Diagnostic Reagents Inc., Sunnyvale, CA. USA.

The Gravity-Detect assay is based on a linear relationship between the urine chloride concentration and the specific gravity. The chloride concentration is determined colorimetrically and the absorbance measured is directly proportional to the urine specific gravity. The pH-Detect assay is based on a specifically engineered linear relationship of enzyme (G6PDH) activity with the pH of the reaction. A higher enzyme activity indicates a higher pH value of the sample. A linear calibration curve is established for both methods. The unknown sample specific gravity/pH values are calculated with the corresponding absorbance from the calibration curve.

Clinical urine samples were exposed to various commonly encountered adulterating agents (i.e. bleach, water, ammonia, hydrogen peroxide, liquid soap, sodium nitrate, table salt, vinegar and gluteraldehyde) to simulate urine adulteration.

The pH of these treated (adultered) samples were determined by a pH meter and subsequently assayed with the pH-Detect test. An average recovery of 103% was observed. A correlation with a regression equation of pH-Detect (y) = 2.62 + (0.56)pH meter (x) and a correlation coefficient (r) of 0.90 was obtained using 120 samples.

The specific gravity of the above samples was determined by a digital urine specific gravity refractometer and with the Gravity-Detect test. An average recovery of 99.9% was observed. A correlation with a regression equation of Gravity-Detect (y) = 0.10 + (1.10)refractometer (x) and a correlation coefficient (r) of 0.90 was obtained using 120 samples.

Used together, both pH-Detect and Gravity-Detect are effective and convenient in determining urine adulteration.