A Closer Look at Urine Casts
Urine sediment analysis is frequently performed as a component of routine urinalysis in veterinary laboratories. While identification of cellular elements and crystals may become routine to most, urine casts can be challenging to discern. This article will discuss the mechanism of cast formation and provide guidelines for microscopic identification of casts.
Fundamentals of Cast Formation
Casts are the only formed element of urine that originate solely in the kidney.1 Four criteria are necessary for cast formation: high salt concentration, acidic pH, reduced tubular flow rate, and a matrix mucoprotein. Tamm-Horsfall mucoprotein is an albuminous mucoprotein secreted in small amounts by normal tubular epithelial cells in the loop of Henle, distal tubule, and collecting ducts; these are the sites from which most casts form. Precipitation of Tamm-Horsfall mucoprotein is the first event in cast formation. It provides a meshwork of fibrils to which crystals, cells, or cellular elements surrounding the mucoprotein can adhere.2,3
Since the cast is formed by using the renal tubular lumen as its mold, it is cylindrical, with parallel sides and consistent diameter throughout the length. In general, the cast has rounded or tapered ends and is several times longer than it is wide, although the exact shape and length of the cast depend on the morphology of the renal tubular lumen from which it is formed.3 Variation in cast size and shape may contribute to low precision for cast identification in some laboratories.1
Types of Urinary Casts
Hyaline Casts
Hyaline casts consist of Tamm-Horsfall mucoprotein and are colorless, semitransparent cylinders (unstained) with rounded ends; they contain no cells. These casts are best viewed microscopically with subdued light.
During prolonged stasis, the hyaline cast serves as the structural unit to which crystals, cells, or cellular elements attach, thus reflecting the malady of the nephron from which the cast is generated.3 They are named according to the predominant element that they contain. The following are examples:
Erythrocyte Casts
Erythrocyte casts form when red cells are present in the tubular lumen (Figure 1). These casts indicate intrarenal bleeding but may also be seen in glomerulonephritis. They are very fragile and appear orange-red to yellow-orange in fresh urine.
Leukocyte Casts
Leukocyte casts connote a suppurative process in the kidney; usually the predominant cell type is the neutrophil (Figure 2).
Epithelial Cell Casts
Epithelial cell casts result from desquamation of tubular cells that have not disintegrated (Figure 3). They occur in any disease that damages the tubular epithelium. If the epithelial cells have degenerated, these casts may be difficult to distinguish from leukocyte casts.
Granular Casts
Granular casts form from degeneration of epithelial cell casts (Figure 4). Epithelial cell demise causes cell boundaries to diminish and disintegrate, giving rise to opaque granules that vary in size and shape. These casts are called coarsely granular casts. Further disintegration of coarse granules results in fine granules attached to casts, termed finely granular casts, although the distinction between these two types of casts is clinically insignificant.3 Aside from size of granules, coarsely granular casts also are darker in color (often dark brown), and are typically shorter and more irregular in outline than finely granular casts; they frequently have broken ends. Finely granular casts contain granules that are grayish to pale yellow. In addition to tubular epithelial cell damage, granular casts can be associated with proteinuria of glomerular or tubular origin.
Fatty Casts
Fatty casts (lipid casts) are coarsely granular casts containing fat droplets that accumulate as a result of cell lipid degeneration (Figure 5). These droplets are highly refractile and may adhere to Tamm-Horsfall protein in degenerative tubular disease.
Waxy Casts
Waxy casts are colorless to gray and highly refractile (Figure 6). They are broader and appear denser than hyaline casts. The body of the cast may be convoluted or contain cracks or fissures; the ends are usually square. These casts are formed in the collecting tubule when the urine flow is decreased; considerable time and intrarenal stasis are necessary for formation of a waxy cast.
One popular theory is that granular, fatty, and waxy casts are products of epithelial cell casts in different stages of degeneration.3
Frequently Asked Questions
Q. What preanalytical variables help find urine casts?A. For sediment analysis, urine specimens should be centrifuged at 400g for 5 minutes.2 Excessive centrifugation speed or time may disrupt casts. In addition, urine specimens more than 2 hours old or alkaline pH may contribute to disintegration of urine casts.4Q. What is the best microscopic magnification to use?A. Microscopic analysis should be performed at low magnification (10×) with subdued light to delineate casts in the urine. View 10 fields, average the number found per field, and report the number and type of casts per low-power field.2Q. Which casts are commonly seen?A. Up to 2 hyaline and 1 granular cast per low-power field are considered normal in urine that is moderately concentrated. Excessive numbers of casts imply a disease process in the kidney.2
AcknowledgmentFigures 1, 2, 3A, 3B. Reprinted from Interpretation of Canine and Feline Urinalysis. Chew D, DiBartola SP-Wilmington, DE: The Gloyd Group Inc, for Ralston Purina, 1998, pp 26-28, with permission.
A CLOSER LOOK AT URINE CASTS • Carolyn Sink
References1. Basic examination of urine. McPherson RA, Ben-Ezra J, Ahao S. In McPherson RA, Pincus MR (eds): Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed-Philadelphia: Saunders Elsevier, 2007, pp 410-414.2. Laboratory Urinalysis and Hematology for the Small Animal Practitioner. Sink CA, Feldman BF-Jackson: Teton New Media, 2004, pp 8, 20, 27-30.3. Urine sediment: Under the microscope. Osborne CA, Stevens J. Urinalysis: A Clinical Guide to Compassionate Patient Care-Yardley: Veterinary Learning Systems, 1999, pp 136-141.4. Urine analysis. Archer J. In Villiers E, Blackwood L (eds): BSAVA Manual of Canine and Feline Clinical Pathology-Gloucester, UK: British Small Animal Veterinary Association, 2005, pp 156-157.