Urinary sediment is the solid material that settles at the bottom of a urine sample after it has been spun in a centrifuge. When viewed under a microscope, this material provides a direct look at particles from the urinary tract. This examination is a component of a complete urinalysis, complementing the physical and chemical analysis by allowing for the direct visualization of microscopic elements. This process helps identify and quantify components not normally present in significant numbers.
The Urinary Sediment Examination
Examining urinary sediment begins with collecting a fresh urine sample, ideally the first or second morning urine, to ensure the elements are well-preserved. In the laboratory, 10 to 15 milliliters of the urine is placed into a conical tube. This tube is then spun in a centrifuge at high speeds, around 1,500 to 2,000 revolutions per minute (rpm), for about five minutes.
This rapid spinning separates the urine into a liquid top layer called the supernatant and a pellet of solid material at the bottom, which is the sediment. The supernatant is poured off, leaving the concentrated sediment and a small amount of residual urine. This mixture is then gently resuspended by flicking the tube or using a pipette.
A drop of this resuspended sediment is placed onto a glass microscope slide and covered with a coverslip. The slide is then ready for microscopic analysis. The examination starts at a low power magnification (100-200x) to scan for larger elements, followed by a switch to high power (400x) for detailed identification. Some laboratories may use a special stain to make the elements easier to see.
Cellular Components
The microscopic examination of urinary sediment reveals various cells, including red blood cells (RBCs), white blood cells (WBCs), and epithelial cells. Seeing more than a few RBCs can indicate bleeding somewhere along the urinary tract. These cells appear as small, pale discs under the microscope. Their presence, a condition known as hematuria, can be associated with issues ranging from kidney stones and infections to conditions affecting the kidneys or bladder.
White blood cells, which are part of the body’s immune system, may also be present. A small number of these cells is not unusual, but an increased count, known as pyuria, often signals inflammation or infection. A high WBC count points toward a urinary tract infection (UTI), especially when bacteria are also observed. These cells are larger than RBCs and have a granular appearance.
The sediment also contains epithelial cells, which line the urinary tract. It is normal for some of these cells to be shed into the urine. Their shape can suggest their origin within the urinary tract. While a few squamous epithelial cells are common and indicate contamination from the skin, an abundance of other types, like transitional or renal tubular cells, may suggest conditions affecting the bladder or kidneys.
Urinary Casts
Urinary casts are cylindrical molds formed within the kidney’s tubules. They are composed of a protein called Tamm-Horsfall, which is secreted by the tubule cells. Because they are formed inside the kidney, their presence in urine is a direct indicator of conditions affecting the kidney itself. The shape of the cast provides a microscopic snapshot of the tubule where it originated.
Hyaline casts, composed only of protein, are the most common type. Seeing a few hyaline casts can be normal, especially after strenuous exercise or dehydration, but large numbers may point to kidney disease. Their appearance is clear and colorless, making them difficult to see without adjusting the microscope’s contrast.
Other casts contain cellular elements trapped within the protein matrix, offering more specific diagnostic clues. Red blood cell casts, for example, are a sign of bleeding from the glomeruli, the kidney’s filtering units, and are associated with glomerulonephritis. White blood cell casts are indicative of inflammation or infection within the kidney, such as pyelonephritis. The presence of these cellular casts is always an abnormal finding.
Crystals and Microorganisms
Crystals and microorganisms are also common findings. Crystals are solid, geometric structures that form when urine is supersaturated with certain minerals. Their presence is not always a cause for concern, as many types, such as calcium oxalate and uric acid, can be found in the urine of healthy individuals. Factors like diet, hydration status, and urine pH can influence their formation.
However, large numbers of certain crystals can be associated with a higher risk of developing kidney stones. For instance, an abundance of calcium oxalate crystals might be seen in individuals prone to forming this common type of stone. In some cases, specific types of crystals, like cystine or leucine, can indicate metabolic or liver disorders.
Microorganisms, primarily bacteria and yeast, can also be identified in the sediment. While some bacteria may be present due to contamination during sample collection, their presence with a significant number of white blood cells is a strong indicator of a urinary tract infection (UTI). Yeast may also be seen, particularly in individuals with diabetes or those taking certain antibiotics.