Casts are clusters of urinary sediment elements (red blood cells, white blood cells, fat bodies, etc.) wrapped in a protein matrix. Cast formation necessarily occurs in the kidney tubules, and the the contents of the casts indicate the nature of the abnormalities occurring in the kidney itself.
Hyaline casts are the only casts that should be observed in normal urine and are not of particular clinical interest. They can be observed after intense exercise, in very concentrated low-volume urine, or during diuretic treatment.