What is urinalysis?

Indications and Procedures

Urinalysis is one of the oldest and most useful of noninvasive clinical tests. In addition to aiding in the diagnosis of urinary tract or kidney disease, the procedure may be applied to the analysis of most metabolic by-products that pass through the kidneys. Thus it may be applied to observations of kidney or liver abnormalities and metabolic diseases such as diabetes mellitus.

For routine analysis, approximately 10 to 15 milliliters of urine are collected in a clean jar, though larger volumes are preferable. Initial examination involves the physical appearance of the urine sample: color, turbidity, and possible odor. Normal urine is generally pale yellow in appearance, though variation from such color is not necessarily abnormal. Bacteria may cause alterations in this color, as can simple by-products of the diet. Normal urine is generally clear, though as with color, turbidity (cloudiness) may be associated with a variety of causes. Fresh urine also has a characteristically mild odor.

The specific gravity of the urine may be analyzed at this time, though the usefulness of this test is limited to those circumstances in which the water intake of the patient is known. Generally, the only specimen of use for this test is one utilizing the first urine output of the day. The pH is most accurately determined using a pH meter, though dipstick pads impregnated with colored pH indicators can be used when frequent (or inconvenient) monitoring is necessary.

Hematuria
, the presence of blood in the urine, is never normal, though its detection need not indicate a significant pathology. Hemoglobin may be detected using a dipstick method with follow-up necessary to determine the specific cause.

The microscopic examination of urine consists of centrifugation of a volume of urine under specified conditions followed by resuspension of the sediment in a standard volume of liquid. The presence of any blood cells, bacteria, yeast, or other types of sediment can then be determined.

Chemical analysis can be utilized for determination of the presence of a wide variety of chemicals or drugs. Routinely, chemical procedures are used to detect sugar, protein, or by-products of fat metabolism such as ketones. Dipsticks are available for routine analysis.

Uses and Complications

Diagnosis of urinary or metabolic problems cannot necessarily be made from a single abnormal test result, as a variety of factors have a potential impact on test results. Rather, analysis of a combination of tests is often necessary in diagnosis of a problem.

Urinalysis involves the physical, chemical, and microscopic analysis of urine. Physical examination centers on the color, turbidity, and odor of urine. A pink or red color can be indicative of the presence of blood, though microscopic or chemical examination is needed for confirmation. (For example, a red color may simply indicate that the patient recently ate beets.) An increase in turbidity can result from the presence of yeast or mucus, indicating infection, or from diet by-products such as lipids. Likewise, abnormal odors can result from urinary tract infection (elevated levels of ammonia) or certain metabolic diseases; however, ingestion of asparagus may also result in unusual odors.

Chemical analysis of urine ranges from the determination of pH to the detection of any of a variety of chemicals. On a routine basis, this usually involves examination for sugar, protein, or ketones. Normal urine is usually acid (pH 6), though the patient’s diet will often affect such values as well. A high pH may be indicative of urinary tract infection; microscopic detection of microorganisms may be used to confirm this diagnosis.

Small quantities of protein in the urine are normal. Elevated levels of proteinuria, however, can result from kidney disorders, particularly those associated with glomerular damage, or from urinary tract disease. Likewise, small quantities of sugar in the urine are generally of no clinical significance. In the case of diabetes, however, with resultant high levels of glucose in the bloodstream, significant quantities of glucose may be found in the urine. Persons with severe diabetes are unable to remove and utilize glucose from the blood; metabolism in such individuals will switch to the utilization of fat, with resultant breakdown products such as ketones being secreted in the urine. Such products are volatile and may disappear from urine if the sample is not analyzed within sufficient time. Since fat metabolism is employed as a source of energy in the absence of carbohydrates, severe dieting may also result in the excretion of ketones.

Perspective and Prospects

Analysis of urine for diagnosis of disease was among the earliest of medical procedures. Greek physicians at the time of Hippocrates observed the color of urine and its taste. Pouring urine on the ground to see if insects were attracted to it could be used to test for sugar.

Until the mid-twentieth century, chemical tests on urine utilized a variety of liquid reagents. The introduction of dipsticks significantly improved the efficiency and convenience of such analysis. The dipstick consists of a thin strip of plastic with a cellulose pad attached. Impregnated in the pad are the chemicals necessary to carry out the specific test. For example, the dipstick used in the analysis of pH contains an indicator that will change color depending on the degree of acidity or alkalinity.

Instrumentation is available that allows the analysis of a combination of tests simultaneously, much as a blood sample can be analyzed. Either the dipstick or the urine sample itself may be inserted into a machine for urinalysis. For simple home analysis in which only a single test is necessary, commercial production began in the 1980s of analogous materials for detection of urinary chemicals. For example, home pregnancy kits are available and are home drug testing kits, and in theory, similar kits could be used for the detection of any substance in urine.

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