Skip to Main Content

Urine Casts

++

Urinary casts can provide important information corroborating a suspected diagnosis. In some cases, a careful examination of the urine sediment can point to a possible diagnosis that may not have initially been given prominent consideration. Urine that is to be examined for casts and other urinary elements should be fresh and should be examined as soon as possible after it is obtained. The procedure for preparing urine for examination is as follows. Ten milliliters of urine is centrifuged at 1500–2000 rpm for 5 to 10 minutes. Suction is used to remove 9.5 ml of the supernatant. The sediment at the bottom of the tube is resuspended in the remaining 0.5 ml of supernatant by tapping on the tube. Using a pipette, a single drop of the resuspended urine is placed on a slide and then a coverslip is placed on the urine drop. The sample should be examined under the low-power objective and the high-power objective of the microscope.

++

Not all casts are pathologic. For instance, hyaline casts can be a normal finding. The presence of most casts, however, suggests renal disease. The absence of casts cannot be used to exclude a diagnosis, as casts may be missed or may degrade due to specimen processing. In general, casts are composed of the diagnostic element within a matrix of Tamm-Horsfall glycoprotein. Hyaline casts, which are composed primarily of Tamm-Horsfall glycoprotein, are faint, nearly colorless casts that can be seen in patients without renal disease. Renal tubular epithelial cell casts, which are composed of renal tubular epithelial cells shed in the setting of tubular injury, are most commonly seen in patients with ATN. Granular casts represent a degradation product that can contain broken down renal tubular epithelial cells and also other cellular elements. Thus granular casts may be seen in ATN, but are nonspecific and may also be seen in other types of renal disease. A scoring system by Perazella et al based on the number of renal tubular epithelial cells and granular casts was found to be useful in distinguishing prerenal AKI from ATN, as well as in predicting the worsening of either prerenal AKI or ATN. RBC casts are seen in pathologic states in which there is blood of glomerular origin, such as glomerulonephritis or vasculitis affecting the kidney. Leukocyte casts occur in pyelonephritis and acute interstitial nephritis, as well as in glomerulonephritis. Fatty casts, which contain lipid droplets and have a “Maltese cross” appearance under polarized light, may be seen in the setting of lipiduria, such as in patients with nephrotic syndrome. Table 104-2 summarizes different casts and their clinical significance.

++
Table Graphic Jump Location
Table 104-2 Urinary Casts
++

An important limitation in the use of casts as a diagnostic tool relates to the skill it takes to correctly identify urinary casts. One study by Tsai et al comparing the performance of nephrologists with that of the clinical laboratory found that the nephrologists were more likely to identify renal tubular epithelial cells, granular casts, renal tubular epithelial cell casts, and dysmorphic RBCs, as compared to the clinical laboratory. The clinical laboratory identified more squamous epithelial cells than the nephrologists, raising the possibility that the clinical lab was incorrectly identifying other urinary elements, such as renal tubular epithelial cells, as squamous epithelial cells. Overall, the nephrologists performed better than the clinical laboratory in deriving information from the urinalysis. Therefore, it is advisable to review a urinary sediment with a nephrologist, rather than relying on the clinical laboratory, if the results of the sediment examination are crucial.

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.