Urinary Casts

Composition: Casts are cylindrical structures composed mainly of mucoprotein (the Tamm-Horsefall mucoprotein) which is secreted by epithelial cells lining the loops of Henle, the distal tubules and the collecting ducts. The factors responsible for the precipitation of this mucoprotein are not fully understood, but may relate to the concentration and pH of urine in these areas. Casts may form in the presence or absence of cells in the tubular lumen. If cells (epithelial cells, WBC) are present as a cast forms, they may adhere to, and subsequently be surrounded by, the fibrillar protein network.

Formation: A commonly-held theory is that cellular, granular, and waxy casts represent different stages of degeneration of cells in a cast. The appearance of a cast observed in a urine sediment depends largely upon the length of time it remained in situ in the tubules prior to being shed into the urine, as well as where it forms in the tubules. A cast recognizable as "cellular", for example, was shed shortly after it was formed. A waxy cast, in contrast, was retained longer in the tubular system prior to being released (see image below).

With renal tubular injury, epithelial cells slough into the lumen of the renal tubules and are caught up within a mucoprotein matrix made from Tamm-Horsfall protein (hyaline cast). This forms a cellular cast. With time, the epithelial cells degenerate and can no longer be recognized as cells within the hyaline matrix, thus forming coarsely granular, then finely granular, casts. Waxy casts are the final step in the formation of casts and usually indicate chronic tubular disease. Representative images of these various casts as seen under light microscopy of a wet preparation of urine sediment can be viewed by clicking on the link in the table below.

1. An absence of casts does not rule out renal disease. Casts may be absent or very few in cases of chronic, progressive, generalized nephritis. Even in cases of acute renal disease, casts can be few or absent in a single sample since they tend be shed intermittently. Furthermore, casts are unstable in urine and are prone to dissolution with time, especially in dilute and/or alkaline urine.
   
   
2. Although the presence of numerous casts is solid evidence of generalized (usually acute) renal disease, it is not a reliable indicator of prognosis. If the underlying cause can be removed or diminished, regeneration of renal tubular epithelium can occur (provided the basement membrane remains intact).