Protein, as well as epithelial cells, white blood cells (leukocytes), red blood cells (erythrocytes) and fat particles, may accumulate in the renal tubules in renal disorders. They cover the renal tubules from the inside, as if taking a cast of them or forming cylinders. Urinary casts appear in the urine only if it contains protein, because protein acts as an adhesive substance. The presence of casts in the urine is defined as cylinduria.
Urinary casts do not dissolve if the urine is acidic. If it is alkaline, they either do not form at all or dissolve fast. Thus, a little or no urinary casts found in the alkaline urine may fail to reflect the actual state of affairs.
Urinary casts may have different shapes, colors and sizes, which depends on their composition. As a result, not only the amount of urinary casts but also their type is indicated in the urine test. Casts can be of different types as follows: hyaline casts, granular casts, waxy casts, red blood cell (RBC) casts, white blood cell (WBC) casts, renal tubular epithelial cell casts.
Hyaline casts are composed only of protein. They may be detected in the urine in proteinuria (amount of protein in the urine), even if there is no damage to the kidneys (physiological proteinuria), as well as in all renal diseases associated with elevated protein levels in the urine (glomerulonephritis, pyelonephritis, interstitial nephritis).
Granular casts are protein casts of renal tubules with renal tubule epithelial cells, disintegrated or regenerated, sticking to their surfaces and thus giving the casts their granular shape. Granular casts may be found in diseases associated with damage to renal tubules and protein in the urine (acute and chronic glomerulonephritis, diabetic nephropathy, renal amyloidosis, pyelonephritis, viral infection with fever).
Waxy casts form from hyaline and granular casts which stay in the renal tubules. They are composed of unstructured mass that resembles wax. As a rule, waxy casts in the urine are indicative of a severe damage to the kidneys or an advanced chronic renal disease. Waxy casts are considered a negative prognostic indicator.
Red blood cell (RBC) casts form from protein and red blood cells (erythrocytes). They often fail to be detected in the urine due to their fragility. Red blood cell casts indicate the renal origin of hematuria (red blood cells in the urine). They are found in acute glomerulonephritis, renal infarction, renal vein thrombosis, diabetic nephropathy, renal tuberculosis, kidney tumors.
White blood cell (WBC) casts are made from protein and white blood cells (leukocytes). They are quite rare and are more common with acute kidney infections (pyelonephritis).
Epithelial cell casts are composed of exfoliated renal tubular epithelial cells. Their presence in the urine indicates pathological processes in the kidneys. Most often, they are detected in the urine in nephrosis and acute nephritis. Epithelial cell casts found in the urine within a few days after surgery indicate the rejection of the kidney transplant.