The Chemical Examination
To perform the chemical examination, most clinical laboratories use commercially prepared test strips. These are narrow plastic strips that hold small squares of paper called test pads, arranged in a row. The test pads have chemicals impregnated into them. When a strip is briefly, but completely, dipped into urine, the test pads absorb the urine and a chemical reaction changes the color of the pad within seconds to minutes.
The laboratorian compares the color change for each reaction pad to a color chart provided with the test strips to determine the result for each test. Each reaction pad must be evaluated at the appropriate time for that chemical. If too little time or too much time has passed since the reaction, the laboratorian may get incorrect results. To reduce timing errors and eliminate variations in color interpretation, automated instruments are frequently used to “read” the reaction color on each test pad.
The degree of color change on a test pad can also give an approximation of the amount of substance present. For example, a slight color change in the test pad for protein may indicate a small amount of protein present in the urine whereas a deep color change may indicate a large amount.
The most frequently performed chemical tests using reagent test strips are:
Some reagent test strips also have a test pad for ascorbic acid [vitamin C].
There are no "abnormal" specific gravity values. This test simply indicates how concentrated the urine is. Specific gravity measurements are actually a comparison of the amount of solutes (substances dissolved) in urine as compared to pure water. If there were no solutes present, the SG of urine would be 1.000 – the same as pure water. Since all urine has some solutes a urine SG of 1.000 is not possible. If a person drinks excessive quantities of water in a short period of time or gets an intravenous (IV) infusion of large volumes of fluid, then the urine SG may be as low as 1.002. The upper limit of the test pad, an SG of 1.035, indicates a concentrated urine, one with many solutes in a limited amount of water.
Knowing the urine concentration helps health care providers decide if the urine specimen they are evaluating is the best one to detect a particular substance. For example, if they are looking for very small amounts of protein, a concentrated morning urine specimen would be the best sample.
As with specific gravity, there are typical but not "abnormal" pH values. The kidneys play an important role in maintaining the acid-base balance of the body. Therefore, any condition that produces acids or bases in the body such as acidosis or alkalosis, or the ingestion of acidic or basic foods, can directly affect urine pH.
Diet can be used to modify urine pH. A high-protein diet or consuming cranberries will make the urine more acidic. A vegetarian diet, a low-carbohydrate diet, or the ingestion of citrus fruits will tend to make the urine more alkaline.
Some of the substances dissolved in urine will precipitate out to form crystals when the urine is acidic; others will form crystals when the urine is basic. If crystals form while the urine is being produced in the kidneys, a kidney stone or “calculus” can develop. By modifying urine pH through diet or medications, the formation of these crystals can be reduced or eliminated.
The protein test pad measures the amount of albumin in the urine. Normally, there will not be detectable quantities. When urine protein is elevated, a person has a condition called proteinuria; this can be an early sign of kidney disease. Albumin is smaller than most other proteins and is typically the first protein that is seen in the urine when kidney dysfunction begins to develop. Other proteins are not detected by the test pad but may be measured with a separate urine protein test. Other conditions that can produce proteinuria include:
- Disorders that produce high amounts of proteins in the blood, such as multiple myeloma
- Conditions that destroy red blood cells
- Inflammation, malignancies (cancer), or injury of the urinary tract - for example, the bladder, prostate, or urethra
- Vaginal secretions that get into urine
- An excessively high glucose concentration in the blood, such as may be seen with people who have uncontrolled diabetes mellitus
- A reduction in the "renal threshold." When blood glucose levels reach a certain concentration, the kidneys begin to excrete glucose into the urine to decrease blood concentrations. Sometimes the threshold concentration is reduced and glucose enters the urine sooner, at a lower blood glucose concentration.
Some other conditions that can cause glucosuria include hormonal disorders, liver disease, medications, and pregnancy. When glucosuria occurs, other tests such as blood glucose are usually performed to further identify the specific cause.
Ketones are not normally found in the urine. They are intermediate products of fat metabolism. They can form when a person does not eat enough carbohydrates (for example, in cases of starvation or high-protein diets) or when a person's body cannot use carbohydrates properly. When carbohydrates are not available, the body metabolizes fat instead to get the energy it needs to keep functioning.
Ketones in urine can give an early indication of insufficient insulin in a person who has diabetes. Severe exercise, exposure to cold, and loss of carbohydrates, such as with frequent vomiting, can also increase fat metabolism, resulting in ketonuria.
This test is used to detect hemoglobin in the urine (hemoglobinuria). Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). Its presence in the urine indicates blood in the urine (known as hematuria). The small number of RBCs normally present in urine (see microscopic examination) usually result in a "negative" test. However, when the number of RBCs increases, they are detected as a "positive" test result.
Even small increases in the amount of RBCs in urine can be significant. Numerous diseases of the kidney and urinary tract, as well as trauma, medications, smoking, or strenuous exercise can cause hematuria or hemoglobinuria.
This test cannot determine the severity of disease nor be used to identify where the blood is coming from. For instance, contamination of urine with blood from hemorrhoids or vaginal bleeding cannot be distinguished from a bleed in the urinary tract. This is why it is important to collect a urine specimen correctly and for women to tell their health care provider that they are menstruating when asked to collect a urine specimen.
Sometimes a chemical test for blood in the urine is negative, but the Microscopic Exam shows increased numbers of RBCs. When this happens, the laboratorian may test the sample for ascorbic acid (vitamin C), because vitamin C has been known to interfere with the accuracy of urine blood test results, causing them to be falsely low or falsely negative.
Leukocyte esterase is an enzyme present in most white blood cells (WBCs). Normally, a few white blood cells (see microscopic examination) are present in urine and this test is negative. When the number of WBCs in urine increases significantly, this screening test will become positive.
When the WBC count in urine is high, it means that there is inflammation in the urinary tract or kidneys. The most common cause for WBCs in urine (leukocyturia) is a bacterial urinary tract infection (UTI), such as a bladder or kidney infection.
This test detects nitrite and is based upon the fact that many bacteria can convert nitrate to nitrite in the urine. Normally the urinary tract and urine are free of bacteria. When bacteria find their way into the urinary tract, they can cause a urinary tract infection (UTI). A positive nitrite test result can indicate a UTI. However, since not all bacteria are capable of converting nitrate to nitrite, someone can still have a UTI despite a negative nitrite test.
Bilirubin is not present in the urine of normal, healthy individuals. Bilirubin is a waste product that is produced by the liver from the hemoglobin of RBCs that are removed from circulation. It becomes a component of bile, a fluid that is secreted into the intestines to aid in food digestion.
In certain liver diseases, such as biliary obstruction or hepatitis, bilirubin leaks back into the blood stream and is excreted in urine. The presence of bilirubin in urine is an early indicator of liver disease and can occur before clinical symptoms such as jaundice develop.
Urobilinogen is normally present in urine in low concentrations. It is formed in the intestine from bilirubin, and a portion of it is absorbed back into the bloodstream. Positive test results help detect liver diseases such as hepatitis and cirrhosis and conditions associated with increased RBC destruction (hemolytic anemia). When urine urobilinogen is low or absent in a person with urine bilirubin and/or signs of liver dysfunction, it can indicate the presence of hepatic or biliary obstruction.