Table 4. A Summary of Urine Reagent Strip Test Principles, Reactions, and Interfering Substances.
| Test | Principle | Reaction | Interfering Substances/Sources of Error |
| pH | Double-indicator system of methyl red and bromothymol blue | pH 4-6: color change from red to orange or yellow pH 6-9: color change from yellow to green or blue | No known interfering substances Falsely acidic: runover from highly acidic protein reagent in protein pad Falsely alkaline: bacterial growth by urease-converting organisms |
| Specific gravity | Change in pKa of a polyelectrolyte | Appears blue (1.000, alkaline) to green to yellow (1.030, acidic) | False positive: high concentration of protein False negatives: alkaline urine (>6.5%) |
| Protein | Protein error of indicators | Appears yellow in the absence of protein and green to blue depending on the amount of protein present | False positive: alkaline urine, highly pigmented urine, high specific gravity False negative: proteins other than albumin, microalbuminuria |
| Glucose | Glucose oxidase | Produces a color change depending on amount of glucose Multistix: green to brown Chemstrip: yellow to green | False positive: oxidizing agents, detergents False negative: high concentration of ascorbic acid, high ketones, high specific gravity |
| Ketone | Sodium nitroprusside | Appears purple in the presence of ketones (mainly acetoacetic acid) | False positives: highly pigmented urine, some medications and dyes False negative: improperly stored specimens (volatilization of acetone, breakdown by bacteria over time) |
| Blood | Pseudoperoxidase activity of hemoglobin | Appears yellow if negative and blue to green if free hemoglobin and/or myoglobin are present If intact RBCs are present, hemoglobin produces a speckled pattern on the pad | False positive: strong oxidizing reagent, menstrual contamination False negative: high concentration of ascorbic acid or nitrite, high specific gravity |
| Nitrite | Greiss reaction | Appears in varying shades of pink (shade of pink does correlate with degree of bacteriuria) | False positive: improperly preserved specimen (growth of bacteria over time), highly pigmented urine False negative: high concentration of ascorbic acid, high specific gravity, presence of bacteria and yeast that cannot reduce nitrate, lack of urine nitrate, treatment with antibiotics, urine that did not sit in the bladder for at least four hours (insufficient time for bacteria to reduce nitrate), nitrite is reduced to nitrogen |
| Leukocyte esterase | Leukocyte esterase (catalyzation of an acid ester) | Appears in varying shades of purple depending on the amount of esterase present | False positive: strong oxidizing agents, highly pigmented urine False negative: high concentration of ascorbic acid, glucose, oxalate, protein, some medications |
| Bilirubin | Diazo reaction | Appears in varying shades of tan or pink to purple depending on the amount of bilirubin | False positive: highly pigmented urine, phenazopyridine False negative: high concentration of ascorbic acid or nitrite, specimen exposure to light |
| Urobilinogen | Multistix: Ehrlich reaction Chemstrip: Diazo reaction | Multistix: appears light pink (normal) to darker shades of pink (increased) depending on the amount of urobilinogen Chemstrip: appears white to pink depending on amount of urobilinogen | False positive: highly pigmented urine Multistix: various Ehrlich-reactive compounds (e.g., certain medications) False negative: improperly preserved specimens (photooxidation of urobilinogen), presence of formalin Chemstrip: high levels of nitrite |
