[PDF] Casts in the Urine Sediment

Casts in the Urine Sediment

Identification and Composition of Types of Granular Urinary Casts

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Casts are the result of solidification

of material (protein) in the lumen of the kidney tubules, more specifically in the nephron. Once formed, these molds (or casts) of the tubule are eliminated via the urine and may be seen in the urine sedi- ment. Such structures are extremely im- portant clinically, as they show the conditions in the nephron where they were formed. They may be considered to represent a biopsy of the kidney. If a cast is seen in the urine, kidney disease or in- volvement exists; the presence of casts indicates kidney (renal) disease rather than lower urinary tract disease.

All casts have a matrix of Tamm-

Horsfall mucoprotein to which other pro-

teins or elements may be added. They are generally associated with a positive reagent-strip test result for protein. They may contain RBCs, WBCs, renal epithe- lial cells, fat globules, bacteria, and degen- erated forms of any of these structures, which are seen as granules. Aggregates of plasma proteins, including fibrinogen, im- mune complexes, and globulins, may also be seen as granules within a cast. Although size and diameter vary, casts are generally cylindrical structures that have a definite outline, showing parallel sides and 2 rounded ends. They are approximately 7 or 8 times the diameter of an RBC and several times longer than wide. Casts are classified primarily on the basis of mor- phologic features; various types have dif- ferent clinical implications.

Hyaline Casts

Hyaline casts are both the most diffi-

cult to visualize and least important typeof casts encountered in the urine sediment. They are the result of solidifica- tion of Tamm-Horsfall mucoprotein, which is secreted by the renal tubular cells, and they may be present without significant proteinuria. A few may be seen in the urine of healthy persons. They may be seen in increased numbers after strenu- ous exercise and in some renal diseases.

Cellular Casts

A cellular cast may be composed of

any of the cells found in the urine sediment, such as RBC, WBC, or renal tubular epithelial cell. Bacterial cell casts have also been described as have casts consisting of a mixture of cell types. The cellular cast appears to result from a clumping, or conglutination, of cells that are incorporated in a protein matrix. In some instances, a few cells are found em- bedded in a hyaline matrix. If a cellular cast is seen in the urine, it is kno wn that the cells were present in the kidney (nephron). Although causes and severity differ, the presence of cellular casts indi- cates a disease process. It may be difficult or impossible to determine what cell type is present in a cast, because the cells are subject to deterioration. Such casts are generally reported as "cellular casts," and the cell type is generally suggested by other findings in the urine sediment.

RBC Casts

RBCs may be found in a cast either as

the result of leakage of RBCs through the glomerular membrane or by bleeding into the tubules at any point along the nephron [I1]. According to Rose and Rennke, 1 "Red cell casts . . . are virtually diagnostic of some form of glomerulonephritis or vasculi- tis." RBC casts are most often associated with diseases that affect the glomerulus, such as acute poststreptococcal glomer- ulonephtitis and other acute glomerulo- nephritides, IgA nephropathy, and lupus nephritis. They may also be seen in cases ofsubacute bacterial endocarditis, renal infarc- tion, and rarely in severe pyelonephritis (tubulointersititial disease).

Other forms of RBC casts include

blood casts and hemoglobin casts, which are the result of degeneration of RBCs within the cast matrix. The cells are no longer visible, yet the remaining hemo- globin pigment imparts a characteristic orange-yellow or red-brown color, which distinguishes them from waxy casts. This breakdown of cells within the cast sug- gests urinary stasis and a condition more chronic than acute. Reagent-strip findings associated with RBC casts are positive test results for protein and blood.

WBC Casts

WBC casts are generally composed

of neutrophils. Theoretically, they may enter the nephron at any point, but they are generally associated with tubulointer- stitial disease such as acute pyelonephri- tis. In this case, they are associated with the presence of WBCs and bacteria in the urine sediment; the occurrence of WBC casts locates an infection within the kid- ney rather than the lower urinary tract.

Occasionally, the WBCs enter the urine at

the glomerulus, and WBC casts or mixed

RBC and WBC casts may be seen in

cases of acute glomerulonephritis. They may also be seen in acute interstitiallaboratorymedicine>april 2001>number4>volume32191 ?your lab focus? interpretation [cytology]

Casts in the Urine Sediment

Karen M. Ringsrud, MT(ASCP)

From the Department of Laboratory Medicine and Pathology, University of Minnesota Medical School, Minneapolis, MN

Casts in the urine sediment and

their disease associations ?Cast origins or mechanisms of formation ?Reagent-strip findings or other sediment findings associated with casts[I1] RBC cast. RBC casts are particularly fragile

and prone to disintegration (×400).Downloaded from https://academic.oup.com/labmed/article/32/4/191/2504211 by guest on 13 June 2023

laboratorymedicine>april 2001>number4>volume32 192
nephritis, lupus nephritis, and acute papil- lary necrosis.

Reagent-strip findings associated

with WBC casts are positive test results for protein, leukocyte esterase, and nitrite (if certain bacteria are present).

Bacterial Casts

Bacterial casts are casts composed of

bacteria in a protein (hyaline) matrix. The bacteria might be closely packed, diffusely scattered, or concentrated in an area of the cast; WBCs are also frequently present. Bacterial casts may be misidenti- fied as granular or cellular casts. They may be identified by Gram staining the dried (cytocentrifuged) sediment. Bacter- ial casts are diagnostic of acute pyelonephritis or intrinsic renal infection.

Epithelial Casts

Epithelial casts are composed of renal

epithelial cells [

I2]. A serious pathologic

finding, epithelial casts are associated with acute tubular necrosis, viral disease such as infection with cytomegalovirus, and exposure to nephrotoxic substances such as mercury, ethylene glycol, and var- ious drugs (the same conditions associated with renal epithelial cells). They may be difficult to distinguish from WBC casts, especially as they begin to degenerate into granular casts. Other sediment findings and clinical symptoms are helpful.

Reagent strips show positive reactions for

protein, but test results for leukocyte es- terase are negative unless sufficient neu- trophils are also present.

Granular Casts

Granular casts are generally the result

of degeneration of cells in cellular casts

I3]. Their significance lies with the cast

from which they were formed. Granular casts may also be the result of direct ag- gregation of serum proteins and other sub- stances into a matrix of Tamm-Horsfall mucoprotein. In general, the presence of granular casts suggests stasis in the nephron. The casts are associated with tubulointerstitial disease.

Waxy Casts

Waxy casts represent the final stage

in the degeneration of cells within acast - from cellular to granular to waxy

I3]. They suggest renal stasis or

nephron obstruction and represent seri- ous disease. Associated with severe chronic renal disease and renal amyloi- dosis, waxy casts are only rarely seen in acute renal disease. They are often seen as broad casts (having a greater diameter than most casts). Such casts probably form in the larger collecting tubules or dilated tubules where they are able to form when there is significant stasis and tubular atrophy. For this reason, they have been referred to as renal failure casts.

Waxy casts are homogeneous, like

hyaline casts, but they are more refrac- tile, with sharper outlines, and they tend to have broken or blunt ends and fissures or cracks along the sides. It is important that waxy casts not be confused with fibers from disposable diapers or other contaminants. The presence of protein on the reagent strip and lack of polariza- tion of the waxy cast with polarized light help with this distinction. Fatty Casts

Fatty casts contain globules of fat,

either as triglyceride or neutral fat, which stains with fat stains, or as choles- terol, which will polarize as a Maltese cross [

I4a, I4b]. Fatty casts are associ-

ated with oval fat bodies and free fat and with massive proteinuria (300 mg/dL [3g/L] or more on reagent strips). The fatty cast may contain free globules of fat or oval fat bodies. These findings are associated with the nephrotic syndrome.

Fatty casts may also be found in patients

with diabetic nephropathy or toxic renal poisoning.

Other Casts

There are a variety of other casts

that may or may not be of clinical sig- nificance. Myoglobin casts are of clini- cal importance because they occur with myoglobinuria as a result of acute mus- cle damage, which may result in acute renal failure. They appear much like a hemoglobin cast, but they are dark red to brown because of the myoglobin pigment. Associated findings in the uri- nalysis are a positive reagent-strip test for "blood" (actually myoglobin) with- out the presence of RBCs in the sediment.

Casts may also be stained with

bilirubin or intensely colored drugs such as phenazopyridine. Such staining is helpful in the microscopic visualization of these casts. Hemosiderin casts may be seen in the urine sediment 2 or 3 days after an acute hemolytic episode. The ?your lab focus? [I2] Epithelial cell cast (Sedi-Stain, ×400). [I3] Waxy cast (A) and cellular-to-granular cast (B) (×100.) [I4] a, Fatty cast and dysmorphic RBC (arrow) (×400). b, Same fatty cast as in image 4a viewed with polarized light, showing the

Maltese cross appearance of cholesterol

(×400). A B abDownloaded from https://academic.oup.com/labmed/article/32/4/191/2504211 by guest on 13 June 2023 laboratorymedicine>april 2001>number4>volume32 193
?Practice parameters, including standards, practice guidelines, and options, directing clinical decision making ?Rationales for practice parameters ?How practice parameters aredeveloped ?The effectiveness of practiceparameters and barriers to theiracceptance coarse yellow-brown granules of hemo- siderin stain blue with a Prussian blue stain for iron. Hemosiderin granules may also be seen in the urine sediment, either free or within macrophages or renal ep- ithelial cells, as the result of intravascular hemolysis.

Finally, crystal casts of various uri-

nary crystals have been described. There is disagreement about whether they are casts or merely crystals adhering to a cast form or artifacts. The clinical signifi- cance is that of the crystal in question.

Conclusion

Casts in the urine sediment have

various origins or mechanisms of forma-

tion and clinical relevance. The presenceof casts of all types indicates renal in-volvement. Casts represent different dis-ease states; eg, RBC casts are mostassociated with glomerular disease,whereas WBC casts are indicative oftubular disease, especially infection andacute pyelonephritis. Casts must beproperly identified so that meaningfuldiagnoses can be made.

1. Rose BD, Rennke HG. Renal Pathophysiology -

The Essentials. Baltimore, MD: Williams &

Wilkins; 1994:200.

?your lab focus?

Suggested Reading

College of American Pathologists. Surveys

Hematology Glossary. Northfield, IL: College

of American Pathologists; 1999.

Haber MH. Urinary Sediment: A Textbook

Atlas.Chicago, IL: ASCP Press; 1981.

Henry JB, Lauzon RL, Schumann GB. Basic

examination of urine. In: Henry JB, ed.

Clinical Diagnosis and Management by

Laboratory Methods. 19th ed. Philadelphia,

PA: Saunders; 1996.

Linné JJ, Ringsrud KM. Clinical Laboratory

Science: The Basics and Routine Techniques.

4th ed. St Louis, MO: Mosby; 1999.

Ringsrud KM, Linné JJ. Urinalysis and Body

Fluids: A ColorText and Atlas. St Louis, MO:

Mosby; 1995.Downloaded from https://academic.oup.com/labmed/article/32/4/191/2504211 by guest on 13 June 2023

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