3.3: Specific Gravity of Urine

Last updated
Save as PDF

Page ID: 38661

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

RELATED READING: Chapter 26. SEE URINALYSIS INFOBASE.

Specific gravity is a property of urine that is dependent upon the amount of solids dissolved in urine. The specific gravity is primarily a measure of the salts excreted in final urine. Most commonly these salts include the ions sodium, potassium and chloride. Other commonly excreted metabolic products such as urea, and creatinine also make up a portion of the dissolved solids. All urine including that from healthy individuals contains dissolved solids. However, urine output is a consequence of fluid intake, thus there is a considerable variance of specific gravity in urine in the same individual, or between individuals. The dipstick is a rapid method of determining the specific gravity.

Principle of the Method

The dipstick employs a specially treated polyelectrolyte, whose pKa changes in response to increased concentration of ions. The changed pKa results in a changed pH in the environment of the pad, which causes a dye, also impregnated in the pad, to change color. The amount of change in dye color is proportional to the amount of dissolved ions present in the urine.

The reactions are as follows:

\[\text{Polyelectrolyte} + \text{changing ionic strength} \rightarrow \text{change pKa} + \Delta H^{+}\]

\[\Delta H^{+} + \text{indicator dye} \rightarrow \text{color change}\]

The dye requires a minimum pH change before a color change is visible; for many dipsticks this is in the range of 1.000. There is also a maximum color change that can be obtained with the strip reagent. This is commonly set at 1.030.

Reagents

All the reagents for the reaction are embedded in the pad of the dipstick. As with all reagents stored in a dried form, their stability is affected by moisture. Therefore the reagents must be protected from moisture. In addition, careful attention must be given to the manufacturer’s shelf life, which should appear on the label of every dipstick container.

Specimen

Freshly voided urine is the preferred specimen. The first urine specimen of the day is considered the most desirable, because it is the most concentrated. Urine specimens are acceptable up to four hours after voiding. Refrigerated specimens are acceptable up to 24 hours after excretion.

Procedure

Collect the urine in an appropriate specimen container. After verifying that the strips are working (see quality control) quickly dip the strip in the urine, removing excess liquid by moving the edge of the strip against the rim of the container as you remove the strip from the container. After this initial pass to remove excess liquid, remove any remaining liquid by touching the entire edge of the strip to a gauze pad or a paper towel. There should be no visible liquid on the strip except for that on the pad.

Results

Start timing the reaction as soon as the strip is placed in the urine. After 45 seconds read the strip visually. Match the observed color of the pad with the chart color on the bottle of strips or with a color chart, if that is available. Record your observation. If a Clinitek or other instrument is available, place the strip in the device as soon as you dip the strip and record the printed result. If reading visually, continue reading the results after 60 seconds, 120 seconds, and 300 seconds.

Calculations

No calculations are necessary for the visual readings. If instrument readings are made, the calibrated instrument calculates the specific gravity. If an instrument is available, correlate the visual reading with that of the instrument.

Quality Control

When visual readings are taken, be certain that the reader is not color blind. Before testing a test sample, take two strips and test a positive and a negative control sample. These results should be within accepted values. Ideally, a positive and negative control should be tested along with each batch of patient specimens tested.

Expected values

Urine from healthy individuals should give positive results within the range specified on the bottle. The most common abnormal results are from diluted urine.

STUDENT REPORT

Solution	Visual Color Score
Time	30 sec.	60 sec.	300 sec.
QC negative
QC positive
1 g/L
500 mg/L
250 mg/L
125 mg/L
62 mg/L
31 mg/L

Test sugar solution 1
Test sugar solution 2
Test sugar solution 3

Discussion Questions

What is the range of linearity of the visual method?
What is the range of linearity of the instrument?
Does the test result change with time?
If the result does change with time, Why?
Does the range of linearity include all healthy and disease conditions?