| Procedure Caution Although the procedure is simple to perform, accurate results depend on careful adherence to manufacturer’s directions and adequate quality control. Normal and abnormal controls should be tested whenever a new lot of strips is opened, and at the frequency defined by the laboratory's procedure. If quality control results do not correspond to the published control values, the problem must be resolved before patient samples are tested. High levels of ascorbic acid (Vitamin C) in the urine may inhibit some reagent strip reactions, such as glucose, blood, bilirubin, nitrate and leukocyte esterase. The urine dipstick's package insert will provide information about potential interfering substances, including ascorbic acid. Intensely colored urine may make it difficult to correctly interpret color reactions on the dipstick. The affected tests should not be reported from the dipstick. It would be necessary to use an alternative method of testing if available. | View Page |
| All the following statements about the urine specimen are true EXCEPT: | View Page |
| Basis of the Urine pH Test Dipsticks measure pH using methyl red and bromthymol blue indicator dyes. The color change that occurs in this test area correlates with the urine's pH. Sensitivity to pH ranges from 5.0 (acid pH) to 9.0 (alkaline pH) on a urine reagent strip. | View Page |
| Protein Error of Indicators Testing for protein is based on the phenomenon called the "Protein Error of Indicators" (ability of protein to alter the color of some acid-base indicators without altering the pH). In a solution void of protein, tetrabromphenol blue, buffered at a pH of 3, is yellow. However, in the presence of protein (albumin), the color changes to green, then blue, depending upon the concentration. This method is more sensitive to albumin than to globulin, detecting as little as 5 mg albumin/dL urine. Bence Jones protein and mucoprotein are examples of globulin components that are sometimes present in urine, but are not distinguishable by the dipstick method for protein. | View Page |
| Follow-up Testing of Urine Dipstick Protein Results A 24-hour urine protein may be ordered if a large amount of protein is detected with the dipstick method or if protein persists in the urine. A 24-hour urine protein may also be ordered if the physician suspects the release into the urine of protein other than albumin. | View Page |
| Confirmatory Testing for Protein Semiquantitative tests are used in some laboratories to confirm the presence of protein in the specimen when the result is positive on the urine dipstick. Tests that are used for confirmation include: sulfosalicylic acid (SSA); heat and acetic acid; nitric acid ring test; and Roberts' Ring Test. Any one of these procedures may be used for confirmation of the presence of protein. A protein dipstick result that is greater than a trace may be an indication of proteinuria. | View Page |
| Glucose Test The test for glucose is a double sequential enzyme reaction, utilizing the glucose-oxidase/peroxidase method. In the first reaction, glucose oxidase catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide. Then, the peroxidase catalyzes the oxidation of a chromogen by the hydrogen peroxide to form a colored product. This method does not react with lactose, fructose or galactose. Study the dipstick color chart to become familiar with the range of color changes. The urine specimen should be at room temperature for these enzyme reactions to occur properly. | View Page |
| Test for Reducing Substances Other than Glucose Urine specimens from certain pediatric patients should be tested for other reducing substances, such as galactose, when the results for glucose are negative using the routine dipstick method. The laboratory's procedure should define when additional testing is needed. | View Page |
| False Negative Results False negative bilirubin dipstick results are often due to testing a specimen that is not fresh. Bilirubin breaks down when exposed to light. Indoxyl sulfate (Indican) can produce a yellow orange-to-red color response which may interfere with the interpretation of a positive or negative reaction. Positive nitrites due to a urinary tract infection may also cause a false negative result. | View Page |
| Confirmatory Testing Confirmatory testing using an alternative method such as Ictotest reagent tablets can be performed when positive results are seen on the dipstick strip, when a red color forms on the strip, or when atypical color changes occur that are the result of bilirubin-derived bile pigments in the urine masking the bilirubin reaction. | View Page |
| False Positive Urobilinogen Results A false positive urobilinogen reaction may occur with the dipstick method when substances known to react with Ehrlich's reagent such as sulfonamides and p-aminosalicylic acid are present in the urine. Drugs that contain Azo dyes, such as Azo Gantrisin®, have a gold color that masks the reaction, causing a false positive reaction. Atypical color reactions may be obtained in the presence of high concentrations of p-aminobenzoic acid. The dipstick urobilinogen test cannot detect porphobilinogen in a urine specimen. Porphobilinogen is a molecule formed during the synthesis of the heme portion of hemoglobin. | View Page |
| False Negative Urobilinogen Result Due to the instability of urobilinogen, a false negative result may occur using a dipstick method if the urine specimen has remained at room temperature for an extended period of time in the light. A false negative result may also occur if formalin is present. | View Page |
| Leukocyte Esterase Dipstick Test If leukocyte esterase is detected, a color change occurs on the reagent pad after the strip is dipped in the urine sample. Be sure to follow the manufacturer's directions for read-time and test interpretation. A positive leukocyte esterase test indicates the presence of granulocytic white blood cells. Lymphocytes do not contain granules, and would not produce a positive leukocyte esterase test. Positive results should be confirmed by performing a microscopic examination on the sediment; being aware that white blood cells may be absent if they are lysed, yet releasing their esterases into the specimen. Positive results may occasionally be found in random specimens from females due to contamination of the specimen by vaginal discharge. | View Page |
| Advantages and Limitations of the Reagent Strip Method for Specific Gravity Specific gravity measured with the reagent strip method correlates well with gravimetric measurement, and, unlike the gravimetric or refractometer methods, does not need to be corrected for glucose or protein. Cloudy/turbid urines do not need to be clarified before measuring specific gravity with the reagent strip method. It is the recommended method for determining specific gravity if a urine specimen contains x-ray contrast media or plasma expanders. Alkaline urine can affect the indicator system and lower the specific gravity result on the reagent pad. If the result is being read visually, it is recommended that .005 be added to the result when the pH is alkaline. Most dipstick readers, however, will automatically adjust the specific gravity reading for pH. A specific gravity reading higher than the reagent strip range would need to be measured by another method, and may require dilution. | View Page |
| Semi-Automated Instruments Several manufacturers offer semi-automated instruments (dipstick readers) for reading reagent strips. Use of an instrument removes the subjectivity of visually interpreting color changes on reagent strips, and assures that tests will be read at the correct time. Transcription errors will also be avoided if the instrument is interfaced with the laboratory information system. The technology employed is based on the principle of reflectance, with the amount of light reflected being inversely related to the concentration of substances present. An example of reflectance is the light which is scattered after light strikes an unpolished surface. Since each component on the dipstick produces a different color reaction, the light source for each test must be at the appropriate wavelength. This is accomplished either by using filters or monochromatic light sources. The percent reflectance is determined by dividing the test reflectance by the calibration reflectance and multiplying by 100. Algorithms are used to change the results obtained into a linear relationship with concentration of analyte. | View Page |
| Semi-automated and automated reagent strip readers: | View Page |
| Urine reagent strips readers use any manufacturers dipsticks. | View Page |
| Quality Control Both a normal and an abnormal urine control must be tested with each new lot of reagent strips, and at least every day of patient testing to confirm the accuracy of the reagent strips and the dipstick reader. Some dipstick readers also require periodic calibration. Follow the manufacturer's instructions for calibration procedure and frequency. Quality control results must be recorded, and corrective action must be taken when the results are not in the acceptable range. | View Page |
| Causes for Bilirubinuria A screening test for bilirubin in the urine is included in most urine dipsticks and may be present when liver disease or damage is suspected. Bilirubinuria can be detected before other clinical symptoms such as jaundice are present or recognizable. The detection of small quantities is very important in early diagnosis of obstructive and hepatic jaundice. This test is also useful in the differential diagnosis of obstructive jaundice (positive for bilirubinuria) vs. hemolytic jaundice (negative for bilirubinuria). | View Page |
| Reporting Ictotest® Results Since detectable amounts of bilirubin are not normally present in urine, results of the Ictotest® are reported as “positive” or “negative”, there is no quantitation. The sensitivity of Ictotest® is better than dipstick methods or the Harrison test. Ictotest® will detect as little as 0.05-0.10 mg bilirubin/dl urine, making it the procedure of choice for confirming bilirubin in urine specimens. | View Page |
| Alternate Tests for Sugars There are two basic types of tests that are used to screen or monitor glycosuria -- copper reduction tests and enzyme tests. Most enzyme tests use the enzyme glucose oxidase impregnated on a dipstick along with a chromagen, and are specific for detecting only glucose. The copper reduction tests, however, detect any reducing substance. Clinitest® uses the classic Benedict’s copper reduction reaction. Any reducing substances present in the urine will react with the copper sulfate reagent, and the blue cupric sulfate is subsequently reduced to cuprous oxide. The resultant color change from blue through green to orange is proportional to the amount of reducing substance in the urine sample. | View Page |
| The Acetest® Urine to be screened for ketone bodies should be tested immediately or refrigerated in a closed container since acetone is lost to the air if the sample is left standing at room temperature for any length of time.
The Acetest® can be used for the semiquantitation of ketones in urine, serum, or whole blood, however the reaction times differ depending on the type of specimen tested. The same substances which interfere with the dipstick tests for ketones will also interfere with Acetest® because the same reaction is involved.
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