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/