The microscopic examination of urine, or urine sediment analysis, is a standard diagnostic procedure within a complete urinalysis. This process identifies and quantifies the solid components, or “formed elements,” suspended in the liquid urine. These elements include cells, protein-based cylinders (casts), crystals, and microorganisms. By concentrating and viewing these microscopic particles, healthcare professionals gain insight into the health of the patient’s kidneys and the rest of the urinary tract.
Preparing the Sample for Viewing
Converting a raw urine specimen into a slide suitable for viewing requires a process of concentration to make the formed elements visible. The first step involves placing a measured amount of the well-mixed sample, typically 10 to 15 milliliters, into a conical centrifuge tube. A balancing tube is also placed opposite the sample to ensure the centrifuge spins smoothly and safely.
The tube is then spun at a high speed, usually around 400 RCF (relative centrifugal force) for about five minutes. This force causes the heavier solid particles—the cells, casts, and crystals—to fall to the bottom of the tube, forming a compact mass known as the sediment or “pellet.” The clear liquid portion remaining above the pellet, called the supernatant, is carefully poured off or suctioned out.
It is standard practice to leave a small, residual volume of the supernatant, often between 0.5 and 1.0 milliliter, to keep the sediment moist. This remaining liquid is used to gently re-suspend the pellet. Finally, a single drop of this concentrated sediment is placed onto a glass slide and covered with a thin coverslip, creating the wet mount ready for microscopic analysis.
Expected Findings Under the Lens
A healthy individual’s urine sediment contains several components considered normal, especially when present in low numbers. Cellular elements are commonly seen, including epithelial cells shed from the lining of the urinary and genital tracts. Squamous epithelial cells, the largest type, often originate from the external urethra or vagina; excessive numbers may suggest the sample was not collected cleanly.
Transitional epithelial cells come from the bladder, ureters, or renal pelvis. While a few are expected, a greater quantity might indicate irritation or inflammation in the lower tract. Thin strands of protein known as mucus threads may also be visible, forming a pale, wavy matrix across the slide. Mucus threads are composed of Tamm-Horsfall protein and mucin and are generally considered a non-pathological finding.
Trace amounts of blood cells are acceptable. Finding up to two red blood cells (RBCs) or up to four white blood cells (WBCs) per high-power field (HPF) is typically within the normal range. These low numbers do not necessarily signal disease and can occur after strenuous exercise or minor irritation.
The most common type of cast is the hyaline cast, which is a translucent cylinder made purely of Tamm-Horsfall protein. Hyaline casts are not always a sign of kidney disease and can be found in a healthy person, particularly after dehydration or physical activity.
Crystals are also a frequent sight, most often calcium oxalate, which appear as tiny squares with an “X” pattern, or amorphous urates and phosphates. The formation of these crystals is often influenced by diet, temperature changes after collection, or urine pH. Their presence alone does not usually indicate a medical problem.
Findings That Signal Disease
The presence of certain elements, or an excessive concentration of normal components, strongly suggests an underlying medical condition. An abnormally high count of white blood cells, called pyuria, often exceeds five WBCs per HPF. Pyuria, especially when accompanied by visible bacteria or yeast, is a strong indicator of a urinary tract infection (UTI) or inflammation.
Red blood cells found in high numbers, known as hematuria, signal bleeding in the urinary tract, which can be caused by kidney stones, trauma, or a tumor. The presence of pathological casts is significant because these cylindrical molds are formed exclusively within the kidney’s microscopic tubules. Cellular casts, such as red blood cell casts, are a definitive sign of bleeding originating within the filtering units of the kidney, suggesting a condition like glomerulonephritis.
White blood cell casts confirm that an infection or inflammatory process, such as pyelonephritis, is localized within the kidney tissue itself. Other casts represent the degeneration of cellular material over time, including granular casts and the more serious waxy or broad casts. Broad casts form in dilated tubules, and their presence, along with waxy casts, often signals advanced or chronic kidney failure.
Finally, certain crystal types are almost always associated with disease. Cysteine crystals, which are typically hexagonal, indicate the rare genetic disorder cystinuria. Tyrosine and leucine crystals can suggest severe liver disease, providing a direct clue to the nature and location of the patient’s pathology.