Urine Dipsticks
Urine dipsticks:
Urine dipstick tests are routine tests in the urinalysis department. The dipstick is a quick test to get a general idea of any problems in the urine. The urine dipstick tests for: glucose, pH, protein, ketones, blood, bilirubin, urobilinogen, nitrite, leukocyte esterase, and specific gravity. Here we’ll breakdown the general dipstick reactions and any false positive or false negative reactions that can occur. The urinalysis discrepancies are really important!
Glucose:
Glucose + 2H2O + O2 --> gluconic acid + 2H2O2 then
H2O2 + chromogen catalyzed by peroxidase --> color change
The amount of glucose present will correlate with the color change.
False positives:
bleach
False negatives:
ascorbic acid (vitamin C)
pH:
H+ interacts with methyl red at low pH --> color change
H+ interacts with bromothymol blue at high pH à color change
The dual indicator system covers the spectrum of pH.
Protein:
The ‘Protein Error of Indicators Method’ is used. Protein will bind to the dye shifting the pH and color.
False positives:
Highly alkaline urine
False negatives:
Proteins other than albumin such as Bence Jones protein may not bind as well causing less of a color change.
Ketones:
Ketones (acetoacetic acid) + nitroprusside à color change
Increased ketones can be indicative of diabetes mellitus because of increased use of fat and fat metabolism (diabetic ketoacidosis).
Blood:
Blood + peroxide = O2, O2 + indicator à color change
Blood in the urine can be due to hemolysis and nephritis amongst other conditions.
Bilirubin:
Bilirubin + diazonium salt à azobilirubin (causes color change)
Urine bilirubin will be increased in hepatic and post-hepatic liver conditions. This test only detects conjugated bilirubin, unconjugated bilirubin does not react with the diazonium salt. The chemistry section gives a complete review of bilirubin.
The Ictotest is a more sensitive tablet version of the dipstick assay and can be used for confirmation.
False negatives:
Ascorbic acid (vitamin C), light
Urobilinogen:
Urobilinogen is measured using ‘Ehrlich’s reagent’ which is:
Urobilinogen + dimethylaminobenzaldehyde à pink color change
False negatives can be caused by nitrite and formalin.
Urine urobilinogen is increased in pre-hepatic and hepatic liver conditions. The Watson-Schwartz test can be used to differentiate urobilinogen from porphobilinogen. The chemistry section gives a complete review of urobilinogen.
False negatives:
Nitrite, formalin, light
Nerdy Note
Ehrlich’s reagent is named after the scientist Paul Ehrlich (1854-1915) who was one of the pioneers of cell staining. Ehrlich’s reagent was originally used to distinguish typhoid from diarrhea.
Nitrite:
Nitrite + p-arsenilic acid à diazo compound
diazo compound + tetrahydrobenzoquinolinol à color change
Positive samples are indicative of a bacterial infection, specifically of bacteria that can reduce nitrate to nitrite. Make sure you know you microbiology!
False negative:
ascorbic acid (vitamin C)
Leukocyte esterase:
Esterase splits pyrrole amino acid ester à 3-hydroxy-5-phenyl pyrrole
3-hydroxy-5-phenyl pyrrole + diazo salt à color change
Positive samples indicate the presence of WBCs. This test usually correlates with nitrite.
Specific gravity:
Specific gravity can also be measured on the urine dipstick.
A polyelectrolyte with maleic anhydride will ionize in proportion to the ionic strength of the urine. The indicator bromothymol blue then interacts with hydrogen ions to form a colored complex. Blue indicates a low specific gravity and green a higher specific gravity.