9.2 Microscopic Urinalysis: Cells, Casts, Crystals, and Renal Pathology

Microscopic Urinalysis: Cells, Casts, Crystals, and Renal Pathology

Quick Answer: The microscopic examination of urine sediment relies on the identification of cellular elements, casts, and crystals. Casts are formed in the renal tubules from a matrix of Tamm-Horsfall protein. Crystals are classified by the pH of the urine and can indicate benign states, metabolic disorders, or impending renal damage.

Standardization and Technique

Microscopic urinalysis requires a standardized volume (usually 10-12 mL), centrifuged at exactly 400 RCF for 5 minutes. The sediment is resuspended in a standard volume (e.g., 0.5-1.0 mL). Sternheimer-Malbin stain (crystal violet and safranin O) is often used to enhance cellular detail.

Cellular Elements

Red Blood Cells (RBCs)

• Appearance: Non-nucleated biconcave disks.
• In Hypertonic Urine: RBCs shrink and appear crenated.
• In Hypotonic Urine: RBCs swell and lyse, leaving empty membranes known as ghost cells.
• Dysmorphic RBCs: Fragmented or irregular RBCs, such as acanthocytes (Mickey Mouse cells), are strongly associated with glomerular bleeding (e.g., glomerulonephritis).

White Blood Cells (WBCs)

• Appearance: Neutrophils are the most common WBCs in urine, appearing as granular spheres with lobed nuclei.
• Glitter Cells: In hypotonic urine, neutrophils swell, and their granules exhibit Brownian motion, creating a sparkling appearance known as glitter cells.
• Clinical Significance: Pyuria indicates inflammation or infection (UTI). Eosinophils (demonstrated via Hansel stain) suggest acute interstitial nephritis, often drug-induced.

Epithelial Cells

1. Squamous Epithelial Cells: The largest cells, with abundant irregular cytoplasm and a small, central nucleus. Originating from the lower urethra or vagina, large numbers indicate a poorly collected, non-midstream clean-catch sample. Clue cells (squamous cells covered with Gardnerella vaginalis) indicate bacterial vaginosis.
2. Transitional (Urothelial) Cells: Spherical, polyhedral, or pear-shaped with a central nucleus. Originating from the renal pelvis, ureters, or bladder. Increased numbers or syncytia (clumps) are seen following catheterization or in transitional cell carcinoma.
3. Renal Tubular Epithelial (RTE) Cells: The most clinically significant epithelial cell. Slightly larger than a WBC with a large, eccentric nucleus. Increased numbers indicate tubular necrosis (e.g., heavy metal toxicity, ischemic injury).

Oval Fat Bodies

When RTE cells absorb lipids that pass across the glomerular membrane, they become highly refractile oval fat bodies. Under polarized light, cholesterol droplets within these cells form a characteristic "Maltese cross" pattern. They are the hallmark of Nephrotic Syndrome.

Urinary Casts

Casts are the only elements uniquely formed in the kidney (specifically, the distal convoluted tubules and collecting ducts). Their matrix consists of Tamm-Horsfall protein (uromodulin), a glycoprotein secreted by the RTE cells. Cast formation is enhanced by urinary stasis, acidic pH, and high solute concentration.

Cast Types and Clinical Correlations

• Hyaline Casts: Composed entirely of Tamm-Horsfall protein. Colorless and transparent (requires low light to see). Seen in normal individuals following strenuous exercise, dehydration, or stress.
• Red Blood Cell Casts: Contain entrapped RBCs. Their presence specifically localizes bleeding to the nephron, cementing a diagnosis of glomerulonephritis.
• White Blood Cell Casts: Contain entrapped WBCs. Their presence indicates inflammation or infection within the kidney itself, differentiating pyelonephritis (upper UTI) from cystitis (lower UTI).
• Epithelial Cell Casts: Contain RTE cells. Indicative of advanced tubular destruction, specifically acute tubular necrosis (ATN).
• Granular Casts: Result from the degradation of cellular casts or the aggregation of plasma proteins. "Muddy brown" granular casts are classic for ATN.
• Waxy Casts: Highly refractile with blunt, fractured ends and notched edges. Represent the final stage of cast degeneration. Indicates extreme urinary stasis and chronic renal failure.
• Broad Casts: Formed in widened, dilated collecting ducts. Known as "renal failure casts," they indicate advanced tubular dilation and destruction.

Urinary Crystals

Crystal formation is largely determined by the pH of the urine, the concentration of the solute, and the temperature. Most normal crystals are clinically insignificant.

Normal Crystals in Acid Urine (pH < 7.0)

• Uric Acid: Yellow-brown, pleomorphic (diamonds, rhombic plates, rosettes). Polarizes light. Associated with gout or chemotherapy (tumor lysis syndrome).
• Amorphous Urates: Yellow-brown "brick dust" sediment. Dissolves upon heating (60 degrees C).
• Calcium Oxalate: Often found in acid/neutral urine.
  • Dihydrate: Colorless, envelope or octahedral shape.
  • Monohydrate: Oval or dumbbell shaped. Classic finding in ethylene glycol (antifreeze) poisoning.

Normal Crystals in Alkaline Urine (pH > 7.0)

• Amorphous Phosphates: White precipitate. Dissolves in dilute acetic acid.
• Triple Phosphate (Ammonium Magnesium Phosphate): Colorless, "coffin lid" prisms. Highly associated with urea-splitting bacteria (Proteus species) and staghorn calculi.
• Calcium Carbonate: Small, colorless dumbbells. Effervesces (bubbles) with acetic acid.
• Ammonium Biurate: Yellow-brown "thorny apples." Often seen in old, poorly preserved urine.

Abnormal Crystals (Always in Acid Urine)

Abnormal crystals are clinically significant and are strictly found in acid or neutral urine.

• Cystine: Colorless, highly refractile hexagonal plates. Seen in cystinuria (a metabolic defect in renal tubular reabsorption of dibasic amino acids). Confirmed with the cyanide-nitroprusside test.
• Cholesterol: Rectangular plates with notched corners. Highly birefringent. Seen in nephrotic syndrome, accompanying oval fat bodies and fatty casts.
• Leucine: Yellow-brown spheres with concentric circles and radial striations. Associated with severe liver disease.
• Tyrosine: Fine, colorless to yellow needles arranged in sheaves or rosettes. Often seen alongside leucine in severe liver disease (e.g., cirrhosis, viral hepatitis).
• Bilirubin: Clumped needles or granules with a characteristic golden-yellow color. Correlates with a positive chemical reagent strip for bilirubin.

Exam Trap: Never identify abnormal crystals in alkaline urine. If you see a hexagonal plate in pH 8.0 urine, it cannot be cystine.