Urinalysis Health Article

Definition

A urinalysis is a group of manual and/or automated qualitative and semi-quantitative tests performed on a urine sample. A routine urinalysis usually includes the following tests: color, transparency, specific gravity, pH, protein, glucose, ketones, blood, bilirubin, nitrite, urobilinogen, and leukocyte esterase. Some laboratories include a microscopic examination of urinary sediment with all routine urinalysis tests. If not, it is customary to perform the microscopic exam, if transparency, glucose, protein, blood, nitrite, or leukocyte esterase is abnormal.

Purpose

Routine urinalyses are performed for several reasons:

• general health screening to detect renal and metabolic diseases
• diagnosis of diseases or disorders of the kidneys or urinary tract
• monitoring of patients with diabetes

In addition, quantitative urinalysis tests may be performed to help diagnose many specific disorders, such as endocrine diseases, bladder cancer, osteoporosis, and porphyrias (a group of disorders caused by chemical imbalance). Quantitative analysis often requires the use of a timed urine sample. The urinary microalbumin test measures the rate of albumin excretion in the urine using laboratory tests. This test is used to monitor the kidney function of persons with diabetes mellitus. In diabetics, the excretion of greater than 200 μg/mL albumin is predictive of impending kidney disease.

Voided specimens

All patients should avoid intense athletic training or heavy physical work before the test, as these activities may cause small amounts of blood to appear in the urine. Many urinary constituents are labile, and samples should be tested within one hour of collection or refrigerated. Samples may be stored at 36–46°F (2–8°C) for up to 24 hours for chemical urinalysis tests; however, the microscopic examination should be performed within four hours of collection, if possible. To minimize sample contamination, women who require a urinalysis during menstruation should insert a fresh tampon before providing a urine sample.

Over two dozen drugs are known to interfere with various chemical urinalysis tests. These include:

• ascorbic acid
• chlorpromazine
• L-dopa
• nitrofurantoin (Macrodantin, Furadantin)
• penicillin
• phenazopyridine (Pyridium)
• rifampin (Rifadin)
• tolbutamide

The preservatives that are used to prevent loss of glucose and cells may affect biochemical test results. The use of preservatives should be avoided whenever possible in urine tests.

Description

Routine urinalysis consists of three testing groups: physical characteristics, biochemical tests, and microscopic evaluation.

Physical tests

The physical tests measure the color, transparency (clarity), and specific gravity of a urine sample. In some cases, the volume (daily output) may be measured. Color and transparency are determined from visual observation of the sample.

COLOR. Normal urine is straw yellow to amber in color. Abnormal colors include bright yellow, brown, black (gray), red, and green. These pigments may result from medications, dietary sources, or diseases. For example, red urine may be caused by blood or hemoglobin, beets, medications, and some porphyrias. Black-gray urine may result from melanin (melanoma) or homogentisic acid (alkaptonuria, a result of a metabolic disorder). Bright yellow urine may be caused by bilirubin (a bile pigment). Green urine may be caused by biliverdin or certain medications. Orange urine may be caused by some medications or excessive urobilinogen (chemical relatives of urobilinogen). Brown urine may be caused by excessive amounts of prophobilin or urobilin (a chemical produced in the intestines).

TRANSPARENCY. Normal urine is transparent. Turbid (cloudy) urine may be caused by either normal or abnormal processes. Normal conditions giving rise to turbid urine include precipitation of crystals, mucus, or vaginal discharge. Abnormal causes of turbidity include the presence of blood cells, yeast, and bacteria.

SPECIFIC GRAVITY. The specific gravity of urine is a measure of the concentration of dissolved solutes (substances in a solution), and it reflects the ability of the kidneys to concentrate the urine (conserve water). Specific gravity is usually measured by determining the refractive index of a urine sample (refractometry) or by chemical analysis. Specific gravity varies with fluid and solute intake. It will be increased (above 1.035) in persons with diabetes mellitus and persons taking large amounts of medication. It will also be increased after radiologic studies of the kidney owing to the excretion of x ray contrast dye. Consistently low specific gravity (1.003 or less) is seen in persons with diabetes insipidus. In renal (kidney) failure, the specific gravity remains equal to that of blood plasma (1.008–1.010) regardless of changes in the patient's salt and water intake. Urine volume below 400 mL per day is considered oliguria (low urine production), and may occur in persons who are dehydrated and those with some kidney diseases. A volume in excess of 2 liters (slightly more than 2 quarts) per day is considered polyuria (excessive urine production); it is common in persons with diabetes mellitus and diabetes insipidus.