I am not too much help on this subject, I do not study those brands of diets.
I do want to let you know that there are other brands of prescription diets available now. I don't know anything about them, other than they are for renal disease. Your vet should be able to order them for you, or possibly call in the prescription to another vet office or an online store.

Purina, NF, kidney function

Eukanuba, kidney conditions, (2 kinds): Early stage/canine dry and advanced stage/canine dry.

Royal Canin, modified formual (renal disease) renal 11p (late stages) and renal mp14 (early stage)