2. Casts

2.1 Hyaline Casts

Morphology: Hyaline casts are colorless and have a low refractive index.  Hyaline casts are composed of a protein called uromodulin, which is secreted by RTE cells. The shape of the cast is derived from the renal tubule it was molded in.  They have parallel sides with rounded ends.  The casts may have rare to occasional granules.  Few to many granules within the cast matrix is classified as a granular cast.

Disease correlation and clinical significance: Pathogenic causes for hyaline casts include chronic renal disease, congestive heart failure, fever and dehydration.  Non-pathogenic causes for hyaline casts found in healthy individuals occur due to strenuous exercise, dehydration and stress.

Helpful Hints:  Hyaline casts are often missed due to their low refractive index, to prevent this use low light and lower the condenser.  Fibers and mucus are often misidentified as casts due to their similar appearance.  Fibers have a higher refractive index than hyaline casts, and will appear refractile under high light.  Mucus is typically thinner than hyaline casts and it lacks the parallel sides with rounded ends.

2.2 Cellular casts

Morphology:

RBC casts: RBC casts have parallel sides with rounded ends and contain multiple RBCs within the cast matrix.  The cells will be embedded in the matrix of the cast not attached to the sides.  The latter is most likely a hyaline cast with free floating RBCs attached.  The casts range in color from pink to red to brown.

WBC casts: WBC casts have parallel sides with rounded ends and contain multiple WBCs within the cast matrix.  The cells will be embedded in the matrix of the cast not attached to the sides.  The latter is most likely a hyaline cast with free floating WBCs attached.

RTE casts: RTE casts have parallel sides with rounded ends and contain multiple RTE cells within the cast matrix.

Disease correlation and clinical significance:

RBC casts:  Pathologic causes associated with RBC casts include glomerular nephritis and glomerular damage.

WBC casts: Pathologic causes associated with WBC casts include pyelonephritis, interstitial nephritis and inflammation.

RTE casts: RTE casts are associated with acute tubular necrosis or damage to renal tubules.

Helpful Hints:  WBC casts and RTE casts can often be mistaken for one another.  For WBC casts look for the presence of free-floating WBCs in the urine.  Additionally look for the segmented nucleus found in WBCs vs the mono nucleus of RTE cells.  Supravital stains can be used to help distinguish nuclear details for differentiation of RTE vs WBC casts.

2.3 Granular Casts

Morphology: Granular casts have parallel sides with rounded ends and contain multiple granules.  Casts can be either fine or coarsely granulated. Coarsely granulated casts tend to be dark in color whereas finely granulated will be lighter in color.

Disease correlation and clinical significance: Pathologic causes associated with granular casts include urinary stasis, glomerulonephritis, tubular damage and pyelonephritis.  Non-pathologic causes associate with granular casts can arise from stress or strenuous exercise.

Helpful Hints: All casts are enumerated under low power but are differentiated on high power.

2.4 Waxy and Broad Casts

Morphology:

Waxy Casts: Waxy casts have parallel sides; their ends can be rounded or blunt.  The casts are typically darker than hyaline casts and are highly refractile.  The casts have a sliced wax appearance.

Broad casts: Broad casts have parallel sides; their ends can be rounded or blunt.  Broad casts get the name “broad” because they are significantly wider than other casts, as they are produced in the collecting duct.  Broad casts can be granular or waxy in appearance.

Disease correlation and clinical significance:  Pathologic causes for both waxy and broad casts are associated with extreme urinary stasis, chronic renal failure and end stage renal disease.

Helpful Hints: The presence of waxy and/or broad casts will be accompanied by the presence of other cast types.  Waxy casts are often misidentified as they look similar to fibers.  Look for the presence of other cast types in the sample before calling a waxy cast.