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 urinalysis is 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 for diagnosis of many specific disorders, such as endocrine diseases, bladder cancer, osteoporosis, and phorphyrias. This often requires the use of a timed urine sample. Examples include the d-xylose absorption test for malabsorption, creatinine clearance test for glomerular function, the 24-hour urinary metanephrine test for pheochromocytoma, and the microalbumin test. The urinary microalbumin test measures the rate of albumin excretion in the urine using immunoassay. This test is used to monitor the renal vascular function of persons with diabetes mellitus. In diabetics, the excretion of greater than 200 μg/mL albumin is predictive of impending glomerular 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 2-8°C for up to 24 hours for chemical urinalysis tests; however, the microscopic exam should be performed within four hours, 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

Preservatives used to prevent loss of glucose and cells may affect biochemical test results. The use of preservatives should be avoided whenever possible.

Description

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

Physical tests

Physical tests are color, transparency (clarity), and specific gravity. In some cases, volume (daily output) may be measured. Color and transparency are determined from visual observation.

Color: Normal urine is straw 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). Bright yellow urine may be caused by bilirubin. Green urine may be caused by biliverdin or medications. Orange urine may be caused by some medication

or excessive urobilinogen. Brown urine may be caused by excessive amounts of prophobilin, or urobilin.

Transparency: Normal urine is transparent. Cloudy or turbid urine may be caused by both normal or abnormal processes. Normal conditions giving rise to turbid urine include precipitation of crystals (usually urates or phosphates), mucus, or vaginal discharge. Abnormal causes of turbidity include the presence of blood cells, yeast, and bacteria. Turbidity is typically graded by visual comparison to standard solutions of barium sulfate.

Specific gravity: The specific gravity of urine is a measure of the concentration of dissolved solutes, and it reflects the ability of the renal tubules to concentrate the urine (conserve water). It 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 failure, the specific gravity remains equal to that of the plasma (1.008-1.010) regardless of changes in salt and water intake. Urine volume below 400 mL per day is termed oliguria, and may occur in persons who are dehydrated and those with glomerular disease owing to reduced glomerular filtration. Volume in excess of 2 liters per day is termed polyuria and is common in persons with diabetes mellitus and diabetes insipidus.