Nephropathology Essentials – Urine Microscopy by Dr. Seltzer

Nephropathology Essentials – Urine Microscopy by Dr. Seltzer

In this session of Nephropathology Essentials, Dr. Seltzer provided a comprehensive review of the art and science of urine microscopy. Our Moderator’s Notes are derived from his live presentation, which you can watch here: https://glomcon.org/general-gn/urine-microscopy/

Moderator’s Notes
Author: Dr. Pravir V. Baxi
Editors: Dr. Ali Poyan Mehr

Key points:

  • Review of the urinary sediment can (1) provide insight into the etiology of a patient’s acute kidney injury (AKI) and potentially help differentiate between the various causes, (2) help guide the decision to pursue a kidney biopsy and (3) assess the renal response to therapy
  • Staining
    • Sternheimer-Malbin (SM) stain facilitates identification of WBCs, epithelial cells, and casts
    • Sudan III stain (not used routinely) helps identify lipids and is used an adjunct to polarization
  • Microscope
    • X100, x400 and x1000 magnification options are recommended
    • Condenser: focuses the light on a single point in the plane of view; different illumination modalities are changed via the modification of the light coming from the condenser
    • Use glass slides and coverslips – not plastic
    • Verify proper illumination of the specimen - Kohler Illumination (= a process through which the microscope setup is optimized to provide the best visual quality)
      • Produces uniform bright light which focuses on the specimen
      • Restricts light exposure of the specimen to the observed field
  • Illumination modalities – if available, utilize all four
    • Bright Field
      • Simplest of all optical microscopy illumination techniques
      • Darker sample on a bright background
      • Provides the best resolution in a stained specimen
    • Dark Field
      • Excludes the direct light from the image and thus the field around the specimen is dark
      • Helps illuminate unstained or transparent elements against a dark background
      • Lower refractive index elements are seen more readily via this modality (such as lipids, crystals, and casts)
      • Provides lower resolution than the Bright Field modality
    • Phase Contrast
      • Enhances contrast of transparent and colorless objects by altering the optical path of light and thus objects will shine out of contrast to adjacent structures
      • Good for identifying dysmorphic RBCs
    • Polarized Light
      • Helps with viewing specimens that are visible primarily due to their optically anisotropic character
      • Useful for the identification of lipids, crystals, and contaminants (starch, synthetic fibers)
  • Urine Sediment
    • Cells
      • Size can help differentiate the type
        • Cell diameter: Squamous epi cell > transitional epi cell > renal tubular cell > neutrophil > erythrocyte > bacteria
      • RBCs
        • Normal RBCs will have a round appearance
        • Acanthocytes are seen in glomerular hematuria
        • Not all dysmorphic RBCs are acanthocytes, and while acanthocytes are considered to be relatively specific for glomerular hematuria, dysmorphic RBCs, in general, are rather non-specific and include crenated RBCs, schistocytes, poikilocytes, etc.
        • RBC mimics that may be visible under the microscope: yeast, air bubbles, starch, calcium oxalate, neutrophils, sperm, fat droplets, and pollen
      • Neutrophils
        • Found in infection and/or inflammation
        • These cells have a granular cytoplasm and a segmented multi-lobed nucleus
        • The SM stain facilitates visualization of the segmented nucleus
        • Glitter Cells – pale staining WBCs with granular motility but these are a non-specific finding
    • Lipids
      • Oval fat bodies are renal tubular epithelial cells containing fat globules
        • They are formed when lipids coalesce into refractile globules on absorption by renal tubular cells
        • These oval fat bodies can also be found within a cast
        • Sudan III stains are helpful in that it colors the lipids orange
    • Casts
      • Formed via solidification of Tamm-Horsfall mucoprotein with other cells/structures within the renal tubules
      • Hyaline casts: most common, not pathologic, normal in concentrated urine
      • Granular casts
        • Can result from the breakdown of cellular casts or degenerative products of tubular cells and proteins
        • Classified as fine or coarse, but this has no diagnostic significance
        • These are indicative of acute tubular necrosis (ATN)
      • Pigmented casts: hemoglobin, myoglobin, bilirubin, and drug pigments
      • WBC casts
        • Indicative of inflammation or infection
        • WBC casts can be present in patients with proliferative glomerulonephritis (GN)
      • RBC Casts
        • Signify the presence of a proliferative GN or vasculitis process
      • Renal Tubular Epithelial casts: indicative of ATN
      • Waxy casts: seen in chronic renal failure and thought to represent the end product of cast revolution
      • Lipid casts: seen in nephrotic syndrome
      • Pseudo-casts: cylindrical appearing structures that resemble a true cast
        • Formed when cells or particles adhere to a mucous thread
    • Crystals
    • Artifacts

Selected References:
Fogazzi, G. B., Ponticelli, C., & Ritz, E. (1999). The Urinary Sediment: An Integrated View. Oxford University Press.
Haber, M. H., Blomber, D., Galagan, K., Glassy, E. F., & Ward, P. C. (2010). Color Atlas of the Urinary Sediment: An Illustrated Field Guide Based on Proficiency Testing. College of American Pathologists.
Seltzer, J., Velez, J. C., Buchkremer, F., & Tesser, J. A. (n.d.). Renal Fellow Network (RFN) . Retrieved from https://www.renalfellow.org/category/urine-sediment-of-the-month/.