Urinary casts

Urinary casts are a compacted collection of protein and other substances formed in the tubules of the kidneys.

Urinary casts assume the shape of the tubule or collecting duct in which they were formed. The ones formed in the distal tubules have a narrow caliber whereas those formed in the collecting tubules tend to be very broad.

Urinary casts are formed by proteins (mainly Tamm-Horsfall protein) and sometimes by other substances that were present within the tubule or collecting duct when the cast was formed such as bacteria, cells or diffuse granules.

There are certain conditions that contribute to the formation of the urinary casts:

• Reduced urine flow.
• Increased amounts of dissolved substances in the glomerular filtrate.
• Acidic urine (pH urine below 6.5).

Urinary casts are classified using the names of the trapped substances within the protein that form the compacted structure.

• Hyaline casts: They are transparent and difficult to differentiate from mucus. They consist almost entirely of Tamm-Horsfall proteins. Hyaline casts are the most frequently seen in urinary sediment.
• Granular casts: They can contain fine or coarse granules. They may result from the disintegration of structures (fats, cells, proteins or degenerated WBC) present in the tubules of the kidneys at the time that the cast was formed.
• Cellular casts: They may contain different types of cells:
  • Epithelial casts: Contain renal epithelial cells. They represent the presence of advanced tubular destruction.
  • RBC casts: Contain red blood cells and have a distinct orange-red color. RBC casts are primarily associated with damage to the glomerulus (glomerulonephritis).
  • WBC casts: Contain white blood cells. The appearance of WBC casts in the urine signifies infection or inflammation within the nephron.
• Waxy casts: Probably represent the last stage of granular casts degeneration. They are smooth, with an irregular outline and very refractive.
• Fatty casts: Formed by the breakdown of cholesterol lipid-rich epithelial cells.
• Bacterial casts: Contain bacilli both within and bound to the protein.

Urinary casts found in the urinary sediment are relevant for the diagnosis of kidney diseases because casts are the only elements found in the urinary sediment that are unique to the kidney.

Besides, depending on the type of the casts found they can give additional information about the possible diagnosis.

This test is performed in the urine sediment examination. To study the casts under the microscope, it is necessary to obtain previously the urinary sediment. For this matter, the urine sample should be placed at rest for some hours waiting for the precipitation of the different elements of the urine (white and red blood cells, casts, crystals, etc.). As this process is very slow, the urine is centrifuged for about 5 minutes at 1500 or 2000 revolutions per second to obtain a sample of the sediment in less time.

Once the sample sediment is obtained, it can be studied under the microscope and the presence of different types of urinary casts is reported.

For a better visualization of urinary casts, the intensity of the light source on the microscope must be decreased and the urine must be examined with the low-power objective.

A few hyaline casts in the urine sediment are considered normal (1 or 2 per low-power field LPF).

The presence of any other type of casts need to be evaluated by the physician because they are suggestive of kidney disease (damage to the kidney tubules).

The presence of concrete types of urinary casts can be useful to get a diagnosis. The different types of urinary casts are related to:

• Hyaline casts: Must be considered pathological when they appear in a great number, (more of 2 per LPF low-power field)
  • Strenuous exercise
  • Dehydration
  • Heat exposure
  • Emotional stress
  • Acute glomerulonephritis
  • Pyelonephritis
  • Chronic renal disease
  • Congestive heart failure
• Granular casts
  • Strenuous exercise
  • Pyelonephritis
  • Glomerulonephritis
  • Acute tubular necrosis
• Cellular casts
  • Epithelial casts (contain renal epithelial cells)
    • Acute tubular necrosis
    • Nephritis associated with cytomegalovirus infection
    • Hepatitis
    • Heavy metal, chemical or drug-induced toxicity
    • Viral infection
  • RBC casts (contain red blood cells)
    • Glomerulonephritis
    • Renal infarction
    • Goodpasture syndrome
    • Kidney involvement in subacute bacterial endocarditis
    • Lupus
  • WBC casts (contain white blood cells)
    • Pyelonephritis
    • Inflammation of the kidney
    • Interstitial nephritis
• Waxy casts:
  • Chronic renal failure
  • Decrease in the urine flow
• Fatty casts
  • Nephrotic syndrome
  • Toxic tubular necrosis
  • Diabetes mellitus
  • Crush injuries
• Bacterial casts
  • Pyelonephritis

The use of certain diuretics such as furosemide or ethacrynic acid may increase the number of hyaline casts in the urine.