Urine Test Overview
Urinalysis, also called a routine urine test, is used to check for abnormalities in the urine. Urine is liquid waste, which is made by the kidneys and stored in the bladder until it is eliminated by the body through urination.
A number of diseases and conditions can result in abnormalities in the urine. These abnormalities often may be detected through physical, chemical, and microscopic examination.
Urinalysis may be performed as part of a regular physical examination to screen for early signs of disease. In patients who have kidney disease or diabetes, urine tests are used to monitor the condition.
Urinalysis also may be performed to detect a urinary tract infection (UTI), protein in the urine (proteinuria), or blood in the urine (hematuria), and to diagnose pregnancy. Toxicology screening can be used as part of urinalysis to detect illegal drug use. This is not part of routine urine testing.
History of Urinalysis
Urinalysis was the very first laboratory test developed—the value of urine testing to diagnose medical conditions was established over 6000 years ago. There is evidence of visual testing of the urine in the earliest civilizations.
As advances in medicine proved visual testing to be unreliable, chemical evaluation of the urine replaced this method as a more accurate means of diagnosis.
Further understanding of the structure of the body (anatomy) and of the physical and chemical processes involved in organ function (physiology), as well as the invention and development of the microscope, led to additional advances in urine testing as a diagnostic tool.
Urinalysis was first introduced as part of the routine physical examination in 1827, and this test remains a valuable method of diagnosis. The ability to test large numbers of urine samples (through automation) and recent advances in chemical and microscopic techniques (e.g., monoclonal antibody testing [used to identify infectious organisms]) have improved the effectiveness of urine testing.
Urinalysis Procedure and Results
Urinalysis may be performed using a "clean catch" sample of urine. To achieve this, the area surrounding the opening of the urethra is cleansed and rinsed well. A small amount of urine should then be allowed to fall into the toilet and the rest should be collected midstream in a clean container.
Urinalysis should be performed within 15 minutes of collecting the urine sample. If that is not possible, the urine should be refrigerated.
Some parts of routine urinalysis are performed using a "dipstick" test or tablet test, and others are completed using microscopic (under a microscope) evaluation in a laboratory.
Dipstick tests involve placing papers that contain small pads of chemicals into the urine sample. These papers change color when exposed to various substances in the urine. This type of urine testing can result in false-positive or false-negative results. Tablet tests use the same principle—the tablets are moistened with urine and react in response to various substances.
Urinalysis involves checking the appearance and color of the urine (called gross examination). Urine can appear cloudy (turbid), clear, or layered. It can vary in color from pale (almost colorless) yellow to dark yellow, and even red, green, or blue. Infection may cause the urine to be abnormal in color and cloudy.
Urine color usually is determined by concentration, but it can be affected by certain foods (e.g., beets, asparagus, blackberries), ingested dyes (e.g., food coloring), and a number of medications. The presence of bilirubin, which may indicate liver disease, can make the urine dark in color (even olive green), and hemoglobin, which may indicate injury to the urinary tract, can make the urine pink or red.
Green or blue urine may result from dyes (e.g., in medications) or the presence of riboflavin (B complex vitamin). Orange urine may be caused by medications (e.g., pyridium [used to treat urinary tract infections], warfarin, laxatives), B complex vitamins, and carotene (found in yellow vegetables).
Urinalysis also involves performing chemical tests, including checking the pH level (i.e., acidity/alkalinity) and the specific gravity (i.e., concentration) of the urine.
The pH level of normal urine ranges from 4.5 to 8.0. The average level is slightly acidic (6.0). Highly acidic or alkaline urine may indicate a number of metabolic diseases. Alkaline urine also may indicate infection.
The concentration of the urine provides information about the hydration level of the patient. An inability to affect the concentration of the urine (e.g., by compulsive water drinking) may indicate potassium deficiency, high levels of calcium in the blood (hypercalcemia), acute renal failure, or renal tubular defects.
The urine is also examined for the presence of the following, which are not normally found in urine and may indicate disease or another condition:
- Bilirubin (pigment in bile; may indicate liver disease)
- Glucose (type of sugar; may indicate diabetes)
- Hemoglobin (iron-containing pigment in red blood cells; may indicate injury of the urinary tract, anemia, or infection)
- Ketones (e.g., acetone; may indicate diabetes or metabolic disease)
- Protein (may indicate kidney disease)
- Urobilinogen (by-product of bilirubin; may be found in trace amounts in normal urine)
Microscopic tests include checking for bacteria and other microorganisms, as well as the following:
- Blood cells (e.g., red blood cells, white blood cells; indicate hematuria or infection)
- Casts (e.g., hyaline, granular; may indicate kidney disease)
- Cells from the lining of the urinary tract (epithelial cells)
- Crystals (may indicate metabolic disease)
- Fat (may indicate nephrotic syndrome or diabetic nephropathy)
- Renal tubular cells (may indicate acute tubular necrosis)