Urinalysis - The Carter Center

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Calibration
To obtain the correct specific gravity readings in urine, the urinometer
must be weighted to read exactly 1.000 in distilled water. The reading
on the urinometer scale should be exactly 1.000. If it is not, a correction
must be applied to all values obtained for urine specimens with the
urinometer. For Example, Suppose the urinometer reads 1.002 in
distilled water. The specific gravity of water is 1.000. Therefore the
urinometer correction is 0.002 must be subtracted from the subsequent
relative specific gravity. If a urine specimen has an apparent specific
gravity of 0.037, this value minus 0.002 results in the corrected specific
gravity of 1.035 for the urine specimen.
Temperature Correction
The specific gravity of a solution is dependent on temperature. Most
urinometers are calibrated for use at 15 o C. For each 3 o C difference
0.001 must be added if above, or subtracted if below than the calibration
temperature. For example, if the specific gravity of the urine is 1.022 at
23 o C, and the urinometer has been calibrated at 20 o C, the correct
reading is 1.022+0.001= 1.023.However, significant error will result if
the reading is taken on the urine specimen that has been refrigerated.
Instead of applying this correction, the urine specimen should be allowed
to warm up to room temperature before its specific gravity is determined.
Correction for abnormal Dissolved Substances
The specific gravity increases by 0.004 for every 1% glucose in urine
and 0.003 for every 1% protein in solution. Therefore subtract 0.004 from
the specific gravity reading for every 1% glucose in urine. And subtract
0.003 from the specific gravity reading for every 1% protein in the urine.
It is not usual however for the Laboratory Technician to correct specific
gravity readings for the presence of sugar or protein when laboratory
results are reported. Instead, the clinician will be aware that the specific