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Hyaline Casts

What Are They, Causes, Diagnosis, Treatment, and More

Author: Anna Hernández, MD

Editors: Ahaana Singh, Lisa Miklush, PhD, RN, CNS

Illustrator: Jillian Dunbar

What are hyaline casts?

Hyaline casts are the simplest and most common type of urinary cast. Urinary casts are microscopic clusters of urinary particles, such as cells, fat bodies, or microorganisms, wrapped in a protein matrix and found in the urine. Urinary casts serve as clinical indicators of kidney condition and can be assessed to determine the functioning of the kidneys. 

The kidneys are two bean-shaped organs located in the abdomen, at either side of the lower spine. They’re composed of millions of filtering units called nephrons. Each nephron is composed of a glomerulus (a ball-shaped network of blood vessels involved in the formation of urine) and kidney tubules (a series of tubules that reabsorb and modify urine composition according to the body’s necessities).

Cast formation takes place in the final portion of the kidney tubules, which consists of the distal convoluted tubules and collecting ducts. It results from the precipitation of Tamm–Horsfall protein (also known as uromodulin) which is secreted by the epithelial tubule cells. Aggregation of Tamm–Horsfall protein into a protein matrix can then attract the adhesion of other tubular particles (e.g. cells, bile, hemoglobin, albumin, immunoglobulins). Once the urinary casts have developed, they can dislodge from the tubular lumen and travel through the urinary tract, before being excreted in the urine. 

Depending on the composition, urinary casts can be categorized into cellular casts and non-cellular casts. Examples of cellular casts include renal tubular epithelial casts, red blood cell (RBC) casts, and white blood cell (WBC) casts. Examples of non-cellular casts include hyaline casts, granular casts, fatty casts, and waxy casts, among others. 

What do hyaline casts in urine indicate?

Hyaline casts are considered to be a nonspecific finding— they can be found in both healthy individuals and individuals with pathological conditions. The presence of hyaline casts typically indicates a decreased or sluggish urine flow, which can be a result of strenuous exercise, diuretic medications, severe vomiting, or fever. In combination with other clinical findings, large amounts of hyaline casts may indicate kidney damage due to decreased blood flow to the kidneys. 

Are hyaline casts dangerous?

Hyaline casts are typically not considered a dangerous finding. Unlike hyaline casts, other types of urinary casts are generally associated with kidney disease. For instance, epithelial cell casts reflect severe damage and death of the tubule cells, which is also known as tubular necrosis. In most cases, red blood cell casts reflect damage to the glomerulus, known as glomerulonephritis, or the presence of a microscopic bleeding within the kidney. White blood cell casts are typically associated with an underlying kidney infection (such as pyelonephritis) or inflammatory processes. Finally, fatty casts are typically detected in individuals with nephrotic syndrome, which causes proteinuria (excess protein in the urine), along with tissue swelling, low blood protein levels, and increased blood levels of cholesterol.

Are hyaline casts normal in urine?

Normally, the presence of casts in the urine is considered to be an unusual finding. However, small amounts of hyaline casts (between 0–2 casts per low power field of the microscope) may be detected in the urine of healthy individuals without necessarily indicating a serious condition like kidney disease. In fact, hyaline casts are the only casts that should be detected in the urine in absence of kidney, or renal, disease. 

What causes hyaline casts?

Hyaline casts are solely composed of aggregated Tamm–Horsfall protein in the tubular lumen of the kidney. Precipitation of Tamm–Horsfall protein increases when the urine is acidic, has decreased or sluggish flow, or is highly concentrated. Consequently, it’s possible to detect a greater amount of hyaline casts after strenuous exercise, during treatment with certain types of diuretic medications, or in individuals with severe vomiting or fever (indicative of dehydration). In addition, hyaline casts may appear alone or in association with other types of casts in certain pathological conditions, including acute kidney injury.

How do you diagnose hyaline casts?

Hyaline casts can be diagnosed by urinary sediment microscopy, which is performed as part of the urine analysis (i.e. urinalysis). This test is performed by centrifuging a urine sample so that the suspended particles in the urine (e.g. casts, cells, pathogenic microorganisms), also known as urine sediment, are separated from the fluid. A small drop of the urine sediment is then placed on a glass slide to be examined under the microscope.

Under bright field microscopy, hyaline casts appear as clear, tiny tubule-shaped particles. They are often easily missed due to their low refractive index, or the lack of contrast between the surrounding urine and the hyaline casts. In most cases, the use of stains, reduced or dim lighting , and phase contrast microscopy allow for a better identification of hyaline casts. Phase contrast microscopy is an optical-microscopy technique that enhances the contrast between two mediums that present a similar refractive index in order to better identify organisms and particles. 

How do you treat hyaline casts?

In healthy individuals, hyaline casts are not always considered to be an unusual finding, and thus may not require treatment at all. However, if hyaline casts are detected in individuals with kidney damage, treatment may be necessary, and will depend on the specific cause of decreased renal function. For instance, treatment of acute kidney injury may include avoiding medications that may decrease kidney function, ensuring adequate hydration with intravenous fluid administration, and addressing the specific cause of kidney damage.

What are the most important facts to know about hyaline casts?

Hyaline casts are a type of urinary cast, which is a cluster of urinary particles, such as cells, fat bodies, or microorganisms, held together by a protein matrix and found in the urine. Depending on the composition, casts can be classified into different types, including hyaline casts, granular casts, renal tubular epithelial casts, and red or white blood cell casts, among others. Hyaline casts can be identified in both healthy individuals and those with certain pathological conditions. In most cases, they indicate a decreased or sluggish urine flow, which can be caused by dehydration from strenuous exercise, diuretic medications, or severe vomiting, or by decreased blood flow to the kidneys. Hyaline casts may be identified by microscopic analysis of the urine sediment, in which they can be seen as clear, tiny tube-shaped particles that are difficult to distinguish from the surrounding fluid. Treatment is typically not required unless kidney damage is suspected.

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