Granular Casts

Friday, December 4, 2009

Granular casts are the second-most common type of cast and they are the result either from the breakdown of cellular casts or the inclusion of aggregates of plasma proteins or immunoglobulin light chains. Depending on the size of inclusions, granular casts can be classified as fine or coarse, though the distinction has no diagnostic significance. Their appearance is generally more cigar-shaped and of a higher refractive index than hyaline casts. While most often indicative of chronic renal disease, granular casts, as with hyaline casts, can also be seen for a short time following strenuous exercise.

When cellular casts remain in the nephron for some time before they are flushed into the bladder urine, the cells may degenerate to become coarsely granular casts, then a finely granular cast, and finally, a waxy cast. Granular and waxy casts are believed to derive from renal tubular cell casts. Broad casts are believed to emanate from damaged and dilated tubules and are therefore seen in end-stage chronic renal disease.

Granular casts type II are hyaline matrix casts filled with granules similar to cytoplasmic degeneration granules. There is a relation between the structure of these granules and the granular cytoplasm of the degenerating tubular cells. Although the causes of this degeneration are unknown, a proteinuria is a usual finding. A protein overload could be responsible for the granular degeneration of the tubular cells. The cytoplasm granulation could then be integrated in a cast as free granules, as cytoplasmic fragments, or as complete cells.

Granular casts type I are casts embedding cellular debris. This type I cast has a variable size granulation with a clumpy distribution. The leukocytes origin of the debris is suspected, but our attempt to stain these casts with the Naphtyl AS-D Chloro-acetate esterase gave deceiving results. We think that these casts are made of cellular debris of different kinds including degenerated leukocytes. The term "cellular debris cast" would be more appropriate and less confusing than the type I granular casts.

Granular casts are a sign of underlying kidney disease. However, they are nonspecific and may be found in people with many different kidney disorders.