The Formation, Appearance, and Composition of Casts

The Formation, Appearance, 

and Composition of Casts 

Casts, uniquely found in the urine, were initially discovered 

in the mid-19th century and were at first thought 

to be associated with renal abnormalities. Time has 

proven this to be a false assumption, however, since a 

few casts are also found in the urine of normal individuals. 

Their presence in disease states is, however, vitally 

important in the diagnosis of the specific etiology of 

renal diseases and the physiological conditions in the 

kidney that prompted their formation. 

All urinary casts are composed of a proteinaceous 

matrix. The proteins forming the cast are secreted by 

the lining cells of the tubules lining the distal nephron—specifically, 

the distal tubule and collecting duct 

lining cells. These cells are nonciliated columnar epithelial 

cells with basally placed nuclei. They, in contrast 

to the lining cells of the proximal tubule and loop 

of Henle, have the propensity to excrete a particular 

type of immunoprotein known as Tamm-Horsfall (TH) 

mucoprotein. Once excreted into the urinary flow, this 

protein, along with other proteins secreted by these lining 

cells, agglutinates and is then molded into a semisolid, 

fibrillar, cylindrical, amorphous shape, which is 

the prototypical hyaline cast. The hyaline cast serves 

as the basic building block for all cast types. In its most 

elementary form, it is homogeneous, translucent, has a 

low refractive index, and does not contain any particulate 

material or cells. 

In the normal state, cells lining the nephron are 

continually dying and regenerating. In many diseases, 

these same lining cells may be affected by chemical 

toxins, such as lead or mercury, or inflammatory 

agents, such as bacteria, and die prematurely. In either 

instance, cell death results. Intrinsic cellular lysosomes, 

which contain enzymes necessary for cellular degradation, 

serve as “garbage disposals” for the cell’s component 

parts, especially mitochondria, and convert 

these organelles into minute, fine, granular particles. 

The granular products of cell death enter the urinary 

flow and may become incorporated into the matrix of 

a forming hyaline cast. When the amount and number 

of these granules occupies the majority of the cast’s 

surface, the cast is called a granular cast. 

Four cell types may, in a wide variety of diseases, 

adhere to the surface of or become incorporated into 

the matrix of a hyaline cast: these are RBCs, WBCs, epithelial 

cells, and bacteria/fungi. Ultrastructural studies 

have shown that some of these cells are bound to the 

cast’s surface by means of bands of thin proteinaceous 

material, most likely TH protein. These cellular casts are 

named according to the type of cell so bound, i.e., RBC 

cast, WBC cast, epithelial cast, and bacterial cast. Cellular 

casts are not found in normal patients. 

Pigment casts are somewhat complex in that the 

usual hyaline proteinaceous matrix amalgamates with 

a component in the glomerular filtrate, most often hemoglobin. 

Other chemical substances that circulate in 

the serum and remain in high concentration in the urinary 

flow after being filtered by the glomerulus may 

Casts 165

Tamm-Horsfall 

protein 

The Tamm-Horsfall protein, 

also known as uromodulin, is 

a very large glycoprotein that 

is the most abundant protein 

in normal urine. Hyaline 

casts, cylindroids, and mucous 

strands are all made, at least 

in part, of Tamm-Horsfall protein 

fibrils. 

Cast Formation: 

Tamm-Horsfall Protein 

also be incorporated into the cast as it forms within 

the lumen of the nephron: these include melanin in 

patients with melanoma, and myeloma protein in patients 

with multiple myeloma. These so-called pigment 

casts are infrequently seen, however, with the exceptions 

of the hemoglobin cast (see “Muddy Casts,” page 

196), which may be prominent in patients with acute 

renal failure, and bile-stained casts in patients with severe 

jaundice such as is found in acute cases of viral 

hepatitis and chronic liver failure. 

The exact origin of waxy casts is not clear, however. 

Although they have a proteinaceous matrix, which includes 

TH immunoprotein, their surface characteristics 

differ significantly from the usual translucent hyaline 

appearance. These casts have a high refractive index 

and appear dense, often with blunted ends and 

notched margins. It is theorized that waxy casts may 

be the evolutionary product of preexisting cellular 

casts in which the cells have completely disrupted and 

166 Casts 

renal tubular epithelial cells 

distal tubule 

hyaline cast 

Tamm-Horsfall protein is excreted by epithelial cells in the thick 

ascending branch of the loop of Henle and the first part of the 

distal tubules. The molecule polymerizes and forms a gel-like 

meshwork. Over time, the meshwork becomes more dense as 

fibrils are added to the initial structure, which begins to take on 

the shape of the tubule. The cylindrical structure matures into 

a hyaline cast as other proteins are added to the gel. The cast 

initially blocks urine flow but hydrostatic pressure eventually 

dislodges the cast from its attachment to the epithelial cells. 

their cytoplasmic and nuclear components have lost 

their definition and merged with TH protein already in 

the cast, increasing their refractive index—hence, the 

waxy appearance. 

In summary, there is one prototypical type of cast 

found in the urinary sediment: the hyaline cast. Other 

casts may be considered variations, except perhaps 

the waxy cast. Casts are named according to their 

microscopic appearance, i.e., hyaline (clear), cellular, 

pigmented, granular, etc. As the cast forms, products 

of cell degeneration (granules), crystals, or intact cells 

may become applied to the cast’s surface or incorporated 

within its matrix. Additionally, abnormal products 

of metabolism in disease states, such as hemoglobin, 

bilirubin, and melanin, that circulate in the blood and 

appear in high concentration in the glomerular filtrate 

may become admixed with TH protein elaborated by 

the renal tubular epithelium and form what have been 

called pigmented casts.

RBC 

renal tubular 

epithelial cell 

crystals 

Intratubular matter and cells may 

become attached to the surface 

or incorporated into the Tamm-Horsfall 

protein matrix as the cast forms. The type of 

material may be a clue to renal pathology, such 

as infection (bacteria and WBCs), glomerular disease 

(fat and RBCs), and acute tubular necrosis 

(renal tubular epithelial cells). 

RBC 

bacteria 


Composition 

Size Comparison of Cast Constituents 

degenerating 

cell 

WBC 

crystals 

bacteria 

Circulating pigments, such as 

hemoglobin, melanin, and bilirubin, 

may add color to the cast. 

neutrophil 

1 µm 7 µm 8-15 µm 10-15 µm 

(may swell to 20-30 µm) 

4-20 µm 2-20 µm 

15-35 µm 

lipid 

RTE 

eosinophil 

lipid 


glomerulus 

proximal 

convoluted 

tubule 

loop of Henle 


Evolution and Cellular Degeneration 

Cellular breakdown 

products produce a 

mixed cast and eventually 

a granular cast. 

mixed granular 

and WBC cast 

WBC cast 

If epithelial cells or WBCs 

are present as the cast 

forms, they may adhere 

to the fibrillary protein 

mesh and be incorporated 

into the cast. 

collecting duct 

cut-away view of distal 

convoluted tubule 

hyaline cast 

loop of Henle 

168 Casts 

Casts form in distal convoluted 

tubule and the first 

segments of the collecting 

ducts (shown in blue) 

waxy cast 

Cells and granules 

tend to break down 

and lose their definition. 

Their proteins 

ultimately fuse with 

other proteins in the 

cast. The result is a 

more highly refractile 

matrix: a waxy cast. 

finely granular cast 

coarsely granular cast 

Cellular, granular, and 

waxy casts probably 

represent different 

stages of cellular 

degeneration in a cast.

Matrix 

Contents 

Size and Shape 

Color 


Summary of Types and Appearance 

The matrix is either hyaline or 

waxy. Casts in the nephron for 

extended periods produce a 

waxy appearance. 

Cellular casts contain 

red blood cells, 

white blood cells, renal 

tubular epithelial 

cells, and bacteria. 

Noncellular casts 

contain granules, 

lipid, crystals, and 

hemoglobin. 

Mixed casts contain 

several of the components. 

Casts can be long or 

short and take on a 

variety of cylindrical 

shapes; size and 

shape have no clinical 

significance except for 

broad casts. These can 

be of any type but are 

typically waxy. They are 

at least 1.5 times the 

width of a normal cast. 

Waxy casts may 

have a faint yellow 

color. Hemoglobin 

stains casts reddish, 

myoglobin brown, 

and billirubin (which 

typically stains any 

cellular contents) 

greenish-yellow. 

“Muddy casts” are 

brick-red or brown, 

Drug pigments may 

also produce colored 

casts. 

hyaline 

waxy 

RBCs WBCs RTEs bacteria granules lipid crystals Hgb 

cellular casts 

broad casts 

noncellular casts 

hyaline casts of different shapes 

waxy RBC Hgb myoglobin muddy bilirubin drug pigment 

(e.g., phenazopyridine)

The Formation, Appearance, and Composition of Casts

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