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The page below is a sample from the LabCE course Chemical Screening of Urine by Reagent Strip. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Measuring Specific Gravity (SG)

The urine reagent strip measures specific gravity (SG) in increments of 0.005 with readings from 1.000 to 1.030.
The test principle is based on a change in pKa (the negative log of the acid disassociation) of certain pretreated electrolytes (methyl vinyl ether/maleic anhydride) in relation to ionic concentration of the urine. These electrolytes in the specific gravity area contain acid groups that dissociate according to the ionic concentration of the specimen. The more ions in the specimen, the more acid groups will dissociate, releasing hydrogen ions and causing a more acid pH.
The reagent area contains a pH indicator (bromothymol blue) which demonstrates the change in pH. The higher the specific gravity of the urine specimen, the more acidic the reagent area will become. The colors of the reagent area will range from deep blue-green in urines of low ionic concentration to green-to-yellow green in urines of increasing ionic concentration, and consequently, higher specific gravity.