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Urine Microscopy by Dr. Seltzer

Updated: Jun 7, 2023


GLOMERULAR DISEASE DIAGNOSIS



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





By Dr. Pravir Baxi




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/




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