Urine albumin and small globulins derived from plasma comprise two-thirds of the protein in urine. Under certain conditions, this protein forms a meshwork of fibrils that can trap cells, cell fragments, or granular material. Casts begin to disintegrate in dilute and alkaline urine or in the presence of bacteria, which probably contribute to a decreasing acidity. The size and shape of casts depends on the site of formation. Large casts are seen in dilated tubules or with stasis in collecting ducts.
Thin casts occur in tubules compressed by swollen interstitial tissue or because of disintegration.
Granular casts are seen in pyelonephritis , tubular interstitial disease, renal transplant rejection, and chronic lead poisoning . Coarsely granular casts occur with hematuria in cases of renal papillary necrosis . Usually they are associated with visible fat droplets containing triglycerides or cholesterol esters. Fatty casts are seen in the condition of heavy proteinuria and high serum lipids , that is, nephrotic syndrome . When stasis has occurred in the nephron, a red cell cast may degenerate and appear in the urine as a reddish-brown, coarsely granular cast, called a blood or hemoglobin cast.