Chemical examination of urine
Tests can be performed on urine to detect the presence of certain chemicals, which can provide valuable information to the provider. In certain situations, these chemical test results can be critical to a diagnosis. Also, some urine dipstick results will turn positive before a patient has specific symptoms of a disease or disorder. That is one reason a routine urinalysis can be a screening test for many conditions.
Reagent strip testing is the most widely used technique for detecting chemicals in the urine. These strip tests are available in a variety of types. Dipsticks are plastic strips with paper pads attached. The paper pads contain chemical reagents. The reagents react chemically with analytes in the urine. The chemical reaction is read by looking for a color change on each individual pad. The presence or absence of these chemicals in the urine provides information on the status of carbohydrate metabolism, liver and kidney function, and the patient’s acid-base balance.
Reagent strips are designed to be used once and then discarded in a biohazard waste container. The directions for each type of reagent strip test are included inside the package. Instructions must be followed exactly if accurate results are to be obtained. A color comparison chart is provided on the label of the container. In addition to reagent strips, a few tablet tests are available for chemically testing urine.
All strips and tablets must be kept in tightly closed containers in a cool, dry area and should be removed just before testing. To prevent contamination of the bottle, never touch a strip that has been exposed to urine against the color comparison chart on the bottle. If both a UA and C&S have been ordered for a specimen, separate the sample. The original sample in the sterile container will go to the microbiology department and be used for the C&S. Pour off an aliquot into a non sterile urine tube for the UA. If you were to introduce a reagent strip into the sterile urine sample, it would contaminate the specimen and make it unfit for C&S testing.
pH
The pH is a measurement of the acidity or alkalinity of the urine. A urine specimen with a pH of 7 is neutral. A value below 7 indicates acidity and above 7 indicates alkalinity. Normal, freshly voided urine may have a pH range of 5.5 to 8. The urinary pH varies with an individual’s metabolic status, diet, drug therapy, and disease. In the case of bacteria in the urine, the urine pH is usually alkaline. Knowing the pH of the sample also helps when identifying crystals that may be found in the urine sediment.
Glucose
Glucose is filtered out of the blood in the glomerulus, and, under normal conditions, most glucose is reabsorbed in the tubules of the nephron. Detectable glycosuria (glucose in the urine) occurs when the filtered glucose in the renal tubules is so high it cannot be reabsorbed into the blood. A positive urine glucose may be the first indication that a patient is diabetic. The glucose pad on a reagent strip test is based on a specific enzyme reaction in which glucose, and no other sugar, causes a positive reaction. Because many types of bacteria use glucose for energy, specimens should be tested for glucose within an hour of collection or within 24 hours if refrigerated. This will reduce the amount of bacteria from overgrowing in the sample.
Glucose reabsorption
The reabsorption of glucose depends on its renal threshold . The renal threshold for blood glucose is below 160 mg/dL. This means that all the glucose will be reabsorbed into the blood if the blood glucose level is below 160 mg/dL. If the blood glucose level is higher than 160 mg/dL, the blood will not reabsorb the glucose. The filtered glucose then remains in the urine, which will show up as a positive test result for glucose during urinalysis.
Ketones
Ketones are the end product of fat metabolism in the body. Ketonuria (ketones in the urine) is commonly seen in patients with diabetes mellitus that is not well controlled, in persons on a very-low-carbohydrate diet (in which case the presence of ketones would be an expected finding and not necessarily considered abnormal.), or after excessive vomiting. Because ketones evaporate at room temperature, urine should be tested immediately, or the specimen should be tightly covered and refrigerated. Examples of ketones include acetoacetate and acetone.
Protein
Protein in the urine in detectable amounts is called proteinuria and is one of the first signs of renal disease. We normally excrete a very small amount of protein every day that is undetectable.
Some causes of proteinuria may include the following:
- Orthostatic proteinuria: protein is excreted only when the patient is in an upright position.
- Pregnancy: a common finding in pregnancy that must be monitored along with excessive weight gain and increased blood pressure (there are three possible symptoms of preeclampsia)
- After heavy exercise
The reagent strip is highly sensitive to urinary albumin and is less sensitive to the other proteins.