Microscopic Urinalysis Notes - Part 1

Equipment and Standardization

  • Purpose: microscopic analysis of urine sediment after standardized preparation.
  • Standardized centrifuge tube and standardized pipette for urines.
  • Bulb pipette used to withdraw and resuspend sediment: blocks off one mL of urine; rest of urine is dumped off.
  • Standardized microscopic slides with slots: urine is placed on slides so the urine wicks through the slot; allows analysis of multiple samples per slide (e.g., up to 10 urines per slide) and provides consistent view across sections.
  • Commercial automated systems exist which: load urine, centrifuge, analyze sediment, and show standardized images; increases standardization and reduces user-error compared to manual methods.
  • Volume considerations:
    • Ideally use 12 ext{ mL} of urine for processing.
    • If less than that, it may be acceptable, but if urine is < 3 ext{ mL}, a comment such as QNS (quantity not supplied) is appropriate.
  • Unspun urine: many labs allow microscopic examination on unspun urine; some comments may reflect this.
  • Centrifugation parameters:
    • Typical setting: 400 ext{-} 500 imes g for 5 ext{ min}.
  • Sediment handling:
    • The sediment concentration should be equivalent to 1 ext{ mL} of urine.
    • Resuspend the pellet/sediment to standardize the view.
    • The volume of sediment viewed should be about one drop; automated systems may standardize this more precisely.
  • Volume checks and standardization:
    • The volume of sediment viewed and the technique should be standardized to enable reliable comparisons across samples.
  • Microscopy objectives:
    • Low-power (e.g., 10× objective) used to examine casts and epithelial cells.
    • High-power objective used to examine the rest of the sediment.
  • Staining and visualization options:
    • Various stains to visualize elements: supervital stain, Sternmeyer Malbun stain, and related variants.
    • Toluidine blue or other nuclear/cytoplasmic stains can help differentiate cell types.
    • Acetic acid can be used to lyse red blood cells and highlight white blood cells and epithelial cells.
    • Fat/lipid stains such as Sudan III (SUDN3) or Oil Red O to identify lipid content; helps differentiate triglycerides vs cholesterol.
  • Stains mentioned (names as stated in transcript):
    • supervital stain
    • Sternmeyer Malbun stain
    • tulidine blue
    • acetic acid
    • SUDN3 or oil red O stain
    • Gram stain
    • Prussian blue stain
    • Hansel stain (methylene blue and ESNY and methanol)
  • Phase contrast, polarization, and interference contrast:
    • Phase contrast improves visualization of refractive index variations; useful for identifying cellular elements.
    • Bright-field is the standard but phase contrast provides better detail for some structures.
    • Polarizing microscopy is especially useful for detecting crystals (e.g., uric acid, cholesterol) due to birefringence; yellow when light is parallel to crystals and blue when perpendicular.
    • Interference contrast (Nomarski) provides a strong 3D-like image but is expensive and not commonly used in many traditional labs.
  • Quick visual comparison example:
    • Left: bright-field image of a cast; Right: phase-contrast image of the same cast showing clearer structural details.
  • Summary: Modern practice combines standardization, appropriate centrifugation, proper sediment handling, and a suite of imaging and staining techniques to maximize reliability and interpretive accuracy.

Urine Sediment Components and Normal Ranges

  • Red blood cells (RBCs):

    • Morphology: small, concave discs with a darker center; moderately refractile.
    • Variants depending on urine tonicity:
    • Hypertonic (concentrated): crenated RBCs (spiky projections).
    • Hypotonic (dilute): ghost cells (swollen, difficult to see).
    • Rare sickle cells possible in sickle cell disease.
    • Normal range: typically zero to three RBCs per high power field (HPF) on average across many HPFs.
    • Correlation: if RBCs are elevated microscopically, expect a positive dipstick for blood; correlate with patient factors.
    • False positives/negatives on dipstick can occur due to ascorbic acid, myoglobin, etc. (correlative assessment needed).
    • Clinical significance: presence of RBCs can be normal in menstruation or after catheterization; high levels suggest hematuria (hematuria can indicate glomerulonephritis, pyelonephritis, cystitis, urolithiasis, tumors, trauma, contamination).
    • Differentiation from look-alikes:
    • Yeasts show budding and should be distinguished from RBCs.
    • Air bubbles and oil droplets are highly refractile and lack the RBC concave disc appearance.
    • Differentiation methods mentioned:
      • Staining with Sternmeyer Malbun (to differentiate RBCs vs other round elements).
      • Polarizing microscopy: calcium oxalate crystals polarize (yellow or blue depending on orientation) while RBCs do not.
      • Acetic acid lysis: lyses RBCs but not yeast or calcium oxalate; helpful for counting WBCs when heavy RBC presence exists.
      • Starch appearance: starch is highly refractile with a puckered/dimpled center; RBCs are concave with a central area of pallor.
      • Yeast appearance: ovoid and buds.

  • White blood cells (WBCs / leukocytes):

    • Morphology: larger than RBCs with a granulated appearance; can appear singly or in clumps.
    • Normal range: typically 0–8 WBCs per HPF.
    • Correlation: positive dipstick leukocyte esterase generally indicates WBCs; however, sometimes leukocytes may be present microscopically with a negative dipstick depending on sample and degradation; staining may be needed to differentiate.
    • Significance: presence often indicates infection or inflammation (pyelonephritis, cystitis, urethritis, prostatitis); leukocyturia may also occur in nonbacterial inflammatory conditions like glomerulonephritis.
    • Note: lymphocytes lack leukocyte esterase; if leukocytes are suspected but esterase is negative, consider staining to identify cell types.
    • Hypotonic urine can cause glitter cells (WBCs that appear to lyse in hypotonic conditions); hypertonic urine can cause shrinkage.

  • Epithelial cells:

    • Squamous epithelial cells: the largest epithelial cells; have a relatively small nucleus compared to cytoplasm.
    • Indicate contamination if numerous; normal to see a few, but large numbers suggest non-ideal sample.
    • Edges may curl or fold; contamination correlates with mixed bacterial presence.
    • Transitional (urothelial) cells: found higher up in urinary tract; align the renal calyces, pelvis, ureters, bladder, and male urethra.
    • Size smaller than squamous cells; nucleus-to-cytoplasm ratio is larger; often described as resembling a fried egg (nucleus like yolk, cytoplasm like white).
    • A few are normal; larger numbers or sheets may be seen in catheterized specimens if the catheter wall is nicked or lineings slough.
    • Renal tubular cells: possible to see a few; increased numbers suggest tubular disease.
    • Variants: oblong/cigar-shaped, round/circular, or cuboidal forms; high nucleus-to-cytoplasm ratio.
    • Associated with acute ischemic/toxic renal disease, nephritis, transplant rejection, etc.
    • Oval fat bodies: renal tubular cells engorged with fat; slough into urine.
    • Always pathologic; associated with glomerular dysfunction and nephrotic syndrome; accompanied by proteinuria and casts.
    • Polarizing microscopy can show a Maltese cross pattern due to fat droplets.

  • Crystals in urine (general):

    • Crystals form more readily in concentrated urine; factors include dehydration, high solute load, stable urine pH, and low urine flow.
    • Clinical significance varies: many crystals are non-significant if urine is freshly voided or if crystals form due to storage conditions (refrigeration).
    • Important to review and correlate with 10 HPF and 10 low-power fields before reporting.
    • Crystals may adhere to casts; differentiate when crystals are seen in isolation vs attached to casts.
    • Common theme: most crystals are not clinically significant unless accompanied by other findings or clinical context.

  • Crystals: overview of clinically significant and common crystals

    • Acidic crystals (common in acidic urine):
    • Amorphous urates: small yellow-brown crystals; not clinically significant; dissolve on heating to ~60^ ext{°C} or in alkali.
    • Amorphous phosphates (amorphous phosphate): small clusters found in alkaline urine; soluble in acid; not significant; differentiate from bacteria by morphology.
    • Acid urates: small yellow-brown spheres similar to leucine; dissolve at ~60^ ext{°C}; not clinically significant.
    • Monosodium urate: slender needle-like crystals; not clinically significant; dissolve at ~60^ ext{°C}.
    • Uric acid crystals: variable shapes (diamond, rectangular/barrow-like shapes); birefringent under polarized light with color changes; present when urine pH < 5.7; clinically significant mainly in gout or certain drug exposures.
    • Calcium oxalate crystals: envelope/dumbbell shapes; colorless; common and often non-significant; may be seen in normal urine or after ethylene glycol exposure (antifreeze) or in severe renal disease; differentiate from others by polarizing microscopy and other tests.
    • Bilirubin crystals: correlate with bilirubin on dipstick; associated with yellow-orange urine; clinically significant.
    • Cystine crystals: hexagonal/crystal shapes; indicate congenital cystinuria or cystinosis; confirm with cyanide-nitroprusside reaction turning purple.
    • Tyrosine crystals: fine delicate needles; indicate overflow aminoaciduria; clinically significant; confirm via appropriate testing.
    • Leucine crystals: round spheres with internal striations; indicate overflow aminoaciduria; clinically significant; confirm.
    • Cholesterol crystals: large sheets with notched edges; not necessarily clinically significant but may indicate nephrotic syndrome.
    • Fat and related lipid crystals (fatty casts/oval fat bodies): indicate nephrotic-range proteinuria; often present with cholesterol; correlate with protein content.
    • Drug-related crystals:
    • Ampicillin crystals: long needle-like shapes.
    • Sulfonamide crystals: bow-tie or “pasta-like” shapes.
    • Radiocontrast media crystals: long needles sometimes with notches; generally associated with recent imaging procedures; characterize by very high specific gravity and absence of protein or fat in urine.
    • Alkaline crystals (common in alkaline urine):
    • Amorphous phosphate: small clusters; soluble in acid; not clinically significant.
    • Triple phosphate (coffin lids): classic shape; common; not usually clinically significant; may be seen with UTIs or renal colic.
    • Calcium phosphate: comes in multiple shapes; including rosette-like prisms (dibasic calcium phosphate) and flat plates (monobasic calcium phosphate); pH-dependent appearance; more common in alkaline urine.
    • Ammonium biurate (thorny apple): yellow-brown spheres with spiny projections; dissolve in acid or heat; not usually significant.
    • Calcium carbonate: dumbbell-shaped crystals; not clinically significant; differentiate from RBCs with acetic acid (RBCs lyse; calcium carbonate does not); polarizing microscopy can help differentiate from bacteria.
    • Other notes on crystals:
    • Heat and acid dissolution tests can help differentiate certain crystals (e.g., amorphous urates dissolve with heat/alkali; cholesterol is not dissolved this way).
    • The clinical significance of crystals depends on the context: a freshly voided urine with crystals can be significant (e.g., uric acid crystals in gout-associated nephropathy); crystals formed after refrigeration are typically not clinically significant.

Staining and Special Visualization Techniques

  • Supervital stain: used to highlight certain cellular components in urine sediment.
  • Sternmeyer Malbun stain: used to differentiate cellular elements (helps distinguish RBCs and other elements).
  • Toluidine blue (toluidine blue): enhances nuclear detail and cytoplasmic differences for better cell differentiation.
  • Acetic acid: lyses RBCs; helps emphasize white blood cells and epithelial cells; aids in accurate WBC counting by removing RBCs.
  • Sudan III (Sudan III) or Oil Red O stains: identify lipids/fat droplets; useful for oval fat bodies and lipid-containing casts.
  • Gram stain: allows identification to the level of bacteria; caution: cannot determine Gram status automatically in urine without further testing; general approach is to report bacteria categories (e.g., Gram-positive rods, Gram-positive cocci, etc.) only when interpretation supports it.
  • Prussian blue stain: detects hemosiderin (iron-containing pigment) in casts or epithelial cells.
  • Hansel stain (methylene blue and ESNY with methanol): primarily used to identify eosinophils; eosinophils may be present in acute interstitial nephritis, often related to penicillin allergy.

Microscopy Techniques: Quick Reference

  • Bright-field microscopy:
    • Traditional method; useful for general observation.
    • Works with lowered condenser to improve visualization of certain structures.
  • Phase contrast microscopy:
    • Enhances differences in refractive index; easier to identify cellular elements and subtle features in urine sediment.
    • Demonstrated example: left image with bright-field cast vs right image with phase-contrast cast showing more detail in the latter.
  • Polarizing microscopy:
    • Detects birefringence of crystals; critical for identifying crystals like uric acid and cholesterol.
    • Typical interpretation: parallel alignment yields yellow color; perpendicular alignment yields blue color under polarized light.
    • Important note: uric acid crystals are birefringent and show color changes under polarization; cholesterol crystals can be confused with other crystals without polarization data.
  • Interference (Nomarski) contrast:
    • Provides strong 3D-like visualization and depth, but is expensive and not routinely used in all labs.

Practical Correlation and Reporting Tips

  • Always correlate microscopic findings with:
    • Dipstick results (e.g., blood, leukocyte esterase, bilirubin, protein).
    • Physical exam notes (urine appearance, cloudiness, odor).
    • Specific gravity and hydration status to interpret potential crystal formation or cell lysis.
  • Sample interpretations:
    • If RBCs are elevated microscopically but dipstick is negative or borderline, reassess sample integrity or test methodology; consider staining or repeat sampling.
    • If WBCs are present with leukocyte esterase positive, infection/inflammation is likely; if granular leukocytes are suspected, staining may be required to differentiate cell types.
    • If epithelial cells are predominantly squamous and sample looks contaminated, interpret cautiously and consider a repeat clean-catch specimen.
    • If oval fat bodies are seen, suspect nephrotic-range proteinuria and glomerular dysfunction; correlate with protein tests.
    • If specific crystals are observed, consider patient history (diet, medications, dehydration, renal disease) and order appropriate confirmatory tests or follow-up.
  • Reporting emphasis:
    • Note the presence and quantity of each element per HPF (e.g., RBCs, WBCs, epithelial cells).
    • Identify any artifacts (air bubbles, starch grains, oil droplets) and differentiate them from true elements.
    • Mention any staining results used to confirm identity (e.g., cyanide nitroprusside for cystine, acetic acid lysis results, polarized light findings).
    • Include suspected clinical scenarios and appropriate follow-up actions (e.g., repeat sampling, additional testing).

Common Etiologies and Clinical Implications (Summary)

  • Hematuria (RBCs in urine): causes include glomerulonephritis, pyelonephritis, cystitis, nephrolithiasis, trauma, tumors, contamination; always correlate with dipstick and other findings.
  • Leukocyturia (WBCs in urine): often indicates infection/inflammation; correlate with leukocyte esterase dipstick; consider bacterial vs nonbacterial etiologies.
  • Epithelial cell findings: squamous cell predominance suggests contamination; transitional/renal tubular cells indicate site of pathology along urinary tract; oval fat bodies indicate nephrotic syndrome and glomerular dysfunction.
  • Crystals: most are clinically insignificant unless associated with clinical history (e.g., gout, ethylene glycol ingestion) or pH context; special stains and polarizing microscopy help differentiate crystals from one another and from artifacts.
  • Eosinophils (Hansel stain): may indicate acute interstitial nephritis, often linked to drug allergy (e.g., penicillin).
  • Phase contrast and polarization: useful tools to improve identification and reduce misinterpretation; choose method based on suspected elements and available equipment.

Quick Reference Formulas and Key Values (for quick study)

  • Centrifugation: 400 ext{-} 500 imes g ext{ for } 5 ext{ min}
  • Sediment volume: 1 ext{ mL} of urine (concentration equivalent)
  • Examination fields: average counts over the range of at least 10 ext{ HPF} (high power fields) for RBCs and WBCs
  • pH threshold for uric acid crystals: ext{pH} < 5.7
  • Normal RBCs in urine: up to 0 ext{ to } 3 ext{ per HPF}
  • Normal WBCs in urine: up to 0 ext{ to } 8 ext{ per HPF}

Notes on Part 1 of Microscopic Urinalysis

  • This completes the Part 1 overview on preparation, techniques, and identification of key elements in urine sediment.
  • Further sections may cover additional elements, diagnostic algorithms, and case-based examples.