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 renal tubular epithelial 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). Casts can form at any levels within the tubules (proximal, distal, convoluted). Narrower casts are thought to originate from proximal or distal tubules, however wide casts are thought to originate from the collecting tubules or ducts.

cast formation

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 below

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).

  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).

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 Cellular Casts
Granular Casts Fatty Casts
Waxy Casts

Hyaline 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.

hyaline cast phase cast
Hyaline casts (black arrows) in a wet preparation of urine sediment as visualized with the condenser racked down or the iris diaphragm closed. Hyaline casts (indicated by the black arrow) as seen under phase contrast microscopy. The edges of the cast are easier to see with this technique.
mucus mucus phase
Mucus strands (arrow) are mostly seen in urine form horses. They mimic casts, however they often have irregular borders, fold over, and taper at one or both ends A phase contrast image of mucus strands. Tapering ends are evident in several of the strands (arrow), which are also more wavy and fold over compared to hyaline casts

Hyaline casts can be present in low numbers (0-1/LPF) in concentrated urine of otherwise normal patients and are not always associated with renal disease.

Greater numbers of hyaline casts may be seen in association with proteinuria of renal (e.g., glomerular disease) or extra-renal (e.g., overflow proteinuria as in myeloma) origin. In such cases it has been proposed that the presence of excessive serum protein in the tubular lumen promotes precipitation of the Tamm-Horsefall mucoprotein.
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Cellular castscellular cast

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.

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Granular castsgranular cast

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.

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"Fatty" castsfatty cast

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.

feline renal epithelium
Feline proximal renal tubular epithelial cells contain numerous fat droplets (100x oil immersion, Wright's stain)

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.

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Waxy castswaxy

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.

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