Refraction is the change in the velocity of a wave that affects a surface as it passes from one medium to another with a different refractive index. The refractive index is the ratio of the speed of light in vacuum to the speed of light in another substance, making the refractive index a specific physical property. This parameter of analysis is a good approximation for media where losses by absorption or other phenomena are negligible.
The formula behind the phenomenon is described with the Snell Law, which is used to describe the relationship between the angles of incidence and light. The measuring instrument associated with this parameter is called a refractometer, able to measure with great precision up to 5 significant figures and is a very used equipment to characterize solutions.
Veterinary medicine and the use of the refractometer
Since the 1960s, refractometry has been used as a rapid technique for the determination of total proteins in serum or plasma. In veterinary medicine, it is used to determine the concentration of total plasma solids and urinary density. Numerous authors have reported that the results of the estimation correlate well with those obtained by Biuret’s methods and traditional uranometry, respectively. The advantages of this technique compared to those used in clinical laboratories are the small amount of sample required for analysis, the speed and simplicity of measurement and the low cost associated with the instrument used (portable refractometer), being considered an acceptable field test for its sensitivity.
How is the refractometer used in veterinary medicine?
As mentioned above, refractometry is used to determine total solids in plasma and urinary density. In the case of blood tests, the angle of refraction produced by a serum sample is due to the combined participation of all its solutes (so-called total solids), where proteins are the predominant solute. In addition, blood plasma is composed of nonprotein substances (electrolytes, glucose, urea, and lipids) that contribute to the refractive angle. So, it can be said that refractometry qualifies as a technique to measure total solids based on proteins. Some instruments are calibrated in protein concentration in g/dL, if not, there are tables that assume the non-protein solutes for conversion.
For urinary density, the gold standard is based on comparing the weight of a urine volume before and after drying (picnometric determinate). Clinical refractometers typically have a scale equivalent to urine density based on data from normal human urine, labeled urine gravity (UG), urine specific gravity (USG), or urine specific gravity (PE).
Measurement scales in veterinary clinical refractometer
There are different specific scales according to the application. The most commonly used scale is Degrees Brix (proportion of sucrose or salts in a solution). On the other hand, veterinary clinical refractometers usually bring two or three scales:
- The serum protein (SP) scale: used to measure total protein levels in a sample of blood serum or plasma. Its units are g/dl (g/100ml).
- The scale of the refractive index (nD or ND): is usually referenced with appropriate conversion tables to measure the concentration of a variety of substances.
- The specific gravity (SG) or urine severity (UG) scale: Usually used to measure specific gravity on urine samples.
It should be noted that the scale on which the instrument is being worked on must be taken into account when calibrating the instrument. It is recommended that the prism and lid are thoroughly cleaned and calibrated with distilled water each time a measurement is made.
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