Lesson 1 Collecting, Preserving, and Examining Urine Samples
Macroscopic and Physical Examination of Urine
Macroscopic analysis deals with those procedures or examinations, which are accomplished without the aid of a microscope. Included in this category are measurement of volume, color, appearance, pH, and specific gravity. Before performing microscopic or chemical tests on a urine specimen, a macroscopic examination is accomplished. As the metabolic waste products filtering into the kidneys are constantly changing in relation to body intake, so the urine is constantly changing with respect to volume, color, appearance, specific gravity, and pH. Therefore, an accurate description of these physical properties furnishes the physician and/or physician extender with valuable information regarding kidney function.
The total 24-hour volume of urine voided by the normal adult is influenced by food and fluid intake, temperature, exercise, seasonal change, and the use of diuretics such as caffeine. Nonetheless, a consistent normal range has been established. The average adult produces between 750 and 2,000 ml of urine during a 24-hour period, with a median of about 1,400 ml. Volume determination is a quantitative analysis and therefore, the 24-hour specimen is used. When the total volume of a urine specimen is to be measured, the smallest graduated cylinder that will hold the entire quantity should be used. The amount of liquid preservative that has been added is not included in the total volume measurement. It should be noted that the amount of urine excreted might fall above or below the normal range without the existence of a pathological condition. However, abnormalities can cause marked deviations in total urinary output, resulting in one of the three following conditions:
Polyuria. This term refers to an abnormal increase in the total volume of urine excreted (more than 2,000 ml/24-hours). Polyuria is associated with such pathological conditions as diabetes mellitus, diabetes insipidus, certain tumors of brain and spinal cord acromegaly, myxedema, and certain kidney diseases. The nonpathologic cause is usually increased fluid intake.
Oliguria. A reduction in the total volume of urine excreted is called oliguria (less than 200 ml/24-hours). This condition is associated with febrile states, excessive vomiting, severe diarrhea, or extreme dehydration. Nonpathological causes are decreased fluid intake and excessive sweating.
Anuria. This term literally means "no urine" and refers to a complete lack of urine excretion. It results from blockage of the kidneys or urinary tract, certain bacterial infections of the kidneys, and prolonged states of dehydration. There are not any tnonpathological causes.
Urine color is another physical property that is evaluated in the routine urinalysis.
The color of normal urine is caused by the presence of various pigments, which are collectively referred to as urochrome. The various shades of yellow in urine specimens vary with the intensity of the urochrome present; the intensity of the color also varies with the specific gravity. Urine can show a typical coloration because of pathological conditions and as a result of the ingestion of certain substances, including
food pigments, dyes, drugs, and so forth. It is important that one note the exact color observed and indicate on the laboratory slip any changes that occur on standing. The physician determines the diagnostic significance of the observed color.
Yellow. Normal urine has a color of straw, yellow, or amber. Urines that are concentrated are usually amber; very dilute specimens may be almost colorless.
(1) In addition, a yellow color may be produced by the following substances:
- Cascara--a laxative.
- Phenacetin--to ease fever or pain.
- Food colors.
- Atabrine® (brand name)--an anti-malarial.
- Azulfidine® (brand name)
(2) A specimen that is a very pale yellow, greenish-yellow, or nearly colorless can be the result of several pathological conditions, specifically:
- Severe iron deficiency.
- Chronic kidney disease.
- Diabetes mellitus.
- Diabetes insipidus.
Green and Blue-Green. The blue-green color is frequently due to the mixture of the color blue with the yellow of the urine. The following can impart a green or blue-green color to the urine:
- Oral contraceptives.
- Bile pigment.
- Diagnex Blue® (brand name).
- Elavil® (brand name).
- Indican in large amounts.
- Vitamin B complex.
- Blue diaper syndrome.
- Evans blue.
- Methylene blue in kidney medication.
- Yeast concentrate.
- Pseudomonas toxemia.
- Increased serum copper concentrations.
Brown and Black. Brown-colored or black-colored urine can be produced by the following:
- Injectable iron compounds.
- Melanin pigment.
- Phenol poisoning.
- Alkapton bodies.
- Tertian malaria.
Red, Pink, or Reddish-Orange. Quite a few substances can give the urine a pink or reddish coloring. These substances include the following materials:
- Dilantin® (brand name).
- Food colors.
- Azo Gantrisin® (brand name).
- Senna in alkaline urines.
- Pyridium® (brand name).
- Povan® (brand name).
- Rhubarb in alkaline urines.
- Chromogenic bacteria.
Orange. The following list of substances can give the urine an orange color:
- Azo Gantrisin® (brand name).
- Furoxone® (brand name).
- Food colors.
- Santonin in acid urines.
| NOTE: A highly concentrated urine resulting from fever, inadequate water intake, or excessive water loss may also appear orange in color. |
1-11. GENERAL APPEARANCE OF THE URINE SAMPLE
The general appearance of a urine specimen should also be evaluated routinely.
Normally, fresh urine is clear, but the specimen can also be hazy or cloudy. Freshly voided urine should be used, since, if allowed to stand, all samples become turbid due to bacterial contamination. This uniform turbidity does not disappear upon heating or acidification.
Clear. Normal, freshly voided urine is usually clear as it has no visible particles.
Hazy. When the sample contains a small amount of particles, it is designated as hazy. Normal urine specimens may have a hazy appearance. Haziness may be due to mucus, epithelial cells, or amorphous urates or phosphates. Amorphous urates can be removed from the urine specimen by gently heating the specimen in warm water or by gently heating the prepared microscope slide. These techniques cause the crystals to redissolve.
Cloudy. Moderate to large amounts of visible particles produce a cloudy urine. Cloudiness may be caused by crystallized mineral salts that have precipitated due to long standing, or to the increase of bacteria when urine is left standing at room temperature. Cloudiness may also result from pathological conditions that produce blood or pus. The bacteria resulting from acute infections may also produce a cloudy urine.
1-12. SPECIFIC GRAVITY
A good test of total kidney function is the determination of specific gravity. Such a determination will measure the kidney's ability to concentrate urine. Specific gravity is a comparison of the density of urine to the density of distilled water, which is regarded as 1.000. Generally, the greater the volume of urine excreted, the lower the specific gravity. There is considerable variation in the specific gravity range of 1.003 to 1.030.
Pathological conditions often result in an elevated or decreased specific gravity. In pathological conditions, the range of urine specific gravity may be 1.001 to 1.060. The determination of specific gravity involves the use of the following two instruments:
Standard Urinometer. The equipment required for the determination of specific gravity includes the urinometer and glass cylinder. A new urinometer should always be checked prior to use. When calibrated using distilled water, this instrument should read 1.000 at the temperature specified by the manufacturer. If a large discrepancy is noted, the urinometer should be discarded. If the discrepancy is small, a correction factor may be used. In addition, if the temperature at which readings are taken differs from the manufacturer's specified temperature, a temperature correction of .001 should be added or subtracted for every three degrees above or below manufacturer's calibration temperature. (See figure 1-1 for an illustration of an urinometer.)
Figure 1-1. A urinometer.
Refractometer (Total Solids Meter). The refractometer is an optical instrument, which is based on the principle of light refraction. As the specific gravity of the urine increases, the degree of light refraction increases proportionally. The refraction is observed through an eyepiece, and results are obtained by noting where a shadow falls on the vertical graph. The actual measurement is the refractive index; however, the scales have been calibrated in terms of total solids (percent composition) for plasma or serum and in terms of specific gravity for urine. This instrument has several advantages: accuracy, simple operation, ability to obtain readings from a single drop of the specimen, lack of need to adjust for room and specimen temperature. However, it must be remembered that the readings of the total solids meter are specific for the two types of samples involved, plasma/serum and urine. Each scale is calibrated for one type of sample and is not a valid measurement of the other. To compensate for this situation, conversion tables are available. (See figure 1-2 for an illustration of a refractometer.)
Figure 1-2. A refractometer.
The determination of the pH of a specimen is part of a routine urinalysis. To be accurate, pH must be measured with fresh urine. Most specimens are acidic in their reaction, but fresh urine may be neutral or alkaline. The usual pH is about 6.0, with a reference range of 4.6 to 8.0. If urine specimens are allowed to stand at room temperature for long periods, they become increasingly alkaline because of the conversion of urea to ammonia by bacteria. This change in pH often causes adeterioration of many of the microscopic structures present in the urine and adversely affects a microscopic analysis. Therefore, if tests on a specimen are to be delayed, the specimen must be preserved. Changes in pH can be used to investigate the electrolyte balance of a patient as well as possible pathological conditions, such as acidosis or alkalosis.
a. Significance of Acidity. Urine with pH below 6.0 is considered to be acidic. Fresh urine is usually acidic and of little clinical significance; persistently acid urine occurs in some metabolic diseases. Formed elements usually remain well preserved if the urine specimen is acid.
b. Significance of Alkalinity. Urine with a pH above 6.5 is considered alkaline. When freshly voided urine is persistently alkaline, it may signify urinary infection, metabolic disorders, or the administration of certain drugs. There is an "alkaline tide" after meals, which is perfectly normal. In alkaline urine, the urinary sediment may be greatly modified by the dissolution of casts and lysis of red blood cells.
c. pH Determination.
(1) pH meter. For exact pH values, the pH meter should be used. However, since this instrument is rather complex, it is not used very often in urinalysis due to time limitations.
(2) pH paper. The pH of urine can be determined by the use of indicator paper such as pHydrion or nitrazine paper. The tip of the paper is dipped into the specimen or a drop may be placed on the paper. The resulting color is compared with the standard chart supplied with the paper. Nitrazine paper has a range of 4.5 to 7.5. The color varies from yellow at 4.5 to blue at 7.5.
(3) Reagent strips ("dipstix"). (See figure 1-3.) Some multiple reagent strips include a test region with the indicators methyl red and bromthymol blue. This combination of indicators gives a pH range from 5.0 to 8.5. The resulting colors range from orange to blue. Care should be taken to follow the directions supplied by the manufacturer. Excessive immersion time will wash the chemicals out of the test regions. This can affect the results of the readings on one or all of the test regions.
d. Report. The pH determination of a specimen is reported as the numerical value obtained or the relative degree of acidity or alkalinity depending upon the procedure used.
Figure 1-3. Reagent strips (dipstix).
Fresh urine from a healthy patient usually has a very slight aromatic odor, which is due to certain volatile constituents. After standing for a long time, the bacterial decomposition of urea produces a characteristic odor of ammonia. The ingestion of certain foods (for example, asparagus) produces a characteristic odor.
A slight amount of foam is formed when normal urine is shaken. This foam is white. The presence of bile pigments in the urine usually produces a yellow foam, but the presence of certain chemicals or drugs (for example, phenylazodiaminopyridine) will also produce a yellow foam. Excess urine protein (proteinuria) causes a marked increase in the foaming quality of urine.