Urine examination: Urine Dipstick testing

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Dipstick testing

Analysis for pH, blood, protein, glucose, and white cells can be done with dipstick testing.

pH
Urinary pH varies between 4.5 to 8, averaging between 5.5 and 6.5.

Blood
Normal urine contains <3 RBCs per high-powered field (~1000 erythrocytes/ml of urine; upper limit of 5000 to 8000 erythrocytes/ml). +ve dipstick for blood indicates the presence of haemoglobin in the urine. Haemoglobin has a peroxidase-like activity and causes oxidation of a chromogen indicator, which changes colour when oxidized. Sensitivity of urine dipsticks for identifying haematuria (>3 RBCs/HPF is >90%); specificity is lower (i.e. a higher false +ve rate with the dipstick, due to contamination with menstrual blood, dehydration (concentrates what rbcs are normally present in urine)).

Protein
Normal, healthy adults excrete about 80 to 150mg of protein per day in their urine (normal protein concentration <20mg/dL). Proteinuria suggests the presence of renal disease (glomerular, tubulointerstitial, renal vascular) or multiple myeloma, but it can occur following strenuous exercise. Dipstick test is based on a tetrabromophenol blue dye colour change (green colour develops in the presence of protein of >20mg/dL).

White blood cells
Leukocyte esterase activity detects the presence of white blood cells in the urine. Leukocyte esterase is produced by neutrophils and causes a colour change in a chromogen salt on the dipstick. Not all patients with bacteriuria have significant pyuria. False -ves: concentrated urine, glycosuria, presence of urobilinogen, consumption of large amounts of ascorbic acid. False +ves: contamination.

Nitrite testing
Nitrites in the urine suggest the possibility of bacteriuria. They are not normally found in the urine. Many species of gram -ve bacteria can convert nitrates to nitrites, and these are detected in urine by a reaction with the reagents on the dipstick which form a red azo dye. The specificity of the nitrite dipstick for detecting bacteriuria is >90% (false +ve nitrite testing is contamination). Sensitivity: 35 to 85% (i.e. lots of false -ves); less accurate in urine containing fewer than 105 organisms/ml.

Cloudy urine which is +ve for white blood cells and is nitrite +ve is very likely to be infected.

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