Chemical Examination of Urine

Reagent Strips

Reagent strips consist of chemical-impregnated absorbent pads attached to a plastic strip. A color-producing chemical reaction takes place when the absorbent pad comes in contact with urine. The reactions are interpreted by comparing the color produced on the pad with a chart supplied by the manufacturer. Several colors or intensities of a color for each substance being tested appear on the chart. By careful comparison of the colors on the chart and the strip, a semiquantitative value of trace, 1+, 2+, 3+, or 4+ can be reported. An estimate of the milligrams per deciliter present is available for appropriate testing areas. Automated reagent strip readers also provide Systme International units.

Reagent Strip Technique

Testing methodology includes dipping the reagent strip completely, but briefly, into a well-mixed specimen, removing excess urine from the strip by running the edge of the strip on the container when withdrawing it from the specimen, waiting the specified length of time for reactions to take place, and comparing the colored reactions against the manufacturers chart using a good light source.

Handling and Storage of Reagent Strips

In addition to using correct testing technique, reagent strips must be protected from deterioration caused by moisture, volatile chemicals, heat, and light. Reagent strips are packaged in opaque containers with a desiccant to protect them from light and moisture. Strips are removed just prior to testing, and the bottle is tightly resealed immediately. Bottles should not be opened in the presence of volatile fumes. Manufacturers recommend that reagent strips be stored at room temperature below 30C. All bottles are stamped with an expiration date that represents the functional life expectancy of the chemical pads. Reagent strips must not be used past the expiration date. Care must be taken not to touch the chemical pads when removing the strips.

Quality Control of Reagent Strips

Reagent strips must be checked with both positive and negative controls a minimum of once every 24 hours. Many laboratories perform this check at the beginning of each shift. Testing is also performed when a new bottle of reagent strips is opened, questionable results are obtained, or there is concern about the integrity of the strips. All quality control results must be recorded following laboratory protocol. Several companies manufacture both positive and negative controls. Distilled water is not recommended as a negative control because reagent strip chemical reactions are designed to perform at ionic concentrations similar to urine. All readings of the negative control must be negative, and positive control readings should agree with the published value by one color block. Results that do not agree with the published values must be resolved through the testing of additional strips and controls.

Demonstration of chemically acceptable reagent strips does not entirely rule out the possibility of inaccurate results. Interfering substances in the urine, technical carelessness, and color blindness also produce errors. Reagent strip manufacturers have published information concerning the limitations of their chemical reactions, and laboratory personnel should be aware of these conditions. A primary example of reagent strip interference is the masking of color reactions by the orange pigment present in the urine of persons taking phenazopyridine compounds. If laboratory personnel do not recognize the presence of this pigment or other pigments, they will report many erroneous results.

Summary of Reagent Strip TestingCare of Reagent Strips1. Store with desiccant in an opaque, tightly closed container.2. Store below 30C; do not freeze.3. Do not expose to volatile fumes.4. Do not use past the expiration date.5. Do not use if chemical pads become discolored.6. Remove strips immediately prior to use.Technique1. Mix specimen well.2. Let refrigerated specimens warm to room temperature before testing.3. Dip the strip completely, but briefly, into specimen.4. Remove excess urine by withdrawing the strip against the rim of the container and by blotting the edge of the strip.5. Compare reaction colors with the manufacturers chart under a good light source at the specified time.6. Perform backup tests when indicated.7. Be alert for the presence of interfering substances.8. Understand the principles and significance of the test, read package inserts.9. Relate chemical findings to each other and to the physical and microscopic urinalysis results.Quality Control1. Test open bottles of reagent strips with known positive and negative controls every 24 hr.2. Resolve control results that are out of range by further testing.3.Test reagents used in backup tests with positive and negative controls.4. Perform positive and negative controls on new reagents and newly opened bottles of reagent strips.5. Record all control results and reagent lot numbers.