Examining urine under the microscopy reveals the presence of cells, crystals, casts and other findings that can suggest the cause of a patient's renal or urinary complaint.
There are many ways in which a urine sample can be collected. The method of collection depends on the type of test performed, and certain patient factors.
Types of Urine Collection
- Random urine - taken at any time; Used for testing of electrolytes etc.
- Morning urine - the first urine of the day is the most concentrated, increasing the yield; Used for urinalysis, microscopy and beta-HCG testing
- Clean catch / midstream urine - the first few mL are discarded and then urine is collected, removing urethral bacterial flora from the sample; Best for urine culture and sensitivity testing
- 24 hour urine - all of the urine passed over 24 hours is collected; Used to measure certain analytes, such as creatinine, protein, electrolytes, uric acid, cortisol, catecholamines / metanephrines
- Catheterised sample - e.g. from an in and out / indwelling / suprapubic catheter
- Suprapubic aspiration - a needle is used to aspirate urine; May be required if a patient cannot be catheterised, or if sterile urine absolutely must be collected
Squamous Epithelial Cells
It is useful to first check whether the urine contains squamous epithelial cells (SECs), which are a marker of contamination.
SECs seen in urine originate from the urethral wall. If these are present in urine then this suggests a contaminated, non-midstream urine sample that likely also contains urethral normal flora.
White Blood Cells
The presence of white blood cells in urine is referred to as pyuria. This tends to be an indicator of urinary tract infection, though there are several non-infective causes.
Causes of Pyuria
- Infection - urethritis, prostatitis, cystitis, pyelonephritis, tuberculosis
- Neoplasm - renal, bladder, protate
- Inflammation - interstitial nephritis, interstitial cystitis, Kawasaki disease, SLE
- Foreign body - renal calculus, indwelling catheter, ureteric stents
- Polycystic kidney disease
- Pelvic irradiation
The presence of eosinophils in urine is classically a sign of acute interstitial nephritis, however the actual correlation between eosinophiluria and AIN is relatively poor.
Red Blood Cells
Urine does not normally contain red blood cells. The presence of red cells in urine suggests glomerulonephropathy, certain other renal conditions, or pathology of the urinary tract.
Macroscopic haematuria is urinary blood that is evident on inspection of the urine; this is more likely to represent a urinary tract cause. Microscopic haematuria refers to urinary blood that is only visible using a microscope.
- Causes of haematuria include:
- Glomerulonephropathy - congenital glomerulonephropathy, primary or secondary glomerulonephritis
- Vascular pathology - arteriovenous malformation, renal vein thrombosis, renal infarct
- Polycystic kidney disease
- Urinary tract infection - urethritis, cystitis, pyelonephritis, prostatitis, tuberculosis
- Neoplasm - renal, bladder, prostate
- Renal calculus
- Inflammation - interstitial cystitis
- Haemorrhagic cystitis (cyclophosphamide)
Urinary casts are tubular structures formed by the precipitation of Tamm-Horsfall mucoprotein. These may contain red cells, white cells or other substances, and their presence may suggest specific disease processes.
- Acellular casts do not contain cells. Examples include:
- Hyaline casts (solidified mucoprotein) - dehydration, exercise, diuretics
- Granular casts (degenerated cells) - glomerulonephritis, acute tubular necrosis, pyelonephritis
- Muddy brown casts (likely denatured haemoglobin) - acute tubular necrosis
- Waxy casts (severely degenerated cells) - severe acute or chronic renal failure
- Fatty casts (lipids) - nephrotic syndrome, diabetic nephropathy
- Pigment casts (haemoglobin, myoglobin, bilirubin) - haemolysis, rhabdomyolysis, liver disease
- Cellular casts contain red cells, white cells or other sloughed cells. These may include:
- Red cell casts (bleeding through the glomerulus or into the tubules) - glomerulonephritis, acute interstitial nephritis
- White cell casts (interstitial or glomerular inflammation) - pyelonephritis, glomerulonephritis, acute interstitial nephritis
- Renal tubular epithelial cell casts (sloughed cells) - acute tubular necrosis, acute interstitial nephritis
- Bacterial casts - pyelonephritis
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