Urine is the most common sample received by clinical laboratories, and is the specimen of choice to diagnose a variety of conditions, including urinary tract infections, bladder stones, kidney failure, prostatitis, and ovarian and prostate cancers, among others. Depending on the diagnostic test, a single random sample or a 24-hour urine sample is collected from the patient and analyzed for the presence of pus cells, bacteria, or substances such as bile and glucose.
Urinalysis
Urinalysis is the physical, chemical and microscopic examination of the urine, and is performed routinely at a doctor's office. The first step of urinalysis involves visual examination of the urine sample for characteristics such as clarity and color. The next step is the microscopic examination in which a small amount of the sample is smeared on a glass slide and observed under a microscope. The presence of pus cells, blood cells, urine crystals or bacteria indicates the possibility of an abnormality. Gram staining may also be done, especially if bacterial infection is suspected. During this procedure, a small amount of urine sample is smeared on a glass slide and stained with dyes such as crystal violet and basic fuchsin, and observed under a microscope for the presence of bacterial cells. According to "Mackie and McCartney's Manual of Practical Medical Microbiology," this test can provide a preliminary identification of any bacteria that might be present in the sample.
The third step of urinalysis is the chemical analysis that is done to look for the presence of substances such as bilirubin, calcium and glucose in the urine. A special stick, containing pads of chemicals that change color when they come in contact with the substances of interest, is dipped in the urine sample and changes in color are observed.
Culture
A small amount of a urine sample is placed on a growth medium plate which contains a gel that promotes the growth of the bacteria. As per "Mackie and McCartney's Manual of Practical Medical Microbiology," these plates are incubated at 37 degrees C for 18 hours and observed for the presence of bacterial growth. The bacteria are further identified and confirmed using biochemical tests.
24-Hour Urine Protein Test
All the urine samples are collected over a period of 24 hours and tested for the presence of proteins to diagnose conditions that affect kidney function, such as nephrotic syndrome and glomerulonephritis. MedlinePlus warns that drugs such as acetaminophen and certain antibiotics may interfere with the results of this test, so it's important to inform the doctor if a patient has used these drugs prior to the test.
Urine Pregnancy Test
A urine pregnancy test is the most common pregnancy test is performed at home or in the laboratory. This test detects the presence of the hormone known as human chorionic gonadotropin, or HCG, which appears in the urine of pregnant women in the first trimester. Although most pregnancy tests just detect the presence of HCG, a quantitative test may be prescribed for certain women to measure the levels of HCG in the urine, because abnormally rising HCG levels may indicate a miscarriage or an ectopic pregnancy.
References
- "Mackie and McCartney's Manual of Practical Medical Microbiology"; J.G. Collee, et al.; 1996
- MedlinePlus: Urinalysis
- MedlinePlus: 24-Hour Urine Protein
