SG measured with the chemical reagent strip method correlates well with gravimetric measurement, and, unlike the gravimetric or refractometer methods, does not need to be corrected for glucose or protein. Cloudy/turbid urines do not need to be clarified before measuring SG with the reagent strip method. It is the recommended method for determining SG if a urine specimen contains x-ray contrast media or plasma expanders.
Alkaline urine can affect the indicator system and lower the SG result on the reagent pad (Table 2). If the result is being read visually, it is recommended that .005 be added to the SG result when the pH is alkaline. Most chemical reagent strip readers, however, will automatically adjust the SG reading for pH.
Table 2. Chemical Reagent Strip Method for Specific Gravity (SG).
|Specific Gravity Reagent Strip
|Change in pKa of certain pretreated electrolytes in relation to ionic strength of urine. Ionic strength is closely related to specific gravity. The more acidic the urine, the higher the SG.
SG 1.001 deep blue- green
| 100-750 mg/dL protein causes falsely elevated SG readings.
For urines with pH >6.5, add 0.005.