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ATLAS - Casts

CD Chapter Outline:


Casts are cylindrically shaped casts of renal tubules, i.e. of the distal segment of the tubule, Henle's loop, and collecting duct. The relevance of casts depends on their localization. They are considered to basically consist of mucoproteins or Tamm-Horsfall protein, a urinary mucoid that is normally present in urine. Casts are formed in case of urine retention which leads to precipitation of urinary mucoids. The increased urinary protein and salt concentrations, and low urine pH contribute to cast formation. As protein precipitation depends on the composition and degree of urine concentration, casts are more easily formed in the distal part of the nephron and in the collecting duct of the kidney, where the urine concentration is high.
Urinary casts mostly are due to inflammatory lesions of the kidney, and may frequently be caused by previous irritation or hyperemia of the kidney. They may also occur in other organ diseases.
A finding of casts in the urine is almost regularly associated with a demonstrable presence of protein in the urine as well. Urinary casts are of a cylindrical shape, and of a varying length (50-150 µm), thickness (width: 5-50 µm) and composition. They have sharply demarcated, parallel rims, while their edges usually are rounded or sharply folded. The casts can be straight or twisted, and in some cases geniculated. The shape and size of casts depends on the renal duct in which they are formed.
According to their shape, appearance and origin, the casts are classified into epithelial, granular, waxy, hyaline, white blood cells, red blood cells, lipid, coma, and mixed casts.

 Hyaline casts
Hyaline casts consist of a homogeneous transparent basic substance. According to composition, they are pure Tamm-Horsfall protein. These casts are usually covered with a thick layer of amorphous urate or phosphate salts that dissolve with the addition of a drop of acetic acid, or are covered with fat droplets.
Hyaline casts generally are short and straight in shape; however, they may also be twisted and extremely elongated so as to extend all across the field or several fields. The variable lengths, parallel sides, rounded ends and cylindrical shape are typical of these casts They have no inclusions, or may have just one or two inclusions that are quite easily overlooked due to the low index of refraction. Their surface appears to be smooth or slightly wrinkled.
 Granular casts
Most probably, granular casts are hyaline casts with rough or fine granules evenly scattered over their surface. Therefore, they are classified into fine- and rough-granular casts. Granular casts have morphological features similar to hyaline casts, however, they are more easily detectable because of the higher refractivity due to the presence of granules. They generally are wider and larger than hyaline casts. The granules are plasma proteins or lysed cell fragments in origin. According to the appearance of granules, some authors distinguish finely (smaller and less dense granules) and roughly (larger and more dense granules) granular casts. Finely granular casts usually are shorter, wider, and less transparent than hyaline casts, and are of greyish or light-yellow color.
Roughly granular casts are darker, shorter, have uneven rims and frequently ragged edges. Granular casts are formed by tubular cell degeneration. There are roughly and finely granular casts, depending on the degree of cellular degeneration. As long as the renal epithelium is not severely damaged, only finely granular casts are found in the urinary sediment, while roughly granular casts occur in severe glomerulonephritis.
Granular casts point to the presence of renal disease, however, they are a non-specific indicator, as they do not indicate the nature and localization of the intrarenal lesion. Renal diseases accompanied by the excretion of granular casts can be of infective, ischemic or toxic origin (e.g., aminoglycosides). These casts are especially found in patients with glomerulonephritis, less commonly in those with pyelonephritis.
 Waxy casts
Waxy casts are rarely found in urinary sediment. These casts are formed in pathologically distended renal ducts, thus they are wider than other casts. Waxy casts consist of a homogeneous basic substance, however, as differentiated from hyaline casts, they have a waxy gloss and are yellow colored.
Waxy casts are simply indiscernible and unrecognizable because of the high index of refraction, which derives from their structure, predominated by plasma proteins rather than Tamm-Horsfall protein. They are wider than hyaline casts, their ends frequently been broken; they are smooth, with no inclusions on their surface, thus they appear yellowish, wax-like, and glittering. They are characterized by deep, sharp notches along their rims, and are acid resistant.
 Red cell casts
Red cell casts are formed by red blood cells binding to the cast surface, whereby the latter frequently lose their regular shape.
The basis of red cell casts is Tamm-Horsfall protein, on which intact, or more or less degraded red blood cells are found. The red blood cells may have a typical, recognizable appearance, or may be completely lysed. If the red blood cells have undergone lysis, the cast has an almost homogeneous structure of a characteristic red-brownish color of hemoglobin. The presence of red cell casts always points to acute lesion of the glomerular basal membrane (active glomerulonephritis).
 White blood casts
White blood cell casts are mucoprotein casts with incorporated white blood cells. They are always formed within the kidney parenchyma (tubules), while individual or small clusters of white cells may be derived from anywhere in the genitourinary tract. In unstained preparation of urinary sediment, white blood cell casts may occasionally be difficult to precisely identify under a light microscope.
White blood cell casts usually contain neutrophililc granulocytes, however, lymphocytes may also be found. They have a cylindrical shape and tightly packed with white blood cells. The number of neutrophils per cast varies. They may be thinly scattered or tight. White blood cells may be intact or altered due to degenerative changes. They usually are quite readily recognizable, however, they may also be mistaken for renal epithelial casts. White blood cell casts usually are not found in a large number, therefore the preparation should be thoroughly examined to find them. Identification of the white blood cell cast is facilitated with the use of the Sternheimer-Malbin stain. The nuclei of the white blood cell in the cast stain purple to orange color and are enmeshed in a pink hyaline matrix.
In acute infection, white blood cells reach the urine transglomerulally and transtubularly. White blood cell casts are mostly found in acute pyelonephritis, chronic pyelonephritis, chronic renal diseases, malignant hypertension, and all other infectious lesions of the kidneys. Occasionally, they are also found in immune disorders such as glomerulonephritis caused by immunocomplexes, however, usually in association with red cell casts.
 Renal epithelial casts
Epithelial casts are covered with renal epithelial cells which are very frequently degenerated to such an extent that they cannot be identified at all. Degenerative changes of the cells occur on the passage of renal epithelial cells through the nephron. They occur in renal tubular lesion.
Renal epithelial casts consist of mucoprotein matrix and renal epithelial cells. The cells can be of a normal appearance, round with eccentric nucleus and homogeneous cytoplasm, or degenerated. Degenerative changes of the cells manifest as an altered shape of the granules or as the occurrence of fat droplets.
 Lipid casts
Lipid casts are sprinkled with markedly round fat, strongly refracting droplets. Lipid casts have free fat droplets on their surface, which are readily recognized and can be verified by dissolving in ether, or by staining with Sudan III, or by examination under a polarizing microscope when these formations exhibit a characteristic appearance of so-called Maltese cross. Besides free fat droplets, such droplets may also be found within degenerated renal epithelial cells or so-called fat corpuscles.
 Mixed casts
Mixed casts are formed from more than one protein and may contain cellular or other inclusions. The term "mixed cast", however, is applied to casts having at least two distinct and often well-demarcated portions.
Mixed casts are often found consisting of epithelial cells, red blood cells and white blood cells at the same time. Most often a mixed cast is composed of one part of hyaline cast and the other part of epithelial cells, red blood cells or white blood cells.
 Broad casts
Broad casts are formed in collecting ducts consequentially to urine retention in the lower parts of intrarenal excretory ducts. Broad casts originate from distended tubules. Broad casts usually are covered with epithelial cells and are frequently waxy. Broad casts have frequently been described as renal casts (renal failure casts).
Besides true casts, cylindrical formations formed outside the kidney can also occur in urinary sediment. Although closely resembling casts, these are not true casts, and are called pseudocasts or cylindroids. Phosphate casts, urate casts and uric acid casts resemble granular casts.
Cylindroids are less sharply contoured, tapered, and split into filaments at the narrow end. The urate, phosphate and uric acid casts consist of amorphous phosphates, urates and uric acid crystals. They very closely resemble granular casts. A urate cast is a mixture of various uric acid salts. Masses of sodium urates may resemble granular casts. However, the urate casts may be differentiated by their yellowish-brown color and lack of outline. Casts of ammonium urate may be recognized by their small size and reddish-brown color. Bacterial casts are mostly made up of cylindrically shaped bacterial agglomerates.