As defined by Dorland's Medical Dictionary, diagnosis is the art of distinguishing one disease from another, the determination of the nature of a case of disease. Certainly a battery of tests are not needed to diagnose all diseases. Most cases require the use of all three diagnostic tools. Think of them as the legs of a tripod, supporting a diagnosis. If one of the legs is missing or faulty, then the diagnosis is less valid.
Veterinarians are unique among medical professionals.
We cannot solely concentrate on our patient's medical conditions and needs. Each patient has a human owner who must also be factored into the equation. Many clients exert considerable pressure upon their veterinarian for a diagnosis, any diagnosis, when their pet is ill. Often this is done without regard for the diagnostic tripod mentioned above. Virtually every day I receive calls from dog owners wanting to have their pet's illness diagnosed over the phone.