Microscopic Examination of Urine Sediment

Introduction to Microscopic Examination of Urinary Sediment

  • The third part of routine urinalysis:

    • Follows physical and chemical examination.
    • Purpose: detect and identify insoluble materials in urine.

  • Sources contributing formed elements to urine:

    • Blood
    • Kidney
    • Lower genitourinary tract
    • External contamination
Formed Elements Detected in Urine

  • Components detected include:

    • Red blood cells (RBCs)
    • White blood cells (WBCs)
    • Epithelial cells
    • Casts
    • Bacteria
    • Yeast
    • Parasites
    • Mucus
    • Spermatozoa
    • Crystals
    • Artifacts

  • Clinical significance:

    • Some components may be of no clinical significance.
    • Others are considered normal unless present in increased amounts.

  • Importance of examination:

    • Includes identification and quantitation of urinary sediment elements.
Procedural Variations in Microscopic Analysis

  • Variations that may affect results include:

    • Methods for sediment preparation
    • Volume of sediment examined
    • Methods and equipment for visualization
    • Reporting of results

  • Protocols have been developed for:

    • Increasing standardization
    • Enhancing cost-effectiveness of microscopic urinalysis

Macroscopic Screening

  • Goal: Enhance cost-effectiveness of urinalysis.
  • Laboratories may only perform microscopic examination on specimens meeting defined criteria.
Criteria for Microscopic Analysis

  • Primary reliance on abnormalities in physical and chemical urinalysis.

  • Term: "macroscopic screening."

  • Significant parameters considered may include:

    • Color
    • Clarity
    • Presence of:
    • Blood
    • Protein
    • Nitrite
    • Leukocyte esterase
    • Possibly glucose

  • Programming of criteria into automated instruments is common practice.

Importance of Laboratory-designated Criteria

  • Variability observed in percentages of abnormal specimens missed across studies.

  • Consideration of patient population is critical when developing macroscopic screening protocols:

    • Pregnant women
    • Pediatric patients
    • Geriatric patients
    • Diabetic patients
    • Immunocompromised patients
    • Renal patients

  • Clinical and Laboratory Standards Institute (CLSI) recommends:

    • Microscopic examination when physician requests.
    • Testing specific patient populations.
    • Whenever abnormal physical or chemical results are found.

Specimen Preparation

  • Timing of examination:
    • Specimens should be fresh or adequately preserved.
Disintegration of Formed Elements

  • Formed elements (RBCs, WBCs, hyaline casts) disintegrate quickly, especially in dilute alkaline urine.

  • Refrigeration effects:

    • May cause precipitation of amorphous urates and phosphates.
    • Formation of nonpathological crystals can obscure sediment elements.

  • Warming specimens to 37°C before centrifugation can dissolve some crystals.

Clean-catch Midstream Specimen
  • Minimizes external contamination of sediment.
  • Dilute random specimens can yield false-negative results.
  • Importance of thoroughly mixing specimens before decanting for centrifugation.
Technical Tip 7-1
  • It is advisable to warm refrigerated urine specimens to 37°C before centrifuging to dissolve amorphous urate crystals.

Specimen Volume

  • Standard urine volume for centrifuge:

    • Typically 10 to 15 mL.
    • Average used is often 12 mL, facilitating immersion of multiparameter reagent strips.

  • For volumes less than 12 mL (as with pediatric cases), it should be noted:

    • Allowing physician corrections of results.
    • Example correction: If only 6 mL is centrifuged, results are multiplied by 2 to adjust for volume.

Centrifugation

  • Importance of consistent centrifuge operations:

    • Speed and centrifugation time should be standardized.
    • Recommended: centrifugation for 5 minutes at relative centrifugal force (RCF) of 400.
    • Results in optimal sediment with minimal damage to elements.

  • Note on centrifuge specification:

    • RCF (rather than revolutions per minute - RPM) accounts for differences in centrifuge head sizes to ensure consistency in sediment production.