LEARNING OBJECTIVE: Recall the three types of urine specimens, the methods used to preserve urine specimens, and the procedure for performing a urinalysis.
Since the physical and chemical properties of normal urine are constant, abnormalities are easily detected. The use of simple tests provides the physician with helpful information for the diagnosis and management of many diseases.
This section deals with the three types of urine specimens, methods used to preserve urine specimens, the procedure for performing a routine and microscopic examination of urine specimens, and some of the simpler interpretations of the findings.
Urine specimens for routine examinations must be collected in aseptically clean containers. Unless circumstances warrant, avoid catheterization because it may cause a urinary tract infection. Specimens of female patients are likely to be contaminated with albumin and blood from menstrual discharge, or with albumin and pus from vaginal discharge. For bacteriologic studies, care must be taken to ensure that the external genitalia have been thoroughly cleansed with soap and water. The patient must void the initial stream of urine into the toilet or a suitable container and the remainder directly into a sterile container. All urine specimens should either be examined when freshly voided, or refrigerated to prevent decomposition of urinary constituents and to limit bacterial growth. In the following sections, we will cover three types of urine specimens: random, first morning, and 24-hour.
Random Urine Specimen
A random urine specimen is urine voided without regard to the time ofday or fasting state. This sample is satisfactory for most routine urinalyses. It is the least valid specimen, since test results may reflect a particular meal or fluid intake.
First Morning Urine Specimen
The first morning urine specimen is the first urine voided upon rising. It is the best sample for routine urinalysis, because it is usually concentrated and more likely to reveal abnormalities. If positive results are obtained from the first morning specimen, the physician may order a 24-hour specimen for quantitative studies.
Twenty-Four Hour Urine Specimen
The 24-hour urine specimen measures the exact output of urine over a 24-hour period. Use the following steps to collect this specimen.
Refrigerate specimen during collection, and, depending on the test being performed, add a preservative to the first specimen voided.
The normal daily urine volume for adults ranges from 800 to 2000 ml, averaging about 1,500 ml. The amount of urine excreted in 24 hours varies with fluid intake and the amount of water lost through perspiration, respiration, and bowel activity. Diarrhea or profuse sweating reduces urinary output; a high-protein diet tends to increase it. Daytime urine output is normally two to four times greater than nighttime output.
PRESERVATION OF URINE SPECIMENS
To delay decomposition of urine, use the following methods of preservation:
Other preservatives used include formaldehyde, toluene, and thymol. The preservative used must be identified on the label of the container. If no preservative is used, this, too, should be noted.
ROUTINE URINE EXAMINATION
A routine urinalysis includes the examination of physical characteristics, chemical characteristics, and microscopic structures in the sediment. A sample for urinalysis (routine and microscopic) should be at least 15 ml in volume (adult), and either a random or first morning specimen. Children may only be able to provide a small volume, but 10-15 ml is preferred.
Physical characteristics evaluated during a routine urinalysis include color, appearance, and specific gravity.
COLOR.—The normal color of urine varies from straw to light amber. Diluted urine is generally pale; concentrated urine tends to be darker. The terms used to describe the color of urine follow.
The color of urine may be changed by the presence of blood, drugs, or diagnostic dyes. Examples are:
APPEARANCE.—Urine’s appearance may be reported as clear, hazy, slightly cloudy, cloudy, or very cloudy. Some physicians prefer the term “turbidity” instead of “transparency,” but both terms are acceptable.
Freshly passed urine is usually clear or transparent. However, urine can appear cloudy when substances such as blood, phosphates, crystals, pus, or bacteria are present. A report of transparency is of value only if the specimen is fresh. After standing, all urine becomes cloudy because ofdecomposition, salts, and the action of bacteria. Upon standing and cooling, all urine specimens will develop a faint cloud composed of mucus, leukocytes, and epithelial cells. This cloud settles to the bottom of the specimen container and is of no significance.
SPECIFIC GRAVITY.—The specific gravity of the specimen is the weight of the specimen compared to an equal volume of distilled water. The specific gravity varies directly with the amount of solids dissolved in the urine and normally ranges from 1.015 to 1.030 during a 24-hour period.
The first morning specimen of urine is more concentrated and will have a higher specific gravity than a specimen passed during the day. A high fluid intake may reduce the specific gravity to below 1.010. In the presence of disease, the specific gravity of a 24-hour specimen may vary from 1.001 to 1.060.
Specific gravity is measured with an index refractometer, available as standard equipment at most duty stations. See figure 7-22. The index refractometer may be held manually or mounted on a stand like a microscope. The specific gravity of urine is determined by the index of light refraction through solid material.
Figure 7-22.—Index refractometer.
Measure the specific gravity with an index refractometer in the following manner:
Chemical characteristics evaluated during a routine urinalysis include pH, protein, glucose, ketones, and blood. Some laboratories also include tests for bilirubin, urobilinogen, and nitrite, depending on the test strip used. Currently, most medical facilities use the Multistix® and Color Chart, which detects pH, protein, glucose, ketones, blood, bilirubin, and urobilinogen. The Multistix is a specially prepared multitest strip. The strip is simply dipped into the urine specimen and compared to the color values for the various tests on the accompanying chart. The color chart also indicates numerical pH values, which should be reported.
Microscopic Examination of Urine Sediment
Microscopic examination of urine sediment is usually performed in addition to routine procedures.
This examination requires a degree of skill acquired through practice under the immediate supervision of an experienced technician. The specimen used for microscopic examination should be as fresh as possible. Red cells and many formed solids tend to disintegrate upon standing, particularly if the specimen is warm or alkaline.
PREPARING SPECIMENS FOR MICROSCOPIC EXAMINATION.—To prepare urine specimens for microscopic examination, follow the steps below.
CLINICALLY SIGNIFICANT FINDINGS.— Leukocytes, erythrocytes, and casts may all be of clinical significance when found in urine sediment.
Leukocytes.—Normally, 0 to 3 leukocytes per high-power field will be seen on microscopic examination. More than 3 cells per high-power field probably indicates disease somewhere in the urinary tract. Estimate the number of leukocytes present per high-power field and report it as the “estimated number per high-power field.”
Erythrocytes.—Red cells are not usually present in normal urine. If erythrocytes are found, estimate their number per high-power field and report it. Erythrocytes may be differentiated from white cells in several ways:
Casts.—These urinary sediments are formed by coagulation of albuminous material in the kidney tubules. Casts are cylindrical and vary in diameter. The sides are parallel, and the ends are usually rounded. Casts in the urine always indicate some form of kidney disorder and should always be reported. If casts are present in large numbers, the urine is almost sure to be positive for albumin.
There are seven types of casts. They are as follows: