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Urine examination :
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 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 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 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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