The Chemical Examination
To perform the chemical examination, most clinical laboratories use commercially prepared test strips with test pads that have chemicals impregnated into them. The laboratorian dips the strip into urine, chemical reactions change the colors of the pads within seconds to minutes, and the laboratorian determines the result for each test. To reduce timing errors and eliminate variations in color interpretation, automated instruments are frequently used to "read" the results of the test strip.
The degree of color change on a test pad can give an estimate 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 chemical examination is often done in conjunction with or may be followed by a microscopic examination of the urine if there are any abnormal results. Results from both sets of tests are then considered together for interpretation. Abnormal findings may be followed by additional urine and/or blood tests.
The most frequently performed chemical tests using reagent test strips are:
- Specific Gravity (SG)
- Blood (hemoglobin) and Myoglobin
- Leukocyte Esterase
Specific Gravity (SG)
Specific gravity is a measure of urine concentration. This test simply indicates how concentrated the urine is. Specific gravity measurements are a comparison of the amount of substances dissolved in urine as compared to pure water. If there were no substances present, the specific gravity of the urine would be 1.000 (the same as pure water). Since all urine has some substances in it, 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 specific gravity may be very close to that of water. The upper limit of the test pad, a specific gravity of 1.035, indicates concentrated urine, one with many substances in a limited amount of water.
Knowing the urine concentration helps healthcare practitioners understand whether a 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 urine is usually slightly acidic, about pH 6, but can range from 4.5-8. 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.
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. (For additional information, see the articles on Kidney Stone Analysis and Kidney Stone Risk Panel.)
The protein test pad provides a rough estimate of the amount of albumin in the urine. Albumin makes up about 60% of the total protein in the blood. Normally, there will be no protein or a small amount of protein in the urine. When urine protein is elevated, a person has a condition called proteinuria.
Proteinuria may occasionally be seen in healthy individuals. Healthy people can have temporary or persistent proteinuria due to stress, exercise, fever, aspirin therapy, or exposure to cold, for example. Repeat testing may be done once these conditions have resolved to determine whether the proteinuria is persistent.
If trace amounts of protein are detected, and depending on the person's signs, symptoms and medical history, a repeat urinalysis and dipstick protein may be performed at a later time to see if there is still protein in the urine or if it has dropped back to undetectable levels.
If a large amount of protein is detected on a urinalysis and/or if the protein persists in repeated tests, a 24-hour urine protein test may be used as a follow-up test. Since the dipstick primarily measures albumin, the 24-hour urine protein test also may be ordered if a healthcare practitioner suspects that proteins other than albumin are being released into the urine.
Protein in the urine may be a sign of kidney disease. Small amounts of albumin may be found in the urine when kidney dysfunction begins to develop. A different test called a urine albumin test detects and measures small amounts of albumin in the urine. The urine albumin test is more sensitive than a dipstick urinalysis and is routinely used to screen people with chronic conditions that put them at risk for kidney disease, such as diabetes and high blood pressure. (See the article on Urine Albumin.)
Proteinuria may also be associated with many other diseases and conditions. A healthcare practitioner may order other types of follow-up tests to help determine the cause of protein in the urine. For more information, see the articles on Proteinuria and Kidney Disease.
- An excessively high glucose level in the blood, such as may be seen with people who have uncontrolled diabetes
- A reduction in the "renal threshold;" when blood glucose levels reach a certain concentration, the kidneys begin to eliminate 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 a fasting 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 are produced when glucose is not available to the body's cells as an energy source. They can form when a person does not eat enough carbohydrates (for example, in cases of fasting, 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. Strenuous exercise, exposure to cold, frequent, prolonged vomiting, and several digestive system diseases can also increase fat metabolism, resulting in ketonuria.
In a person who has diabetes, ketones in urine may also be an early indication of insufficient insulin. With insufficient insulin, a diabetic cannot process glucose and instead metabolizes fat. This can cause ketones to build up in the blood, resulting first in ketosis and then progressing to ketoacidosis, a form of metabolic acidosis. Excess ketones and glucose are dumped into the urine by the kidneys in an effort to flush them from the body. This condition, called diabetic ketoacidosis (DKA), is most frequently seen with uncontrolled type 1 diabetes and can be a medical emergency. (See the articles on Ketones and Diabetes.)
Blood (Hemoglobin) and Myoglobin
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).
A small number of RBCs are normally present in urine and usually result in a "negative" chemical test. An increased amount of hemoglobin and/or increased number of RBCs are detected as a "positive" chemical test result. Results of this test are typically interpreted along with those from the microscopic examination of the urine to determine whether RBCs are present in the urine. A positive result on this test with no RBCs present may indicate the presence of hemoglobin in the urine (which can occur when RBCs have broken apart) or myoglobin from muscle injury. (See the article on Myoglobin.)
Blood in the urine is not a normal finding, but it is not uncommon and not necessarily a cause for alarm. Your healthcare practitioner will investigate further to try to determine the source and underlying cause of the blood and may request repeat testing to determine whether the blood is persistent.
Leukocyte esterase is an enzyme present in most white blood cells (WBCs). A few white blood cells are normally present in urine and usually give a negative chemical test result. When the number of WBCs in urine increases significantly, this screening test will become positive. Results of this test will be considered along with a microscopic examination for WBCs in the urine.
When this test is positive and/or the WBC count in urine is high, it may indicate 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. In addition to WBCs, bacteria and RBCs may also be seen in the microscopic examination. If bacteria are present, the chemical test for nitrite may also be positive (see below).
This test detects nitrite and is based upon the fact that many bacteria can convert nitrate (a normal substance in urine) to nitrite. Normally, the urinary tract and urine are free of bacteria and nitrite. When bacteria enter the urinary tract, they can cause a urinary tract infection. 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. The results of this test will be considered along with the leukocyte esterase (above) and a microscopic examination.
This test screens for bilirubin in the urine. Bilirubin is not present in the urine of normal, healthy individuals. It is a waste product that is produced by the liver from the hemoglobin of RBCs that are broken down and removed from circulation. It becomes a component of bile, a fluid that is released into the intestines to aid in food digestion.
In certain liver diseases, such as biliary obstruction or hepatitis, excess bilirubin can build up in the blood and is eliminated 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.
The results of this test will be considered along with the result of urobilinogen (below). If positive, the healthcare practitioner will likely follow up with other laboratory tests, such as a liver panel, to help establish a diagnosis.
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 blood. Positive test results may indicate liver diseases such as viral hepatitis, cirrhosis, liver damage due to drugs or toxic substances, or 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.
Ascorbic Acid (Vitamin C)
Occasionally, people taking vitamin C or multivitamins may have large amounts of ascorbic acid in their urine. When this is suspected to be the case, a laboratorian may test the sample for ascorbic acid (vitamin C) because it has been known to interfere with the accuracy of some of the results of the chemical test strip, causing them to be falsely low or falsely negative. Examples of tests that may be affected include the urine dipstick tests for glucose, blood, bilirubin, nitrite, and leukocyte esterase.