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
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
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
General Interpretation of casts:
Casts are quantified for reporting as the number seen per low power field
(10x objective) and classified as to type (e.g., waxy casts, 5-10/LPF).
Casts in urine from normal individuals are few or none (and are usually
hyaline or granular in nature).
Recognition of casts: Click on the buttons below for
pictures and discussion of urinary casts of various types.
- 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.
- 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).