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SOP

Urine Microscopy (Sediment Examination)

SOP for the microscopic examination of centrifuged urine sediment to identify formed elements like cells, casts, crystals, and microorganisms.

Last verified 1 month ago 2 min read Urinalysis and Body Fluids Microscopy Laboratory Techniques Specimen Examination #microscopy #urinalysis #urine microscopy #urine sediment #casts #crystals #formed elements
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Document Control
Version
1.0
Effective
Aug 29, 2026
Reviewed
Aug 29, 2026
Department
Urinalysis and Body Fluids
Principle

Centrifugation concentrates the insoluble, formed elements present in urine into a small volume of sediment. Microscopic examination of this sediment allows for the direct visualization and identification of cells, casts (formed in the renal tubules), crystals, and microorganisms, providing critical diagnostic information about the location and nature of renal or lower urinary tract pathology.

Standard Operating Procedure: Urine Microscopy

1. Purpose and Scope

To microscopically examine centrifuged urine sediment to identify and semi-quantify formed elements (RBCs, WBCs, epithelial cells, casts, crystals, and microorganisms) that may not be detected by chemical analysis.

2. Specimen Requirements

  • Type: Mid-stream clean-catch urine (first-morning preferred).
  • Volume: Minimum 10-15 mL.
  • Stability: Examine within 2 hours. If delayed, refrigerate (2-8°C) and bring to room temperature before processing.
  • Rejection: Contaminated or improperly labeled samples.

3. Safety and Precautions

  • Standard PPE. Urine is a biohazard.
  • Centrifuge with balanced, securely capped tubes to prevent aerosol generation.

4. Equipment and Reagents

  • Centrifuge (capable of 400-500 × g).
  • Conical centrifuge tubes (15 mL).
  • Glass slides and coverslips.
  • Light microscope with 10x and 40x objectives (400x total magnification).
  • Optional: Sternheimer-Malbin or Sedi-Stain for enhanced morphology.

5. Step-by-Step Procedure

  1. Mix the urine specimen thoroughly by gentle inversion.
  2. Pour 10-12 mL of urine into a conical centrifuge tube.
  3. Centrifuge at 400-500 × g for 5 minutes.
  4. Decant the supernatant quickly and completely, leaving approximately 0.5 mL of sediment at the bottom.
  5. Resuspend the sediment by gently tapping the bottom of the tube or using a pipette.
  6. Place one drop (approx. 20 µL) of the resuspended sediment onto a clean glass slide and cover with a coverslip.
  7. Examine under the microscope:
  • 10x objective: Scan the entire coverslip to identify casts (which are often located at the edges) and get a general overview.
  • 40x objective: Identify and count specific elements (RBCs, WBCs, epithelial cells, crystals, bacteria, yeast).
  1. Count the number of each element per High Power Field (HPF, 40x) or Low Power Field (LPF, 10x).

6. Quality Control

  • Verify centrifuge speed periodically with a tachometer.
  • Ensure consistent volume of sediment resuspended (e.g., exactly 0.5 mL) for semi-quantitative accuracy.

7. Decision limits (reporting thresholds per HPF)

  • RBCs: 0-2 / HPF
  • WBCs: 0-5 / HPF
  • Epithelial cells: Few squamous cells are normal; renal tubular cells are abnormal.
  • Casts: 0-2 hyaline casts / LPF. Other casts are generally abnormal.
  • Crystals: Few amorphous urates/phosphates may be normal; others require clinical correlation.

8. Reporting Results

  • Report elements semi-quantitatively (e.g., Rare, Few, Moderate, Many, or specific range like 10-15 / HPF).
  • Flag significant findings (e.g., dysmorphic RBCs, WBC casts, renal tubular epithelial cells, bacteria with WBCs) for clinical attention.

Frequently Asked Questions

First-morning urine is more concentrated and acidic, which better preserves formed elements like casts and cells that might lyse in dilute, alkaline urine.
WBC casts indicate that the inflammation or infection is originating in the kidney itself (e.g., pyelonephritis or interstitial nephritis), rather than the lower urinary tract.