- 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).
- Free lipid can be a normal finding in 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 no.) or pathophysiologic (renal tubular injury) conditions.
- Once liberated from degenerating or ruptured cells, fat frequently sticks to hyaline casts.
- 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 .
Fatty Cast and Oval Fat Body
