Microscopic preparation of urine and cellular findings

Microscopic examination of urine consists of categorizing and counting cells, casts, crystals, and miscellaneous constituents in the sediment of a urine sample. The sediment is obtained after a measured portion of urine is centrifuged. The sediment will be pushed to the bottom of the tube containing urine. The sediment will be prepared for a microscopic exam, and the remaining liquid urine will be poured off and disposed of properly.

Many formed elements are found in the urine. Some are significant, and others are not. Most important, the microscopic examination should correlate with the physical and chemical analyses. For example, if the physical examination of the urine showed a reddish color and the chemical reagent strip tested positive for blood, then seeing red blood cells during the microscopic examination would be consistent with the physical and chemical results. Medical assistants should be familiar with the preparation of urine specimens for this test and with the possible test results.

Microscopic preparation of urine

To perform the microscopic UA procedure, a laboratory must be certified to perform CLIA Provider-Performed Microscopy Procedures (PPMPs), a subcategory of CLIA moderate-complexity laboratories. Quality assurance is just as important in the microscopic examination as in the chemical analysis of urine. To ensure consistency and standardization, commercially available systems can be used, such as the KOVA System or the UriSystem. These systems may include specially designed, graduated centrifuge tubes with devices or pipettes that allow easy decanting of supernatant and retention of an exact amount of sediment. They also use specially designed plastic slides with wells or coverslips that accept only a given volume of sediment. Control solutions containing preserved cells are also available from KOVA. This type of solution also provides quality control for cell identification. Whatever system is used, the Clinical and Laboratory Standards Institute (CLSI) recommends the following:

• The urine volume should be 12 mL.
• The specimen should be centrifuged for 5 minutes at a relative centrifugal force of 400 g (i.e., 400 times normal gravity).
• A standardized slide should be used to view the sediment.
• A consistent reporting format should be used.

When a urine sample is centrifuged, the clear upper portion of the specimen is called the supernatant. It is poured off, and a drop of the well-mixed sediment at the bottom of the centrifuged tube is examined under a microscope. The sediment may be stained to give greater contrast to the formed elements. The stain assists in the identification of formed elements by improving the detail of cellular structures.

Microscopic examination of urine

The examination of urine is not categorized as CLIA-waived; therefore it cannot be performed by a medical assistant without additional training, supervision, and rigid compliance with CLIA quality assurance protocols for the laboratory. Periodic proficiency testing must be successfully completed to maintain a PPMP laboratory certification.

The three main categories of microscopic findings are casts, cells, and crystals.

Casts

Casts are created when protein accumulates and precipitates in the kidney tubules and is washed into the urine. The protein takes on the size and shape of the tubules, forming the casts. Casts are cylindric, with flat or rounded ends, and are classified according to the substances observed inside them. Certain types of casts are connected to specific renal diseases and disorders. Other casts are physiologic and are generally caused by strenuous exercise. Casts can dissolve in alkaline urine if the sample is not examined promptly. The microscopic exam should take place as soon as possible after specimen collection. The following paragraphs describe some of the casts that may be seen, but these are not all the possible casts. The presence of casts should be determined using a low-power objective such as the 10x.

Hyaline casts are pale, transparent, cylindric structures with rounded ends and parallel sides. They are formed when urine flow through individual nephrons is diminished. They can be found in the urine of individuals with kidney disease but also in the urine of people without such disease who have exercised heavily. Hyaline casts will be missed entirely if the light is not adjusted properly. Occasionally, hyaline casts have granular or cellular inclusions.

White blood cell casts are hyaline casts that contain leukocytes. White blood cells seen in casts usually have a multilobed nucleus. White blood cell casts are seen in pyelonephritis.

Red blood cell casts always indicate a pathologic condition and are highly diagnostic. These casts occur in glomerulonephritis. They are hyaline casts with embedded red blood cells. Their presence indicates damage to the glomerulus. They may appear brown due to the color of the red blood cells present.

Renal tubular epithelial cell casts contain embedded renal tubular epithelial cells. These casts are easily confused with white blood cell casts, particularly if the cells have started to degenerate. Renal tubular epithelial cell casts are found when excessive damage has occurred in the kidney. Causes include shock, renal ischemia , heavy-metal poisoning, certain allergic reactions, and nephrotoxic drugs.

Finely and coarsely granular casts may indicate renal disease. On close examination, granular casts show a hyaline cast with coarse or fine granular inclusions. The granules are thought to be caused by protein clusters or the breakdown of cellular inclusions.

Waxy casts are rarely seen. They appear as glassy, brittle, smooth, homogeneous structures. They usually are yellowish, have cracks or fissures, and have squared or broken ends. They are considered to be cellular casts that have broken down and are found in individuals with severe renal disease.

Occasionally more than one type of cell is found in a single cast. Mixed cellular casts have been reported, and it may be difficult to absolutely identify the cell types present. Cast identification is not a CLIA-waived procedure.

Cells

Cells found in the urine include epithelial cells, which come from the lining of the genitourinary tract. Red blood cells and white blood cells, which come from the bloodstream, also may be seen. Cells are classified and counted under high-power magnification.

Red blood cells may enter the urinary tract at any point of inflammation or injury. They may be found in normal urine in small numbers. Persistent hematuria should be investigated. Red blood cells are smaller than white blood cells and have no nucleus. If they are in hypotonic (dilute) urine, they swell and burst. In hypertonic (concentrated) urine, they may be bumpy or wrinkle. Another term for this shape is crenated.

Yeast cells in the urine may indicate vaginal contamination or a urinary yeast infection. Yeast is common in the urine of patients with diabetes. Yeast cells are oval shaped and may show budding. Adding a drop of potassium hydroxide (KOH) will assist in distinguishing red blood cells from yeast, as it will cause all red blood cells to lyse but not yeast.

White blood cells may occasionally be found in normal urine, but increased numbers are associated with a UTI or vaginal contamination during specimen collection. White blood cells are larger than red blood cells and have a granular appearance. They may have a multilobed nucleus. Most white blood cells in the urine are neutrophils, but further testing will be needed to actually determine cell type.

Squamous epithelial cells line the lower portion of the genitourinary tract. When present in large numbers in female patients, they usually indicate vaginal contamination. Squamous epithelial cells are large, flat, and irregular. They have a single, small, round, centrally located nucleus and often occur in sheets or clumps.

Transitional epithelial cells line most of the urinary tract. They are round or oval and may have a tail. Occasionally, two nuclei are seen. They may be seen in diseases of the urinary system.

Renal tubular epithelial cells are somewhat larger than white blood cells, are round or oval, and have a nucleus that is single, large, oval, and sometimes eccentric. A few may be found in normal urine specimens, but their presence in increased numbers indicates tubular damage of the nephrons.