Why must veterinary medicine follow the follies of its human counterpart? With shorter lifespans comes a faster research cycle and so animal health should always be in the lead. Sometimes that is just what we see; the use of ivermectin for bovine parasites began years in advance of its several human uses for example. In terms of practitioner attitude however the shoe is applied to the wrong end of the anatomy.
Lab tests are used as a fast proxy for conversation with the patient. For non-human patients lack of language provides the excuse. How is the cat to express the particulars of his discomfort? When the patient is human the tests' predictability and reimbursability are motivators. How long will it take or what the patient might tell you? The test will provide an simple number after a uniform amount of time and will be paid by the insurance company.
Lab tests are objective in a somewhat narrow sense. They are reliable in the sense of being repeatable and comparable: You really do know the the level of creatinine that was in the blood at the time the sample was taken. Test results are good pictures of the actual reality of the patient's blood chemistry.
Lab tests do not measure either the underlying disease in the literal sense of impairment or malady nor the etymological dis-ease of the patient's experience. Lab tests are not reports of the actual reality of the patient's health. Reasoning and analysis may be able to arrive at valid conclusions but often the practitioner will rely on useful presumptions in the manner of rules of thumb: Elevated creatinine is used as if it were a direct measure of kidney function or failure because what is easily measured correlates fairly well with what you really want to know.
This is well and good up to a first approximation but it becomes a problem when leaping directly from presumption to prescription. If the veterinarian or the physician pounces on a pharmacological solution to fix the presumed problem the result is sometimes, too often, too many of the wrong drugs having inappropriate impacts on the patient's actual health.
The point is not to abandon the lab tests or to skip the first approximation but to remember that Fluffy's real experience is the reality being approximated -- and that the actual reality game is an endeavor of sustained commitment and frequent missteps.