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Urinary casts are microscopic cylindrical structures produced by the kidney and present in the
urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellular m ...
in certain disease states. They form in the distal convoluted tubule and
collecting duct The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct system is the last part of nephron and participates in electr ...
s of
nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure c ...
s, then dislodge and pass into the urine, where they can be detected by
microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of mic ...
. They form via precipitation of Tamm–Horsfall
mucoprotein A mucoprotein is a glycoprotein composed primarily of mucopolysaccharides. Mucoproteins can be found throughout the body, including the gastrointestinal tract, reproductive organs, airways, and the synovial fluid of the knees. They are called muco ...
, which is secreted by
renal tubule The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure c ...
cells, and sometimes also by
albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins ...
in conditions of proteinuria. Cast formation is pronounced in environments favoring protein denaturation and precipitation (low flow, concentrated salts, low pH). Tamm–Horsfall protein is particularly susceptible to precipitation in these conditions. Casts were first described by
Henry Bence Jones Henry Bence Jones FRS (31 December 1813 – 20 April 1873) was an English physician and chemist. Early life Bence Jones was born at Thorington Hall, Stoke-by-Nayland, Suffolk, the son of Lieutenant Colonel William Jones, an officer in the 5t ...
(1813–1873). As reflected in their cylindrical form, casts are generated in the small distal convoluted tubules and collecting ducts of the kidney, and generally maintain their shape and composition as they pass through the urinary system. Although the most common forms are benign, others indicate disease. All rely on the inclusion or adhesion of various elements on a mucoprotein base—the hyaline cast. "Cast" itself merely describes the shape, so an
adjective In linguistics, an adjective ( abbreviated ) is a word that generally modifies a noun or noun phrase or describes its referent. Its semantic role is to change information given by the noun. Traditionally, adjectives were considered one of the ma ...
is added to describe the composition of the cast. Various casts found in urine sediment may be classified as:


Overview table

Main types of urinary casts:


Acellular casts


Hyaline casts

The most common type of cast, hyaline casts are solidified Tamm–Horsfall mucoprotein secreted from the tubular epithelial cells of individual nephrons. Low urine flow, concentrated urine, or an acidic environment can contribute to the formation of hyaline casts, and as such, they may be seen in normal individuals in dehydration or vigorous exercise. Hyaline casts are cylindrical and clear, with a low refractive index, so they can easily be missed on cursory review under brightfield microscopy, or in an aged sample where dissolution has occurred whereas, though, phase contrast microscopy leads to easier identification. Given the ubiquitous presence of Tamm–Horsfall protein, other cast types are formed via the inclusion or adhesion of other elements to the hyaline base.


Granular casts

The second-most common type of cast, granular casts can result either from the breakdown of cellular casts or the inclusion of aggregates of plasma proteins (e.g., albumin) or immunoglobulin light chains. Depending on the size of inclusions, they can be classified as fine or coarse, though the distinction has no diagnostic significance. Their appearance is generally more cigar-shaped and of a higher refractive index than hyaline casts. While most often indicative of chronic renal disease, these casts, as with hyaline casts, can also be seen for a short time following strenuous exercise. The "muddy brown cast" seen in
acute tubular necrosis Acute tubular necrosis (ATN) is a medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Because necrosis is often not present, the term acute tubular injury (ATI) is preferred by pathologists ...
is a type of granular cast.


Waxy casts

Thought to represent the end product of cast evolution, waxy casts suggest the very low urine flow associated with severe, longstanding kidney disease such as kidney failure. Additionally, due to urine stasis and their formation in diseased, dilated ducts, these casts are significantly larger than hyaline casts. * They are cylindrical. * They possess a higher refractive index. * They are more rigid, demonstrating sharp edges, fractures, and broken-off ends. Waxy casts are broad casts, which is a more general term to describe the wider cast product of a dilated duct, and are seen in chronic kidney failure. In nephrotic syndrome, many additional types of casts exist, including broad and waxy casts if the condition is chronic (this is referred to as a
telescopic urine A telescope is an instrument designed for the observation of remote objects. Telescope(s) also may refer to: Music * The Telescopes, a British psychedelic band * ''Telescope'' (album), by Circle, 2007 * ''The Telescope'' (album), by Her Space H ...
with the presence of many casts).Mundt and Shanahan (2011)


Fatty casts

Formed by the breakdown of lipid-rich epithelial cells, these are hyaline casts with fat globule inclusions, yellowish-tan in color. If cholesterol or cholesterol esters are present, they are associated with the "Maltese cross" sign under polarized light. They are pathognomonic for high urinary protein nephrotic syndrome.


Pigment casts

Formed by the adhesion of metabolic breakdown products or drug pigments, these casts are so named due to their discoloration. Pigments include those produced endogenously, such as hemoglobin in hemolytic anemia, myoglobin in
rhabdomyolysis Rhabdomyolysis (also called rhabdo) is a condition in which damaged skeletal muscle breaks down rapidly. Symptoms may include muscle pains, weakness, vomiting, and confusion. There may be tea-colored urine or an irregular heartbeat. Some of t ...
, and
bilirubin Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from t ...
in liver disease. Drug pigments, such as phenazopyridine, may also cause cast discoloration.


Crystal casts

Though crystallized urinary solutes, such as oxalates, urates, or sulfonamides, may become enmeshed within a ketanaline cast during its formation, the clinical significance of this occurrence is not felt to be great.


Cellular casts


Red blood cell casts

The presence of red blood cells within the cast is always pathological and is strongly indicative of
granulomatosis with polyangiitis Granulomatosis with polyangiitis (GPA), previously known as Wegener's granulomatosis (WG), is a rare long-term systemic disorder that involves the formation of granulomas and inflammation of blood vessels (vasculitis). It is a form of vasculitis ...
,
systemic lupus erythematosus Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Commo ...
,
post-streptococcal glomerulonephritis Acute proliferative glomerulonephritis is a disorder of the small blood vessels of the kidney. It is a common complication of bacterial infections, typically skin infection by ''Streptococcus'' bacteria types 12, 4 and 1 (impetigo) but also afte ...
, or
Goodpasture's syndrome Goodpasture syndrome (GPS), also known as anti–glomerular basement membrane disease, is a rare autoimmune disease in which antibodies attack the basement membrane in lungs and kidneys, leading to bleeding from the lungs, glomerulonephritis, ...
. They can also be associated with renal infarction and subacute
bacterial endocarditis Infective endocarditis is an infection of the inner surface of the heart, usually the valves. Signs and symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cell count. Complications ...
. They are a yellowish-brown color and are generally cylindrical with sometimes ragged edges; their fragility makes inspection of a fresh sample necessary. They are usually associated with
nephritic syndrome Nephritic syndrome is a syndrome comprising signs of nephritis, which is kidney disease involving inflammation. It often occurs in the glomerulus, where it is called glomerulonephritis. Glomerulonephritis is characterized by inflammation an ...
s or urinary tract injury.


White blood cell casts

Indicative of inflammation or infection, the presence of white blood cells within or upon casts strongly suggests pyelonephritis, a direct infection of the kidney. They may also be seen in inflammatory states, such as acute allergic
interstitial nephritis Interstitial nephritis, also known as tubulointerstitial nephritis, is inflammation of the area of the kidney known as the renal interstitium, which consists of a collection of cells, extracellular matrix, and fluid surrounding the renal tubul ...
, nephrotic syndrome, or post-streptococcal acute glomerulonephritis. White cells sometimes can be difficult to discern from epithelial cells and may require special staining. Differentiation from simple clumps of white cells can be made by the presence of hyaline matrix.


Bacterial casts

Given their appearance in pyelonephritis, these should be seen in association with loose bacteria, white blood cells, and white blood cell casts. Their discovery is likely rare, due to the infection-fighting efficiency of neutrophils and the possibility of misidentification as a fine granular cast.


Epithelial cell casts

This cast type is formed by inclusion or adhesion of desquamated epithelial cells of the tubule lining. Cells can adhere in random order or in sheets and are distinguished by large, round nuclei and a lower amount of cytoplasm. These can be seen in
acute tubular necrosis Acute tubular necrosis (ATN) is a medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Because necrosis is often not present, the term acute tubular injury (ATI) is preferred by pathologists ...
and toxic ingestion, such as from mercury,
diethylene glycol Diethylene glycol (DEG) is an organic compound with the formula (HOCH2CH2)2O. It is a colorless, practically odorless, and hygroscopic liquid with a sweetish taste. It is a four carbon dimer of ethylene glycol. It is miscible in water, alcohol ...
, or
salicylate Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the EPA Toxic Substanc ...
. In each case, clumps or sheets of cells may slough off simultaneously, depending on the focality of injury. Cytomegalovirus and viral hepatitis are organisms that can cause epithelial cell death, as well.


Eosinophilic cast

This type of cast contains
eosinophils Eosinophils, sometimes called eosinophiles or, less commonly, acidophils, are a variety of white blood cells (WBCs) and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. ...
. It is seen in tubulo interstitial nephritis and occurs in allergy, commonly to drugs such as
methicillin Methicillin ( USAN), also known as meticillin ( INN), is a narrow-spectrum β-lactam antibiotic of the penicillin class. Methicillin was discovered in 1960. Medical uses Compared to other penicillins that face antimicrobial resistance du ...
and NSAIDs.


References


Further reading

* Haber, Meryl H. ''Urinary Sediment: A Textbook Atlas''. American Society of Clinical Pathologists. Chicago, 1981. * Lillian Mundt, Kirsty Shanahan, ''Graff's Textbook of Routine of Urinanalysis and Body Fluids'', 2nd edition, Lippincott Williams & Wilkins, Philadelphia, 2011.


External links


Urine Casts
– different types and what they mean.

– Good description of various cast forms. {{Urine tests Nephrology