A stressed animal has elevated cortisol (stress hormone). High cortisol skews white blood cell counts, elevates heart rate, and inhibits healing. By fixing the behavior , we get more accurate medical data.

Journal of Veterinary Behavior | ScienceDirect.com by Elsevier

A veterinarian listens to a frozen cat’s heart rate of 280 beats per minute (normal is 140-220) and notes "QS" (quiet, stable). That cat is not stable. That cat is in a state of catecholamine toxicity—a flood of adrenaline and cortisol.