Urinalysis
Urinalysis is simple, non-invasive, and inexpensive test for the presence and sverity of kidney disease. It is best avoided during menstruation, and within 2-3 days of heavy exercise, to avoid artefacts.
Tests range from point-of-care urine dipsticks, to microscopic examination, to central lab testing.
A clean-catch urine sample should be promptly examined using microscopic and biochemical means, as delays in urinalysis can result in molecular and cellular degradation. Refrigeration at 2-8 C assists preservation.
- Appearance
- Chemical Assessment
- Microscopic Examination
Urine Appearance
Normal urine colour ranges from almost colorless to deep yellow, depending on concentration of urochrome pigment. Abnormal urine colour can be a sign of disease, but can also result from infections, pigments, dyes, or drugs.
Pink/red/brown/black
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yellow-brown
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blue-green
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Cloudy urine can suggest a high concentraion of leukocytes (pyuria) or amprphous phospate precipitation in alkaline urine.
False positives in urinalysis include:
- hemoglobin: myoglobin, microbial peroxidases
- proteinuria: very alkaline pH (>9), chlorhexidine
- glucose: oxidizing agents
False negatives include:
- hemoglobin: ascorbic acid, delayed examination
- proteinuria: tubular proteins, immunoglobin light chains, globulins
- glucose: UTI, ascorbic acid
Chemical Assessment
dipsticks
Chemical assessment is most often done with a dipstick, a plastic stick containing various reagents used to detect various urine components. Assays are semi-quantitative and are based on colour changes. Tested variables include:
pH 4-9
specific gravity 1.005-1.030
protein (trace - 4+)
blood (trace - 4+)
nitrate (+/-)
glucose (trace - 4+)
ketones (trace - 4+)
Leukocyte esterase and nitrites are usually positive in infection, though a negative test does not rule out infection.
laboratory testing
pH - 4-8
osmolarity 50-1200 mOsm/kg
Na+
K+
Cl-
urea 300-600 mmol/d
specific gravity 1.000-1.030
protein < 300 mg/d
creatinine 6.0-17.1 mmol/d
24-hour urine collection is most often done to deduce protein excretion levels with concern over proteinuria.
Microscopic Examination
Microscopic examination is used to detect cellular elements, casts, crystals, and pathogens.
cells
Dysmorphic RBCs usually suggest glomerular origin and damaged basement membrane, while normal RBCs suggest nonglomerular origin.
Leukocytes suggests urinary tract inflammation or infection.
Eosinophils are present with acute interstitial nephritis.
Tubular epithelial cells are often seen with tubular necrosis, glomerulonephritis, or pyelonephritis.
Squamous epithelial cells can be from any site along the urinary tract and may be unimportant.
casts
Urinary hyaline casts are cylindric structures composed of Tamm-Horsfall protein which precipitate in the tubules. They are a normal finding.
Red blood cell casts most frequently indicate proliferative glomerulonephritis but can also be seen in acute interstitial nephritis.
Renal tubular cell casts suggest acute tubular necrosis, interstitial nephritis, or glomerulonephritis.
Leukocyte casts indicate urinary tract infection but are more suggestive of pyelonepritis. Interstitial nephritis is also possible.
crystals
In the absence of specific symptoms, crysal oxalate (envelope shaped) or uric acid (rhomboid shaped) casts are of little significance.