Specific gravity is a measure of dissolved substances present in a urine solution. S.G test strip measures urine density by changing pH depending on electrolyte concentration as an indicator, therefore, compensation of temperature or glucose are needless and it is not influenced by x-ray contrast. Refractive Index is measured with a refractometer. The refractive Index of a solution is the ratio of the velocity of light in air to the velocity of light in solution.

Substances such as non-ionizable glucose, protein, or radiopaque dyes will act to raise the specific gravity when measured with the refractometer as compared with test strip. Since specific gravity is used to determine the concentrating ability of the kidney, it is desirable to correct refractometer readings for abnormal dissolved substances.

As more ions are present in the urine, more protons are released from the polyelectrolyte, resulting in a decrease of the pH test strip and an indicator changes color from blue-green (SG =1.000) to yellow-green (SG = 1.030).  So, that pH control (YD URiSCAN analyzer adjusts automatically) is needed and a factor of urea concentration cannot be measured since urea is not ionic.