The specific gravity assay is intended for use as a screening tool for determining urine specific
gravities on Olympus, Hitachi, Beckman, Cobas and other automated systems for the analysis of urine. This assay is designed to check for attempts at dilution on urines submitted for drugs of abuse testing. Specific gravity values below 1.005 and above 1.030 specific gravity units should be considered suspect for adulteration. Regulatory agencies have determined that urine samples with a specific gravity of < 1.0010 as substituted and urines with a specific gravity between 1.0010 and 1.0030 as dilute as validated by a refractometer. The analysis is based on an indicator principle that gives a broad range of colors throughout the urinary specific gravity range (1.000 to 1.050). This assay is based on a pKa change of pretreated polyelectrolytes in response to ionic concentration of the test sample. A sample of urine is mixed with the reagent and, depending on the specific gravity (ionic strength) of the urine, the indicator will exhibit a change in color. This color change results in a change in the absorbance that is monitored by a detector at 600 nanometers. All abnormal results should be rechecked by a refractometer. Urine specific gravity values, as well as appearance and creatinine, are useful as an adjunct in detecting diluted urine samples.