Urinalysis W3: Urine Crystals and Renal Calculi

Are crystals normally found in freshly voided urine?

No. Crystals in freshly voided urine formed in vivo and are clinically significant

What factors contribute to urinary crystal formation?

Increased solute concentration, urine pH, low urine flow, temperature, and urine stasis

What happens when urine chemicals exceed solubility?

They precipitate as crystals (amorphous or crystalline)

Why are crystals formed in the nephron clinically important?

They can cause tubular damage and contribute to renal calculi

What happens to crystals during urine storage?

They may grow, dissolve, or deteriorate—altering interpretation

What does “amorphous” mean in urine sediment?

Without definite shape; appear as fine granular material

Key features of amorphous urates?

Yellow-brown granules, acidic pH (5.5–7.0), “brick dust,” dissolve with heat and alkaline pH

Key features of amorphous phosphates?

White/gray granules, alkaline pH, dissolve in acid, do NOT dissolve with heat

How do you differentiate amorphous urates from phosphates?

Urine pH and solubility: urates dissolve with heat; phosphates dissolve with acid

Why are amorphous crystals often confused with bacteria?

Both appear granular; use polarized light and solubility tests to differentiate

Calcium oxalate crystal appearance and pH range?

Envelope (dihydrate) or dumbbell (monohydrate); found at any pH

Clinical significance of calcium oxalate crystals?

Common in normal urine but associated with kidney stones when abundant

Uric acid crystal characteristics?

Yellow-red rhomboids or rosettes; acidic urine; associated with gout and high cell turnover

Triple phosphate (struvite) crystal appearance and pH?

Coffin-lid shape; alkaline urine; associated with urease-positive UTIs

Calcium phosphate crystal features?

Rosettes or wedges; alkaline pH; may be normal or pathologic

Ammonium biurate crystal appearance and significance?

“Thorny apple” shape; alkaline urine; often seen in old or improperly stored urine

Calcium carbonate crystal features?

Dumbbells or spheres; alkaline urine; rare and usually insignificant

Cystine crystal appearance and significance?

Colorless hexagons; acidic urine; always abnormal—indicates cystinuria

How do you differentiate crystals from RBCs?

Crystals refract light and have geometric shapes; RBCs do not refract

How do you differentiate crystals from casts?

Crystals are birefringent and free-floating; casts have parallel sides and rounded ends

What must be included when reporting urine crystals?

Crystal type, quantity (few/moderate/many), and urine pH

Why is urine centrifuged before microscopic examination?

To concentrate sediment (12:1) and improve detection of formed elements

Advantage of commercial urine slides?

Standardized sediment volume, not technique-dependent, cost-effective