Urine Dipstick Testing and Possible Reaction Interference

Although laboratory facilities allow for a wide range of urine tests, some types of tablet, tape, and dipstick tests are available for UA outside the laboratory setting. They can be used and read directly by patients and clinicians.

Similar in appearance to pieces of blotter paper on a plastic strip, dipsticks actually function as miniature laboratories. Chemically impregnated reagent strips provide quick determinations of pH, protein, glucose, ketones, bilirubin, hemoglobin (blood), nitrite, leukocyte esterase, urobilinogen, and specific gravity. The dipstick is impregnated with chemicals that react with specific substances in the urine to produce color-coded visual results. The depth of color produced relates to the concentration of the substance in the urine. Color controls are provided against which the actual color produced by the urine sample can be compared.
The reaction times of the impregnated chemicals are standardized for each category of dipstick; it is vital that color changes be matched to the control chart at the correct elapsed time after each stick is dipped into the urine specimen. Instructions that accompany each type of dipstick outline the procedure. When more than one type of test is incorporated on a single stick (eg, pH, protein, and glucose), the chemical reagents for each test are separated by a water-impermeable barrier made of plastic so that results do not become altered. An example of a form used to record dipstick (UA Chemstrip Screen) testing results is shown on the following page.

Possible Reaction Interference Correlations with Other Tests Measurement False-Positive False-Negative pH None Runover from the protein pad may lower Nitrite Leukocytes Microscopic examination Protein Highly alkaline urine, ammonium compounds (antiseptics), detergents High salt concentration Blood Nitrite Leukocytes Microscopic examination Glucose Peroxide, oxidizing detergents Ascorbic acid, 5-HIAA, homogentisic acid, aspirin, levodopa, ketones, high specific gravity with low pH Ketones Ketones Levodopa, phenylketones None Glucose Blood Oxidizing agents, vegetable and bacterial peroxidases Ascorbic acid, nitrite, protein pH < 5.0, high specific gravity, captopril Protein Microscopic examination Bilirubin Lodine, pigmented urine, indican Ascorbic acid, nitrite Urobilinogen Urobilinogen Ehrlich-reactive compounds (Multistix), medication color Nitrite, formalin Bilirubin Nitrite Pigmented urine on automated readers Ascorbic acid, high specific gravity Protein Leukocytes Microscopic examination Leukocytes Oxidizing detergents Glucose, protein, high specific gravity, oxalic acid, gentamycin, tetracycline, cephalexin, cephalothin Protein Nitrite Microscopic examination Specific gravity Protein Alkaline urine None