Monday, July 9, 2012

urine analysis- part 1: procedure & normal values


Urine Analysis Procedure

For a routine urine analysis, a fresh (<1 hr old), clean-catch urine sample is acceptable. If the analysis cannot be performed immediately, refrigerate the sample. (When urine stands at room temperature for a long period, casts and red blood cells undergo lysis, and the urine becomes alkalinized with precipitation of salts.)
  • Pour 5–10 mL of well-mixed urine into a centrifuge tube.
  • Check for appearance (color, turbidity, odor). If a urine sample looks grossly cloudy, it is sometimes advisable to examine an unspun sample. If an unspun sample is used, make note that you have done so. In general, for routine urine analysis, a spun sample is more desirable.
  • Spin a capped sample at 3,000 rpm for 3–5 min.
  • While the sample is in the centrifuge, use the dipstick (Chemstrip, etc.) to perform the dipstick evaluation on the remaining sample. Read the results according to the color chart on the bottle. Allow the correct amount of time before reading the test (usually 1–2 min) to avoid false results. Chemstrip 10 provides 10 tests (specific gravity, pH, leukocytes, nitrite, protein, glucose, ketone, urobilinogen, bilirubin, and blood). Other strips may provide less. Agents that color the urine (eg, phenazopyridine [Pyridium]) may interfere with the reading. Dipstick specific gravity is also available on some assay strips.
  • Decant and discard the supernatant. Mix the remaining sediment by flicking it with your finger and pouring or pipetting 1 or 2 drops onto a microscope slide. Cover with a coverslip.
  • Examine 10 low-power fields (LPFs; 10× objective) for epithelial cells, casts, crystals, and mucus. Casts are usually reported as number per low-power field and tend to collect around the periphery of the coverslip.
  • Examine several high-power fields (HPFs; 40× objective) for epithelial cells, crystals, RBCs, WBCs, bacteria, and parasites (trichomonads). RBCs, WBCs, and bacteria are usually reported as number per high-power field.
Normal Urine Analysis Values
  • Appearance: Yellow, clear, or straw-colored
  • Specific gravity:
    • Neonate: 1.012
    • Infant: 1.002–1.006
    • Child and adult: 1.001–1.035 (with normal fluid intake 1.016–1.022)
  • pH:
    • Newborn/neonate: 5–7
    • Child and adult: 4.6–8.0
  • Negative for bilirubin, blood, acetone, glucose, protein, nitrite, leukocyte esterase, reducing substances
  • Trace: Urobilinogen
  • RBC: The exact definition of microscopic hematuria is debated, but is generally defined as >3 RBC/HPF (40×).
  • WBC: 0–4/HPF
  • Epithelial cells: Occasional
  • Hyaline casts: Occasional
  • Bacteria: None
  • Crystals: Some limited crystals, based on urine pH (see below)

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