The Chemical Examination
Chemical examination is usually done using 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 colour of the pad within a few seconds or minutes.
The nurse, doctor or laboratory scientist compares the colour change for each reaction pad to a colour chart provided with the test strips to give a 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 laboratory scientist may get incorrect results. To reduce timing errors and eliminate variations in colour interpretation, sometimes automated instruments may be used to “read” the reaction colour of each test pad.
The degree of colour change on a test pad can also give an approximation of the amount of substance present. For example, a slight colour change in the test pad for protein may indicate a small amount of protein present in the urine whereas a deep colour change may indicate a large amount.
The most frequently performed chemical tests using reagent test strips are
- specific gravity,
- leukocyte esterase,
- bilirubin, and
Some test strips also have a test pad for ascorbic acid (vitamin C).
The first test, specific gravity, is a measure of urine concentration and is measured using a chemical test.
There are no “abnormal” specific gravity values. This test simply indicates how concentrated the urine is. Specific gravity measurements provide a comparison of the amount of 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 have some substances dissolved, 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.
pH is not generally a very helpful test. 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 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 can precipitate out to form crystals when the urine is acidic, others can 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 pH can change and become less acidic if the urine is not tested within a few hours of collection.
The protein test pad usually measures the amount of albumin in the urine. Normally there will not be detectable quantities of the protein albumin. When urine protein is levels are high, you have 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 generally detected by the test pad but may be measured using a separate urine protein test. Other conditions that can produce proteinuria include
- An excessively high glucose concentration in the blood, such as may be seen with people who have uncontrolled diabetes.
- A reduction in the "kidney threshold". When blood glucose levels reach a certain concentration then the kidneys begin to excrete glucose into the urine. Sometimes the threshold concentration is reduced and glucose enters the urine sooner- at a lower blood glucose concentration.
Some other conditions that can cause glycosuria include hormonal disorders, liver disease, certain drugs, and pregnancy. When glycosuria occurs; other tests such as blood glucose are usaully performed to further indentify 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 metabolises 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 haemoglobin in the urine (haemoglobinuria). Haemoglobin is a oxygen-transporting protein found inside red blood cells (RBCs). Its presence in the urine indicates blood in the urine (known as haematuria). 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 haematuria or haemoglobinuria.
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 haemorrhoids 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 examination shows increased numbers of RBCs. When this happens, ascorbic acid (vitamin C) many be checked because it can cause falsely low or negative test results.
Leukocytes are 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 bladder or kidney infection.
This test detects nitrite and is based upon the fact that many bacteria can convert nitrate to nitrite in your 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 you 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 haemoglobin 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 (haemolytic anaemia). When urine urobilinogen is low or absent in a patient with urine bilirubin and/or signs of liver dysfunction it can indicate the presence of hepatic or biliary obstruction.