Hyaline Casts · What Are They, Causes, Diagnosis, Treatment, and More

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 function and can be assessed to help determine the cause of the kidney damage.   

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 and a set of kidney tubules. The glomerulus is a ball-shaped network of blood vessels involved in the formation of urine, whereas the tubules are a series of microscopic tubules that modify urine composition according to the body’s needs. 

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. The main function of this protein is to provide a protective effect to urothelial cells against infections and other agents, as well as preventing the formation of kidney stones, among other roles. 

Under certain conditions, Tamm–Horsfall protein can aggregate into a protein matrix that attracts the adhesion of other tubular particles (e.g., cells, bile, hemoglobin, albumin, immunoglobulins, etc.), forming a cylindrical shape that matches the shape of the kidney tubules. 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. On the other hand, non-cellular casts include hyaline casts, granular casts, fatty casts, and waxy casts, among others.  

Hyaline casts can be diagnosed by urinary sediment microscopy, which is performed as part of a 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 the 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.