Microscopic urinalysis uses a microscope to examine urine sediment that contains cells, casts, crystals, and microorganisms. In some laboratories, microscopic analysis may not be part of the routine urinalysis, but may be performed only when physical and chemical screening tests yield positive results. Microscopic urinalysis provides information that may be diagnos-tically significant. An early morning, clean-catch specimen is the most accurate urine specimen for microscopic examination. The specimen should be examined as soon after collection as possible to prevent the deterioration of cells contained in the urine.
Sediment is obtained by placing 10 to 12 milliliters of urine in a test tube, which is subsequently centrifuged. The clear portion of the urine, called supernatant, is the liquid that rises to the top of the test tube. The supernatant is poured off and the remaining substance is the urine sediment. The urine sediment is placed on a slide and examined microscopically. Each urine sediment constituent is discussed individually.
The microscopic examination of urine sediment cells is performed to determine the presence of squamous epithelial cells, transitional epithelial cells of the bladder and renal pelvis, renal tubular epithelial cells, red blood cells, and white blood cells. The presence of renal tubular epithelial cells indicates serious pathologic conditions. The other cell types may be detected in small amounts, with greater numbers of these cells indicating a disease state.
Squamous epithelial cells are continuously sloughed off from the lining of the lower urinary tract and vagina and thus are normally found in urine. If many squamous epithelial cells are present, specimen contamination with vaginal secretions during the collection procedure is suspected.
Transitional epithelial cells line the kidney pelvis, bladder, and (in males) the proximal urethra. These epithelial cells are smaller, rounder, and thicker than squamous epithelial cells.
Normal Findings. A few transitional epithelial cells are present in urine samples from healthy individuals.
Variations from Normal. Increased numbers of transitional epithelial cells are seen in bladder infections. Clusters or sheets of these cells indicate a urinary tract lesion.
Renal tubular epithelial cells line the tubules of the nephrons and also the collecting ducts of the kidney. These cells can be round or cuboidal and have a distinct nucleus. Cuboidal-shaped cells have one flat side and originate in the collecting ducts.
Normal Findings. Absence of renal tubular epithelial cells.
Variations from Normal. The presence of renal tubular epithelial cells indicates renal damage, acute tubular necrosis, glomerulonephritis, or pyelonephritis. Figure 6-1 compares the appearance of squamous epithelial cells, transitional epithelial cells of the bladder and kidney pelvis, and renal tubular epithelial cells.
Although small numbers of erythrocytes and leukocytes are occasionally present in urine, an increased presence raises concern. Larger than expected numbers of red blood cells can be associated with renal and genitourinary disorders as well as urinary tract tumors or lesions. Medications and strenuous exercise may also cause hematuria.
A few white blood cells, primarily neutrophils, are normally found in urine. A large number of leukocytes in the urine is called pyuria and is usually a sign of bacterial infection. Increased leukocytes are also seen in urinary tract inflammatory diseases and urinary calculi. Contamination of the urine specimen may also present an increased number of white blood cells.
Normal Findings. Only a few white or red blood cells are normally present in urine.
Variations from Normal. Increased numbers of red blood cells can be seen in urinary tract and kidney diseases. Urinary tract infections or specimen contamination demonstrate an increase in white blood cells.
Interfering Circumstances. Contaminated urine specimens may negatively affect the reliability of the urine test. Exercise, drugs, and smoking may cause red blood cells to be present in the urine.
The types of casts in urine sediment can be identified under a microscope. Casts are formed when protein accumulates and precipitates in the lumen of the renal tubules and assumes the shape of the tubule. Anything that is present in the renal tubules is entrapped within the cast as it is formed. Casts are a means to examine the contents of the renal tubule. Very few casts are found in normal urine and large numbers of casts can indicate renal disease.
Casts are classified according to the substances trapped in them. Three generic types of casts are hyaline casts, which are made up of mucoprotein, the substance found in all connective and supportive tissue; granular casts, which are made up of cell remnants; and cellular casts, which contain epithelial cells, erythrocytes, and leukocytes. Miscellaneous types of casts include broad casts, fatty casts, and waxy casts. These casts are named for their appearance and composition.
An occasional hyaline cast is normally present in urine sediment and is composed of clear, glasslike proteins. Hyaline casts can be caused by vigorous exercise and must be differentiated from other sediment casts. Figure 6-2 shows the difference between hyaline, granular, and cellular casts.
Granular casts contain fragments of disintegrated cells that appear as fine or coarse granules embedded in the protein of the cast. Granular casts are not disease specific, but their presence in urine indicates renal tubular damage.
Cellular casts may contain red blood cells, white blood cells, and epithelial cells. The presence of red blood cell casts in urine is always a sign of disease, primarily of the glomerulus. Glomerular inflammation and bleeding is associated with red blood cell casts. White blood cell casts indicate inter-
stitial inflammation or infection and can distinguish an upper urinary tract infection from a lower urinary tract infection.
Epithelial cell casts result from the destruction of the cells lining the renal tubules. The presence of these casts represents serious nephrotic disease or damage.
Other types of casts include broad casts, fatty casts, and waxy casts. Their presence always indicates renal disease. Broad casts are found only in renal failure, fatty casts indicate fatty degeneration of renal tubular epithelium, and waxy casts are present in several renal diseases.
Normal Findings. Normal urine is free of casts except for occasional hyaline casts.
Variations from Normal. The presence of any cellular, granular, waxy, broad, or fatty casts in urine sediment indicates the need for additional diagnostic testing. These casts are all associated with serious renal diseases, disorders, or infections.
Interfering Circumstances. Casts are difficult to see and can be confused with other substances in urine sediment such as mucus, hairs, molds, and marks or scratches on the microscopic slide.
The microscopic examination of urine sediment crystals is performed to confirm the presence and type of crystals in the sediment. Crystals are formed from salts in the urine. A wide variety of crystals can be found in normal urine and for the most part are clinically insignificant. However, there are some rare forms of crystals that indicate metabolic disorders. Abnormal urine sediment crystals are made up of cystine, a nonessential amino acid found in protein; tyrosine, an amino acid synthesized from the essential amino acid phenylalanine; and sulfonamide, a synthetic medication that is used in treating bacterial infections.
Normal Findings. Presence of normal crystals. Absence of abnormal crystals.
Variations from Normal. Medications and metabolic disorders can cause the formation of crystals that represent abnormal urine sediment. Cystine crystals are associated with cystinuria, an amino acid disorder that can lead to renal tubule damage. Tyrosine crystals indicate liver disease or an inherited amino acid disorder. Sulfonamide crystals, though rarely seen in urine sediment, can be present in sulfonamide therapy.
Interfering Circumstances. Improper collection of the urine specimen may negatively affect identification of urine sediment crystals.
The microscopic examination of urine sediment microorganisms involves the examination of the sediment to determine the presence of bacteria, yeast, and protozoa. Microorganisms should not be present in properly collected, fresh, normal urine. The presence of a moderate or large number of microorganisms is indicative of disease.
Normal Findings. Microorganism are not present in normal urine.
Variations from Normal. The presence of bacteria in urine sediment may indicate infection or inflammation. The most common form of yeast found in urine sediment is Candida albicans, which is associated with yeast infection. Trichomonas vaginalis, shown in Figure 6-3, is a protozoan identified in urine sediment. It causes trichomoniasis, a sexually transmitted disease characterized by a copious, foul-smelling vaginal discharge.
Interfering Circumstances. Urine specimens may be contaminated by external sources such as improper handling of the specimen, vaginal secretions, or fecal materials.
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