Nature of casts in urine: Tests and diagnosis
Casts in urine are found when looking at a urine sample under a microscope. Casts are typically tube-shaped particles that can be made up of white blood cells, red blood cells, epithelial cells, or substances such as fat and protein. Finding a cast in urine can help physicians determine whether the kidney is healthy or abnormal.
Different types of casts in urine
Multiple types of casts in urine have been identified, with each having some sort of diagnostic value to medical professionals. The presence of urinary casts may hint at certain disease states. They typically form in the distal convoluted tubule (DCT) and ducts of the nephrons of the kidney, then dislodge and pass into the urine as sediment, which can then be assessed under a microscope.
Types of casts in urine include:
- Hyaline casts: The most common type of cast and are composed of solidified Tamm-Horsfall mucoprotein. They are formed during low urine flow states, concentrated urine, or urinary conditions, causing an acidic environment. Hyaline casts can be seen in healthy people and may increase in number during dehydration, exercise, or when using diuretic medication. They have a smooth textured appearance under a microscope.
- Granular casts: The second most common type of cast that can develop either from the breakdown of cellular casts or the inclusion of aggregates of plasma proteins, such as albumin or immunoglobulin light chains. While having some relation to chronic renal disease, granular casts may also be seen in healthy individuals following strenuous exercise. They appear is generally textured and cigar-shaped under microscopy.
- Waxy casts: Considered the final stage of cellular cast degeneration, this type is usually seen in tubular injury commonly occurring during chronic renal disease and renal amyloidosis. Waxy casts often appear cylindrical, being more rigid and having sharp broken off edges. These casts are commonly referred to as “renal failure casts.”
- Fatty casts: These are hyaline casts with fat globule inclusions formed by the breakdown of lipid-rich epithelial cells. They can be seen in conditions such as tubular degeneration, nephrotic syndrome, and hypothyroidism. Fatty casts may appear yellowish-tan in color.
- Red blood cell casts: Finding red blood cells within a cast is always pathological and strongly indicative of several conditions including granulomatosis with polyangiitis, systemic lupus erythematosus, post-streptococcal glomerulonephritis, or Goodpasture’s syndrome. They may also be associated with renal infarct and subacute bacterial endocarditis. Red blood cell casts may appear yellowish-brown in color and are generally cylindrical in shape with occasionally ragged edges.
- White blood cell casts: A sign of inflammation or infection, with their presence strongly suggesting a direct infection of the kidney known as pyelonephritis. Inflammatory states such as acute allergic interstitial nephritis, nephrotic syndrome, or post-streptococcal acute glomerulonephritis may also present with white blood cell casts.
- Bacterial casts: Seen in association with loose bacteria, white blood cells, and white blood cell casts. While finding bacterial casts in the urine is generally a rare occurrence they can appear in pyelonephritis
- Epithelial cell casts: Formed by inclusion or adhesion of desquamated epithelial cells of the tubule lining. Typically appearing in random order or in sheets, epithelial cell casts have large round nuclei and have a lower amount of cytoplasm. Acute tubular necrosis and toxic ingestion of such substances as mercury, diethylene glycol, or salicylate can lead to their formation. Cytomegalovirus and viral hepatitis are organisms that may also lead to epithelial cell cast development.
How the test is performed
Urine casts are best assessed by providing a urine sample from your first-morning urination with samples being taken to the lab within an hour. A clean-catch sample is required to prevent the chances of contamination from germs found on penis or vagina. Your doctor may provide you a clean-catch kit that may make this process occur as smoothly as possible.
What abnormal results mean in casts in urine
Urinary casts cannot be seen by the naked eye and will require laboratory testing to identify and quantify. Depending on the particular type of cast found in the urine, having only a few or even many may be seen as insignificant. However, larger numbers may indicate some form of pathological condition occurring elsewhere in the urinary tract system.
Your doctor will be able to interpret the obtained urinalysis results to rule out the possibility of a suspected diagnosis.