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 contents of the casts indicate the nature of the abnormalities occurring in the kidney itself.
As their name suggests, mixed casts contain more than one type of cell; red blood cells, leukocytes or tubular cells. They indicate kidney disease but cannot be used to specify its nature.