Casts Cast in urine

Casts Casts represent a collection of protein and cellular debris in a kidney tubule, especially during low fluid flow. In a normal urine albumin, globulins form about two-thirds of urine proteins; the rest is a glycoprotein produced by the distal part of the convoluted tubule in the nephron. This is known as Tamm Horsfall (TH) protein. The TH protein forms the matrix of all the casts. An occasional cast may be found in normal urine.

When larger amounts of plasma proteins such as albumin or abnormal proteins such as Bence Jones proteins enter the renal tubules, the formation of casts is increased. The plasma proteins combine with the TH protein. These proteins are precipitated in an acidic pH, trapping the cellular debris in them. The size and shape of casts depends on the site of formation. Narrow casts are formed in the renal tubules whereas wider casts are produced in the collecting ducts. They have parallel sides and usually, blunt ends. When the casts begin to disintegrate, they may show tapering ends and are called cylinderoids.

Microscopic appearance and clinical significance-

Casts are very difficult to visualise in an unstained preparation and may be completely missed if the light is not sufficiently reduced. They should be detected under the low power (10x) objective and identified using the high power (40X) objective. A phase contrast microscope is more useful for their detection. Casts can be classified according to their morphological appearance, depending on their matrix, cells, pigments and inclusions. The presence of casts in urine is indicative of renal involvement.

Hyaline casts- Hyaline casts are semitranslucent, non-refractive, homogenous structures and need special care for their detection.

An occasional hyaline cast may be present in normal urine but their number is increased in renal diseases. A transient increase is seen after exercise, congestive heart failure, diuretic therapy and in fever

Cellular casts- These are the casts in which one or more types of cells are trapped during their formation. They are white cell (pus cell) casts, red cell casts and epithelial casts. Mixed cellular casts may be seen occasionally.

White cell casts- The white cells enter the renal tubules through the interstitial spaces between the renal tubular epithelium. The number of white cells trapped in a cast may vary. White cell casts are seen in the inflammations of the kidney, in conditions such as pyelonephritis and nephrotic syndrome. 

Red cell casts Red cell casts appear yellow under the low power objective. Under higher magnification, the trapped red cells may be seen. When these casts are formed in a nephron, the red cells may degenerate and appear in urine as reddish brown cast with coarse granules. Such a cast is known as blood or haemoglobin cast.

Red cell casts may be seen in serious conditions with renal involvement where glomeruli, tubules or renal capillaries are damaged. They are commonly observed in glomerulonephritis, subacute bacterial or infective endocarditis and renal infarction.

 Epithelial casts -Epithelial casts containing renal tubular epithelial cells are difficult to distinguish from the white cell casts. Phase contrast microscopy or Papanicolaou stain are useful in differentiation.The epithelial casts are seen in acute tubular necrosis, viral infections especially with cytomegalovirus, exposure to toxic drugs and heavy metals

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 Granular casts -These casts which contain coarse or fine granules may form as a result of ag. gregation of plasma proteins in Tamm Horsfall

Red cell casts - may be seen in serious conditions with renal involvement where glomeruli, tubules or renal capillaries are damaged. They are commonly observed in glomerulonephritis, subacute bacterial or infective endocarditis and renal infarction. Epithelial casts Epithelial casts containing renal tubular epithelial cells are difficult to distinguish from the white cell casts. Phase contrast microscopy or Papanicolaou stain are useful in differentiation.

The epithelial casts =are seen in acute tubular necrosis, viral infections especially with cytomegalovirus, exposure to toxic drugs and heavy metals. Granular casts These casts which contain coarse or fine granules may form as a result of ag. gregation of plasma proteins in Tamm Horsfall (TH) matrix or more commonly due to disintegration of cells within the cast.

Granular casts- appear in a variety of glomerular, tubular and interstitial diseases; and also in renal allograft rejection.

 Waxy casts- Waxy casts are homogeneous, with sharp margins and blunt ends. They resemble hyaline casts, but are more refractile, and show cracks and convolutions along the lateral margins. Waxy casts are associated with chronic renal failure, renal allograft rejection and indicate tubular inflammation and degeneration. Sometimes waxy casts are unusually broad due to tubular atrophy and dilatation. They are called renal failure casts indicating a terminal stage of renal disease 

Fatty casts- Fatty material such as cholesterol and triglycerides can be trapped into cast matrix when there is heavy proteinuria. They are highly refractile due to the presence of fat droplets.

Their presence in urine represents fatty degeneration and desquamation of renal tubular epithelium. They are seen in nephrotic syndrome, diabetes mellitus with renal involvement and due to toxic drugs such as ethylene glycol.

 Miscellaneous casts- Other miscellaneous structures such as crystals or various pigment granules such as haemosiderin or bilirubin may get trapped into the TH matrix to form casts.