Why Veterinary Specific Scales? 

Although refractometers are used to determine specific gravity, that measurement is not made directly. Instead, the instrument measures the refractive index of the solution that is correlated to specific gravity using a calibration curve built into the unit.

Research published in 1956 by Rubini demonstrated that canine and feline specimens of exactly the same specific gravity (determined by direct methods) yielded different refractive indices. Rubini then postulated that canine and feline specimens of the same refractive index (measured by refractometry) would yield different specific gravity readings. This work demonstrated the greater refractivity of feline urine that would yield falsely elevated values when measured against human scales; especially with concentrated samples.