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
General Interpretation of casts:
Recognition of casts: Click on the buttons below (or scroll down) for pictures and discussion of urinary casts of various types. Please refer to the urine sediment atlas for a compilation of casts.
Hyaline casts are formed in the absence of cells in the tubular lumen. They consist of Tamm-Horsfall protein and have a smooth texture and a refractive index very close to that of the surrounding fluid. They are very difficult to see in wet preparations of urine and must be distinguished from mucus strands. Generally, hyaline casts have parallel sides with clear margins and blunted ends, whereas mucus strands are more variable in size with irregular margins (see below). Reduced lighting is essential to see hyaline casts in urine sediment preparations. Lighting can be reduced by lowering the substage condenser (or close the iris diaphragm). Hyaline casts are also easier to see if other particles (fat, debris) are caught up within them (see image on the left below). Hyaline casts are far easier to visualize using phase contrast microscopy (see image on the right below), but this is not available on standard microscopes.Note that when fat droplets stick to hyaline casts, they are still called hyaline and not fatty casts.
Cellular casts most commonly result when disease processes such as ischemia, infarction, or nephrotoxicity cause degeneration and necrosis of tubular epithelial cells. The presence of these casts indicates acute tubular injury but does not indicate the extent or reversibility of the injury.
A common scenario is the patient with decreased renal perfusion and oliguria secondary to severe dehydration. Ischemic injury results in degeneration and sloughing of the epithelial cells. The resulting casts often are prominent in urine produced following rehydration with fluid therapy. The restoration of urine flow "flushes" numerous casts out of the tubules.
Leukocytes can also be incorporated into casts in cases of tubulo-interstitial inflammation (eg, pyelonephritis). It is rarely possible to distinguish between epithelial casts and leukocyte casts in routine sediment preparations, however, since nuclear detail is obscured by the degenerated state of the cells.
Granular casts, as the name implies, have a textured appearance which ranges from fine to coarse in character. Since they usually form as a stage in the degeneration of cellular casts, the interpretation is the same as that described for cellular casts. Low numbers of granular casts may be seen in the urine of animals without tubular injury, so the presence of casts should be interpreted along with clinical signs, rest of the urinalysis results, biochemistry results and other pertinent diagnostic information.
Fatty casts are identified by the presence of refractile lipid droplets within the protein matrix of the cast. The background matrix of the cast may be hyaline or granular in nature. Fatty casts, like granular casts, are thought to represent tubular degeneration. Note that the term fatty cast is not used for hyaline casts with fat droplets adhered to them (which are frequently seen in urines in which free lipid droplets are present as well). Pictured on the right is a fatty cast with a hyaline matrix. Also notice the free lipid droplets in the background.
Free lipid can be a normal finding in animal urine (as an isolated finding, lipiduria is seldom of clinical significance) and is likely from degeneration of sloughed renal tubular or transitional epithelial cells. Shedding of these cells in urine can occur under physiologic (low numbers can be shed in healthy animals) or pathophysiologic (renal tubular injury) conditions. Once liberated from degenerating or ruptured cells, fat frequently sticks to hyaline casts. This may happen more in cats, because their renal tubular epithelium can normally contain small to moderate amounts of fat (see image to the left). Differentiation of true fatty casts from hyaline casts with adherent fat can be difficult and is based on the character of the cast matrix, rather than on the lipid content per se.
Waxy casts have a smooth consistency but are more refractile and therefore easier to see in a regular wet sediment preparation of urine compared to hyaline casts. They commonly have squared off ends, as if brittle and easily broken, and also have smooth parallel-sided borders.
Waxy casts indicate tubular injury of a more chronic nature than granular or cellular casts and are always of pathologic significance.
Illustrated to the right is urine from a dog with acute and chronic renal tubular injury as shown by the presence of waxy and coarse granular casts in the urine.