3.1: Protein in urine
- Page ID
- 38659
RELATED READING: Chapter 26, 32. See Methods in CD-ROM forTotal protein, urine.
Background to test
Small amounts of protein are found in the urine of healthy individuals. These proteins include albumin and Tamm-Horsfall mucoprotein. Individuals with glomerular or renal tubular damage excrete larger amounts of protein into final urine than do healthy individuals. As in the case of glucose, the cells of the proximal tubule in healthy individuals reabsorb most of the albumin present in the glomerular filtrate. If the reabsorptive capacity of the tubular cells is exceeded by either increased glomerular permeability to albumin or by a loss of proximal tubular reabsoptive capacity, increased amounts of albumin appears in final urine. Loss of the glomerular barrier can result in far greater protein excretion than the loss of tubular reabsorptive capacity. Other serum proteins, such as beta-2-microglobulin, are also present in urine, however their concentration is much lower, therefore albumin by far is the protein most commonly monitored in routine urinalysis. Proteinuria, or more commonly albuminuria, is commonly observed in diabetics, since this disease is found in 5-6% of the U.S. population. Similarly, hypertension can lead to proteinuria. The dipstick is a rapid method of determining the presence of urinary protein above a predetermined concentration.
Principle of the Method
When albumin binds to a dye embedded in the dipstick, there is a release of H+ and a local change in pH. This pH change results in a change in the color of a dye. The amount of color change in the dye is proportional to the concentration of albumin in the urine.
The reactions are as follows:
\[\text{albumin} + \text{dye} \rightarrow \text{albumin-dye complex} + H^{+}\]
\[H^{+}\; \text{dye} \rightarrow \text{color change}\]
The dye requires a minimum pH change before a color change is visible. For many dipsticks this is in the range of 30-100 mg/L of urine. There is also a maximum color change that can be obtained with the strip reagent. This is commonly set at 2000 mg/L.
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 60 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 protein concentration. If an instrument is used, 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 negative results. Urine from known diabetics may give positive results, depending upon the extent of glomerular damage caused by their diabetes.
STUDENT REPORT
| Solution | Visual Color Score | Instrument Value at Set Times | ||
|---|---|---|---|---|
| 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 protein solution 1 | |
| Test protein solution 2 | |
| Test protein 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?
Instrument notes
The thermal paper print out from many of these instruments is not acceptable as a permanent copy. The reason is that this type of paper is unstable, and becomes unreadable with time. Results must either be hand copied to the patient chart, or made permanent in some other manner.