Urine Microscopy

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Overview

  • 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.

Urine Collection

  • 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
    • Pregnancy
  • 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)
    • Trauma

Urinary Casts

  • 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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