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.
Pictured here is a fatty cast with a hyaline matrix. Also notice the free lipid droplets in the background. Such droplets are most commonly seen in feline urine. As an isolated finding, lipiduria is seldom of clinical significance.