Urine dipstick (reagent strip) analysis

Principle

• Each pad contains reagents that produce a color change proportional to the analyte, read visually against a chart or by a reflectance meter.
• pH: double indicator (methyl red and bromothymol blue), range about 5–9.
• Specific gravity: pKa change of a polyelectrolyte in relation to ionic concentration (does not detect glucose or contrast media).
• Protein: protein error of indicators (tetrabromophenol blue); most sensitive to albumin.
• Glucose: glucose oxidase–peroxidase, specific for glucose.
• Ketones: sodium nitroprusside; detects acetoacetate (and acetone with glycine), not β-hydroxybutyrate.
• Blood: pseudoperoxidase activity of hemoglobin and myoglobin.
• Bilirubin: diazo reaction; urobilinogen: Ehrlich or diazonium reaction.
• Nitrite: Griess reaction (bacterial conversion of nitrate to nitrite); leukocyte esterase: esterase from granulocytes cleaves an indoxyl ester.

Specimen

• Fresh, well-mixed, uncentrifuged urine at room temperature, tested within 2 hours; first morning sample preferred for nitrite and protein.

Procedure

• Dip briefly so all pads are wetted, draw the edge along the container rim and blot on absorbent paper.
• Hold horizontally and read each pad at the manufacturer's stated time (leukocyte esterase about 2 minutes), or use an automated reader.

Quality control and pitfalls

• Test positive and negative controls each day and with each new lot; store strips capped with desiccant, away from light and moisture; do not touch pads.
• Ascorbic acid can falsely lower glucose, blood, bilirubin and nitrite.
• Highly alkaline urine or quaternary ammonium cleaners falsely raise protein; Bence Jones proteins are poorly detected.
• Oxidizing cleaners (bleach) give false positive blood and glucose.
• Nitrite is falsely negative with non-reducing organisms (e.g. Enterococcus), short bladder time or dilute urine.
• Leukocyte esterase is falsely low with high glucose, protein or specific gravity; vaginal contamination causes false positives.
• Old urine: pH rises, glucose falls, bilirubin and urobilinogen degrade with light.

Clinical use

• Screening for diabetes, kidney disease, UTI, liver disease, hemolysis and ketosis; abnormal results guide microscopy and confirmatory tests.