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Handling and Storage of Urine Reagent Strips
The following precautions should be observed when handling and storing urine reagent strips:
- Store strips according to the manufacturer's recommendations.
- DO NOT expose strips to moisture, volatile fumes, or direct sunlight. Remove only enough strips for immediate use and immediately recap the bottle.
- Avoid contamination of test strips. Do not touch the test areas with fingers and do not lay test strips directly on the workbench.
- DO NOT use discolored strips. Compare the color of the unused strip to the negative area on the color chart provided by the company. The color should be similar.
- Check the expiration date. Re-label the container with a revised expiration date if the manufacturer states a shortened usage period once the container has been opened.
Procedural Precautions
- Although the procedure is simple to perform, accurate results depend on careful adherence to manufacturer's directions and adequate quality control.
- Normal and abnormal controls should be tested whenever a new lot of strips is opened, and at the frequency defined by the laboratory's procedure.
- If quality control results do not correspond to the published control values, the problem must be resolved before patient samples are tested.
- Intensely colored urine may make it difficult to correctly interpret color reactions on the dipstick, as illustrated below. The affected tests should not be reported from the dipstick. It would be necessary to use an alternative method of testing if available.
Proper and Improper Storage of Reagent Strips
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All the following statements about the urine specimen are true EXCEPT:
Please select the single best answer
The urine specimen should be tested within two hours of collection if the specimen is stored at room temperature. | |
The urine specimen should be at room temperature prior to testing. | |
The urine specimen must be centrifuged before performing the dipstick testing. | |
The urine specimen must be well-mixed before performing the dipstick testing. |
Clinical Significance
Normal urine contains up to 150 mg (1 to 14 mg/dL) of protein each day. This protein originates from the ultrafiltration of plasma and from the urinary tract itself. Proteins of low molecular weight ([MW] <40,000) readily pass through the glomerular filtration barriers and are reabsorbed. Because of their low plasma concentration, only small quantities of these proteins appear in the urine. In contrast, albumin, a moderate-molecular-weight protein, has a high plasma concentration. This fact, combined with its ability (although limited) to pass through the filtration barriers, accounts for the small amount of albumin present in normal urine. Actually, less than 0.1% of plasma albumin enters the ultrafiltrate, and 95% to 99% of all filtered protein is reabsorbed. High-molecular-weight proteins (>90,000) are unable to penetrate a healthy glomerular filtration barrier. The end result is that the proteins in normal urine consist of about one-third albumin and two-thirds globulins. Among proteins that originate from the urinary tract itself, three are of particular interest: (1) uromodulin (also known as Tamm-Horsfall protein), which is a mucoprotein synthesized by the distal tubular cells and involved in cast formation; (2) urokinase, which is a fibrinolytic enzyme secreted by tubular cells; and (3) secretory immunoglobulin A,which is synthesized by renal tubular epithelial cells.2
The presence of an increased amount of protein in urine, termed proteinuria, is often the first indicator of renal disease. For most patients with proteinuria (prerenal and renal), the protein present at an increased concentration is albumin, although to varying degrees. Protein reabsorption by the renal tubules is a nonselective, competitive, and threshold-limited (Tm) process. Basically, when an increased amount of protein is presented to the tubules for reabsorption, the tubules randomly reabsorb the protein in a rate-limited process. As the quantities of proteins other than albumin increase and compete for tubular reabsorption, the amount of albumin excreted in the urine also increases. Proteinuria results from (1) an increase in the quantity of plasma proteins that are filtered, or (2) filtering of the normal quantity of proteins but with a reduction in the reabsorptive ability of the renal tubules. Early detection of proteinuria (i.e., albumin) aids in identification, treatment, and prevention of renal disease. However, protein excretion is not an exclusive feature of renal disorders, and other conditions can also present with proteinuria.
Proteinuria can be classified into four categories: prerenal or overflow proteinuria, glomerular proteinuria, tubular proteinuria, and postrenal proteinuria. This differentiation is based on a combination of protein origination and renal dysfunction; together, they determine the types and sizes of proteins observed in the urine (Table 6.5).
Table 6.5
Classification of Proteinuria
Prerenal | Overflow proteinuria: an increase in plasma low MW proteins leads to increased excretion in urine | Normal proteins: • Myoglobin • Hemoglobin • Acute-phase reactants |