Examination of the urine is probably the oldest type of human diagnostic testing, dating back at least 6,000 years.
It is important to follow the guidelines for urine specimen collection; inaccurate test results may result from improperly collected urine.
The urine dipstick is a screening test to help detect a wide range of diseases. Color changes of the small “pads” on a plastic strip identify abnormalities.
When a urinary tract infection is suspected, a culture can confirm a bacterial cause, identify the type of bacteria, and indicate the antibiotics likely to be effective.
When kidney disease is suspected, 24-hour urine collections can indicate abnormalities of kidney function.
Today, urine tests are among the most valuable types of laboratory studies in general health care. While the types of urine laboratory tests have greatly expanded, the availability of basic urine diagnostic testing is almost universal and easily performed or obtained through medical clinics and offices.
Types of urine collection for testing
Two basic methods are used to collect urine for testing: a sample from a single void (urination) and the collection of all urine made during a specified time interval. Recognize, however, that errors in urine sample collection are common and can significantly affect the results.
Urine from a single void. This is most often used to perform a routine urinalysis or to test for infection. It is easy to obtain a proper sample:
- Begin by gently cleansing the area around the urethra with a nonirritating towelette.
- Do not open the specimen container until ready to use and avoid touching or contaminating the container’s interior.
- Before collecting the specimen, begin by urinating into the toilet. Then, 1 to 2 ounces should be saved “mid-stream.”
These steps are particularly important if the urine is to be tested for infection. When urine is collected at home, it is best to keep the container refrigerated or cooled until it is brought to the laboratory.
Timed collections of urine. Measurement of numerous substances in the urine often requires complete collections, generally over a period of 24 hours. This is because the excretion of tested substances often varies throughout the day and night. As a result, “normal” values are often based on the amounts in 24-hour specimens. The steps for proper collection are:
- Obtain the large collection container from your medical office or laboratory.
- Maintain a good fluid intake before and during the collection (an average of 2 to 2 ½ quarts per day).
- Begin the collection in the morning, noting the time, by urinating completely into the toilet. After that, urinate into the container until the next morning when you conclude the collection by adding the urine to the container as close to the start time as possible.
- Keep the container cool during the testing period.
- It is advisable to avoid heavy exercise just before and during the collection.
Common urine tests and Urinalysis (UA)
Your urine contains a multitude of substances that not only are measured to estimate overall kidney function but also help to identify numerous other organ diseases.
By far, the most common test of the urine is the urinalysis (UA). The UA consists of both chemical tests using a “dipstick” (Figure 1) and, in many cases, a microscopic examination of the urine. The dipstick is a plastic strip with small square pads of different colors, each of which test for certain substances.
Dipsticks containing different combinations of pads also may be purchased for home use. After the strip is briefly dipped into the urine, some of the pads may change color. A chart on the bottle shows how to interpret color changes as well as how long to wait before reading each pad. Those pads that are frequently of interest include:
- pH levels (level of acidity of the urine) that are excessively high or low may enable kidney stone formation. Certain kidney diseases, including chronic kidney injury and renal tubular acidosis – accumulation of acid in the body due to kidney failure – also affects normal pH regulation.
- Specific gravity reflects the degree of urine concentration (lower water percentage). It may be used as a rough estimate of hydration, as high specific gravity values may be associated with dehydration. It also may indicate the inability of the kidneys to appropriately concentrate or dilute the urine.
- The blood (or “heme”) test pad detects red blood cells in the urine, even if it is not discolored. A positive reaction does not, alone, indicate the source of the blood. It may also result from the other substances that cause a positive reaction. These include hemoglobin released from red cell destruction in the circulatory system and myoglobin released from injured muscle. Acute or chronic kidney disease, kidney stones, trauma, and infection may result in blood in the urine. Microscopic examination can help to distinguish kidney disease from bleeding into the lower urinary system or suggest the presence of similarly reactive substances of importance.
- Color changes for protein in the urine are common but rarely exceed a “trace” level (less than “1+”). Modest elevations of protein (proteinuria) may be benign, seen with fever, or occur from vigorous exercise. Positive tests should be confirmed by repeating the test or by other methods. Persistently elevated urine protein levels may indicate serious underlying kidney diseases or be associated with diabetes, obesity, and congestive heart failure among others. Such a finding should be further evaluated by a healthcare professional.
- Leukocyte esterase and nitrites indicate bacterial infections of the urine (UTI). The leukocyte esterase enzyme is released by neutrophils, a type of white blood cell frequently associated with a UTI. Nitrites in the urine are produced by bacteria such as E. coli. A urine culture reveals the type of infection and the optimal antibiotic treatment.
- Glucose in the urine (glycosuria) is not normally found. It is often present in diabetics, especially when blood sugars are poorly controlled. A positive reaction also can indicate undiagnosed diabetes. Some underlying kidney diseases as well as certain drugs also may cause a positive reaction.
- Ketones in the urine indicate excessive breakdown of fats and proteins. Ketogenic diets (low carbohydrate, high fat and protein) may increase urine ketones as can poorly controlled diabetes, starvation, and excessive alcohol among others.
- Bilirubin is not normally detected by a dipstick. Color changes may indicate underlying liver disease, obstruction of the bile duct, or diseases of the pancreas. Urobilinogen is formed from the metabolism of bilirubin and is often found in small amounts in the urine. Elevated levels may indicate liver disease, some forms of anemia and serious infections. Pairing bilirubin negative or positive findings, and urobilinogen negative or positive findings can indicate the source of the problem.
Symptoms of a bladder or kidney infection or abnormal urinalysis findings usually result in a culture of the urine. In the laboratory, a small amount of urine is added to special plates that support bacterial growth. After a period of observation (usually 1-3 days), bacterial growth is noted, after which the type is identified and tested for sensitivity to a panel of antibiotics.
Timed urine collections
Numerous types of analyses can be performed on timed specimens, often in response to an abnormal urinalysis or features of other diseases. Two of the most common tests measure kidney function and the amount of protein.
- Normal or abnormal kidney function is determined by the glomerular filtration rate (GFR). The amounts of certain substances in both the blood and the urine are measured and adjusted for the collection time. The results determine the presence or absence of abnormal kidney function. If abnormal, the degree of kidney dysfunction also can be estimated.
- Although the presence of protein in the urine can be detected by dipstick, it is frequently useful to measure the quantity in a timed specimen. Most of the protein lost in the urine is albumin and its quantity can suggest the type of underlying kidney disease. In some settings, other measured substances can indicate a number of hereditary or metabolic diseases.