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URINALYSIS AND URINE FUNCTION STUDIES

What types of information can be obtained from a urinalysis? What is their importance?
Urinalysis is the most commonly performed laboratory test in a general medical practice. It is routinely performed in two parts: (I) use of a dipstick and (2) microscopic examination of spun urine. The most common dipsticks give results such as pH, specific gravity, presence or absence of blood, protein, and leukocytes, and various other tests, including assessment of glucose, ketones, bilirubin, and urobilinogen.
 
What is the normal pH of urine?
The normal pH of urine is between 5 and 8, depending on diet and other factors. Urine pH is important in determination of renal tubular acidosis, which may cause a pH below 5.5. Uric acid stones may be suspected with acidic urine. Alkaline urine may be observed in the presence of urea-splitting organisms, such as Proteus sp.
 
 

What is the significance of abnormal findings on a urine dipstick examination?

The presence of glucose in urine may indicate glycosuria secondary to undiagnosed diabetes mellitus. Leukocytes may indicate an inflammatory or infectious process within the urinary tract; interpretation, however, must be verified by microscopic examination. Protein in the urine may indicate a reabsorption problem in the kidneys, whereas ketones may indicate ketoacidosis, as seen with diabetes or a state of starvation or malnutrition. Bilirubin may be seen in the urine of patients with early stages ot liver abnormalities. Erythrocytes or free hemoglobin from lysed erythrocytes should alert the investigator to examine the urine microscopically for red blood cells.
 
How is a clean specimen obtained for microscopic examination of urine?
It is important for the investigator to obtain a "clean-catch urine." Women should clean themselves with a sanitary wipe and spread the labia to get a clean, clear stream into the specimen bottle. Uncircumcized men should retract the foreskin before giving the specimen.
 
How is a proper microscopic examination of urine performed?
The clean-catch specimen is centrifuged for approximately 3-5 minutes at 2-3,000 rpm. The sediment (button) at the bottom of the centrifuge tube is suspended in approximately 0.2 ml of urine. The resuspended button then is poured onto a glass slide and covered with a coverslip. First a low-power scanning of the specimen is performed to detect red blood cell casts, crystals and pathogens such as trichomonads. High-power analysis identifies bacteria, yeast, red blood cells, and white blood cells.

 
What is the significance of findings in a microscopic urine examination?
High-power examination may reveal urine casts that contain erythrocytes encased in a noncellular matrix. This finding may indicate glomerular hemorrhage, as seen in glomerulonephritis. Other casts with renal tubular cells may indicate renal tubular damage. The examiner also may observe urinary crystals of phosphate, oxalate, and cystine in stone-forming patients; however, crystals also may be seen in non-stone-forming patients. Leukocytes in the urine sediment may indicate an inflammatory process secondary to infection, calculi, or interstitial renal disease. Microscopic examination also may reveal erythrocytes. The differential diagnosis for erythrocytes in urine is quite extensive. Patients with microhematuria should be evaluated for infection, calculus disease, carcinoma, inflammatory process, trauma, sickle-cell enemia, bleeding disorder, and glomerulonephritis. Epithelial cells are seen frequently in urinalysi1i, especially squamous epithelial cells in postpubertal and nonmenopausaJ women. Abnormal transitional cells in the urine indicate malignant transitional cell carcinoma.
 

What are the most commonly used renal function tests?
The most commonly used renal function tests are urine specific gravity, serum creatinine, blood urea nitrogen, serum electrolytes, and complete blood count.
 
How is urine specific gravity used to test renal function?
Urine specific gravity is a simple test of the ability of the kidneys to concentrate urine. Decreasing ability to concentrate urine parallels a decrease in o~eralf renal function. Urine specific gravity remains the simplest method of determining the~abi)ity to concentrate and dilute urine.
 

How is serum creatigine helpful in determining renal function?
Creatinine, an eniprodUct of metabolism in skeletal muscle, is excreted by the kidneys and is not influenced by hydration status. Because of the constancy of daily creatinine production in a normal, active patient, levels of serum creatinine remain within a fairly reliable range and therefore can be used to determine renal function.

 
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