Microscopic examination

Centrifuged urine sediment examined under the microscope may be helpful in showing evidence of renal disease, as opposed to lower UTI, and in indicating the type and activity of a renal or disease condition. Microscopic findings should be checked against chemical findings before a final
report is issued.

Examining urinary sediment requires as much standardization as possible in order to maximize accuracy. Fresh urine, optimally concentrated rather than dilute, is centrifuged for five minutes, with the supernatant poured off and the sediment resuspended in the remaining few drops of urine. A drop of this unstained material is examined under a coverslip. Low power is used to detect casts that usually are found around the periphery. Further examination is done under high power to identify cast and cell types, crystal elements, and other objects. At least 10 high-power fields (hpfs) should be examined. Other elements are also reported. Staining sediment can make examination easier.


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Normal sediment may have some formed elements, including a few RBCs, WBCs, or epithelial cells, as well as a hyaline cast or a cellular cast. Automated evaluation of sediment is being standardized and may be widely available soon.

RBCs: In theory, there is no detectable blood in normal urine, but RBCs may be seen, even in healthy individuals. An RBC count ≥3/hpf in two of three urine samples is usually accepted as hematuria. Hematuria of renal origin is often associated with casts in the urine and almost always associated with significant proteinuria.

Exercise-induced hematuria is a benign condition often associated with distance running (>10 km). Glomerular hematuria is typically associated with significant proteinuria, RBC casts, and dysmorphic RBCs. Nonglomerular renal hematuria is secondary to tubulointerstitial, renovascular, or metabolic disorders. Urinalysis in nonglomerular hematuria is distinguished from that of glomerular hematuria by the presence of circular RBCs and the absence of RBC casts. Causes of nonglomerular hematuria include tumors, stones, infections, trauma to the urinary tract, renal disease (glomerulonephritis, pyelonephritis), renal or ureteral calculi, bladder lesions (carcinoma, cystitis), prostatitis, prostatic carcinoma, menstrual contamination, hematopoietic disorders (hemophilia, thrombocytopenia), anticoagulant or aspirin use, eclampsia, systemic lupus erythematosus, sickle cell nephropathy, and cirrhosis. Hematuria without RBC casts or significant albuminuria suggests the possibility of renal or bladder cancer.

The American Urological Association (AUA) Best Practice Policy Panel on Microscopic Hematuria has concluded that because of the lack of specificity of urinary dipstick testing, as well as the risk and expense of evaluation, a positive dipstick test should be followed by complete evaluation for hematuria only if ≥3 RBCs/hpf are seen on microscopic evaluation.1 The mainstays of evaluation, says the panel, are voided urinary cytology, cystoscopy, and urinary tract imaging using ultrasonography, CT, and/or IV urography. 

WBCs: Visible under both low and high power, the WBCs seen in urine are typically segmented neutrophils. Pyuria is usually present in bacteriuria; pyuria without bacteriuria may indicate interstitial nephritis or renal tuberculosis.2 Monocytes in the urine soon after kidney transplantation can indicate rejection. Eosinophils may be diagnostic of acute interstitial nephritis, but the accuracy of this finding is uncertain. Urinary lymphocytes can be seen in diseases that cause lymphocytic infiltration of the kidney, e.g., chronic tubulointerstitial disease.

Casts: Casts originate from the distal convoluted tubule or collecting duct of the kidney during periods of urinary concentration, stasis, or low urinary pH. Named for their content, casts define the area of pathology to a specific location in the genitourinary tract. There is no rapid test for detection of casts, making microscopic evaluation critical in many situations.

RBC casts are always pathologic and indicate acute inflammatory or vascular disease in the glomerulus, causing hematuria, renal infarction, or collagen disease. WBC casts may be found in glomerulonephritis, nephrotic syndrome, or pyelonephritis. Epithelial cell casts can be normal, but many such casts indicate excessive cell desquamation, as is seen in amyloidosis, nephrosis, eclampsia, allograft rejection, or toxic poisoning. Granular casts represent plasma protein aggregates that pass into the tubules from damaged glomeruli or remnants from cellular casts. These casts may be normal in exercise or dehydration, but >1/hpf may indicate pyelonephritis or chronic lead intoxication. Waxy casts are seen in chronic renal diseases, including nephron obstruction, tubular inflammation or degeneration, and chronic renal failure. Hyaline casts are formed by a protein gel and may be normal; transient with position, dehydration, or fever; or permanent, secondary to glomerular capillary damage.6 When found in numbers  >1/hpf, hyaline casts may suggest proteinuria of renal origin.Fatty casts come from lipid-laden renal tubular cells incorporated into the cast matrix. These are seen with heavy proteinuria, such as nephrotic syndrome.

Crystals: A variety of crystals may be found, depending on urinary pH. Most are not pathological. Common crystals, which can be seen in healthy patients, include calcium oxalate, triple-phosphate crystals, and amorphous phosphates.

Additional findings: Spermatozoa are sometimes seen following ejaculation. Yeast cells may be contaminants or a sign of infection. Epithelial cells may be normally found, but >1/hpf may indicate active tubular degeneration, acute necrosis, or necrotizing papillitis.

Urinalysis as a screening test

No evidence-based recommendations advocate urinalysis for screening asymptomatic patients. The Institute for Clinical Systems Improvement believes discontinuation of routine urinalysis should be considered.7,8 The American Academy of Family Physicians specifically recommends against urinalysis to screen for bladder cancer or bacteriuria in asymptomatic persons.9 The U.S. Preventive Services Task Force advises against routine screening of men and nonpregnant women for asymptomatic bacteriuria but strongly recommends that all pregnant women be screened for asymptomatic bacteriuria using urine culture at 12-16 weeks’ gestation.10

Dr. Wallace is assistant instructor of family medicine and Dr. Sadovsky is associate professor of family medicine at SUNY Downstate Medical Center, Brooklyn, N.Y.

References

  1. Walsh PC, Retik A, Vaughan ED, Wein A, eds. Campbell’s Urology. 8th ed., Philadelphia, Pa.: WB Saunders; 2002:98,100,104,107,108.
  2. Rakel RE. Textbook of Family Practice. 6th ed. Philadelphia, Pa.: WB Saunders; 2002:1279,1280. 
  3. Brenner BM. Brenner and Rector’s The Kidney. 7th ed. Philadelphia, Pa.: WB Saunders; 2004:1121,1122.
  4. Noble J. Textbook of Primary Care Medicine. 3rd ed. St. Louis, Mo.: Mosby; 2001:1347-1349, 1367-1369.
  5. Cheng I, Zaas A. The Osler Medical Handbook. 1st ed. St. Louis, Mo.: Mosby; 2003:736.
  6. Vaughan G. Understanding and Evaluating Common Laboratory Tests. 1st ed. Stamford, Conn.: Appleton & Lange; 1999:24.
  7. Preventive services for children and adolescents. Bloomington, Minn.:
    Institute for Clinical Systems Improvement (ICSI); 2004. Available at www.icsi.org. Accessed March 11, 2005.
  8. Preventive Services for Adults. Bloomington, Minn.: Institute for Clinical Systems Improvement (ICSI); 2004. Available at www.icsi.org.
  9. Summary of policy recommendations for periodic health examinations. Leawood, Kan.: American Academy of Family Physicians; 2003.
  10. U.S. Preventive Services Task Force (USPSTF). Screening for asymptomatic bacteriuria: recommendation statement. Rockville, Md.: Agency for Healthcare Research and Quality (AHRQ); 2004.