (which is crap).  I wanted to give him Natural Balance Pea & Duck, they say these brands are not guaranteed to be 100% hypoallergenic and that during processing they can be contaminated with other ingredients.

Is this true - or are they trying to rob me of my money???

Thanks for your opinions/thoughts...

I've heard the same about store brands from various vets.

There are pet allergy doctors, but they cost a lot. They do the same tests that people allergy doctors do--draw a grid and then test for allergies by pricking the skin with some of the allergen. 

If you want a truly hypo food go with the vet brand. Some are based on a novel protein ( Royal Canin Hypoallergenic or Sensitivity VR) while others have hydrolyzed protein (Royal Canin Hyooallergenic HP or Hills z/d). When a protein is hydrolyzed, the size of the protein molecules are reduced so the body doesn't recognize it as protein and therefore, doesn't react. These foods are made with strict guidelines to prevent cross-contamination. For example, Royal Canin (formerly Medi-Cal) cleans out their factory every weekend and only makes their hypo foods on Monday. Some pet store foods, while marketed as a novel protein for pets with food allergies, are made in factories where other brands are made. The potential for cross-contamination is great. Since allergies affect your pet's immunity, I would go with the vet food which is guaranteed to be free of cross-contaminants. It is the proteins that cats and dogs are usually allergic to though it is possible they can be allergic to carbohydrates.