Urinary Casts: Definition & Types

Instructor: Artem Cheprasov
This lesson goes over the many different types of urinary casts. You'll learn what urinary casts are, where they form, and some of the reasons for why they might form.

Urinary Casts

When you think of a cast, you probably think of this big, clunking white thing surrounding a part of your body. But did you know that casts can come from inside your body!? And they are anything but big! It's crazy but true.

Urinary casts are tiny tube-shaped structures composed of various substances that are sometimes found in the urine of people with certain medical conditions.

Let's learn more about what they are and what kinds there are.

A Primer on Kidney Structures

So, that definition was a bit vague for simplicity's sake, but we had to keep it short at first. Let's expand on what urinary casts really are by first having a super-quick and basic overview of the kidney's microscopic anatomy.

Your kidneys are the structures in your body that filter your blood. The blood is filtered of whatever the body no longer needs, and all of that filtration that we expel when we run to the bathroom is what we call urine.

Each kidney has about 1,000,000 tiny filters, called nephrons. Each nephron contains a glomerulus, which filters the blood, and a series of ducts and channels connected to the glomerulus that are collectively called the renal (kidney) tubule. The renal tubule modifies the composition of the liquid that was filtered by the glomerulus.

As you just learned, the renal tubule is a long and winding series of ducts which people have sectioned into different names. The last segment of the renal tubule is called the distal convoluted tubule (DCT). The DCT eventually connects to a structure called the collecting duct, which channels the urine into further structures within the urinary system.

More on Urinary Casts

It is in the DCT and collecting duct where urinary casts are formed.

Because the nephron is a microscopic structure, the casts are microscopic as well. Because they are formed in the open space (lumen) of tubular structures, they are tubular in shape. They commonly have long and parallel sides with rounded ends. So, they sort of look like a capsule you might swallow if you're taking medication.

The urinary casts are important in medicine because they give us an overview of some of the microscopic conditions within the nephron when we examine them under a microscope.

Types of Urinary Casts

There are numerous different types of urinary casts. We won't have time in this lesson to get into the nitty-gritty of which conditions cause them to appear, exactly how that happens, and all of the distinguishing features of each cast type, but you will get an overview of them nonetheless.

Hyaline Casts

One type of urinary cast is known as the hyaline cast. This cast is the one we most frequently see in urine, and it is composed of a protein called uromodulin. Hyaline casts can be seen for numerous reasons, including but not limited to:

  • Dehydration
  • Strenuous exercise
  • Chronic (persistent) renal disease

RBC Casts

Another form of urinary cast is called the RBC cast, or red blood cell cast. The name gives away what they are about. These casts are, predictably, yellow-orange-red in color and are associated with bleeding that has occurred within the nephron itself. RBC casts can be seen in a condition known as glomerulonephritis, which refers to the inflammation ('-itis') of the glomerulus.

This inflammatory condition allows for red blood cells to pass into the filtrate, something normally impossible when the glomerulus is healthy. This is because the glomerulus contains pores of a limited size, which under healthy conditions do not allow large structures, like red blood cells, to pass into the renal tubule.

WBC Casts

Just as easy to understand, we can also encounter WBC casts, or white blood cell casts. These casts are composed of white blood cells, namely ones called neutrophils. These casts are most often seen in conditions such as pyelonephritis, or more simply: a kidney infection. This is often caused by bacteria called E. coli.

Bacterial Casts

Sometimes mixed alongside white blood cells, you'll find bacteria in cases of pyelonephritis. Thus, mixed bacterial/WBC casts can be present. Or, bacterial casts alone may be found, although they may be difficult to identify.

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