Urine casts are cylindrical, cigar-shaped bodies that represent molds or "casts" of the lumen of the renal tubule in which they were formed. The common matrix of all casts is a mucoprotein known as Tamm-Horsfall protein which is secreted by the renal tubule at a fairly constant rate.
|Hyaline casts are formed in the absence of cells in the tubular lumen.
They have a smooth
texture and a refractive index very close to that of the surrounding
fluid. Reduced lighting is essential to see hyaline casts. Lower
the substage condenser.
When present in low numbers (0-1/LPF) in concentrated urine of otherwise normal patients, hyaline casts are not always indicative of clinically significant 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.
Here is an excellent example of a hyaline cast. It is cigar-shaped and blends into the background. They can be found in the urine of normal individuals after physical exertion, emotional stress or febrile diseases.
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
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 previously.|
Fatty casts are identified by the presence of refractile lipid droplets.
The background matrix of the cast may be hyaline or granular in nature.
Often, they are seen in urines in which free lipid droplets are present
as well. Fatty casts are the most common type seen in cat urines.
Interpretation of the significance of "fatty" casts should be
based on the character of the cast matrix, rather than on the lipid
content per se.
Pictured here is a fatty cast with a hyaline matrix. Also notice the free lipid droplets in the background.
|Waxy casts have a smooth consistency but are more
refractile and therefore easier to see compared to hyaline casts. They
commonly have squared off ends, as if brittle and easily broken.
Waxy casts indicate tubular injury of a more chronic nature than granular or cellular casts and are always of pathologic significance.