Lecture #4: Crystals & Casts

Why is urine pH important in identifying crystals?

: pH affects crystal solubility and formation; acidic urine favors urates and oxalates, alkaline urine favors phosphates and biurates.

What are the three crystals commonly found in normal acidic urine?

: Uric acid, amorphous urates, calcium oxalate.

Describe uric acid crystals.

: Rhombic or rosette-shaped; associated with gout, chemotherapy, and increased purine metabolism.

What do amorphous urates look like and how are they affected by temperature?

: Brick dust appearance; form pink precipitate when refrigerated, dissolve with heat or alkali.

What are the two forms of calcium oxalate crystals and their significance?

: Dihydrate (envelope-shaped, common in kidney stones); monohydrate (ovoid, seen in ethylene glycol poisoning).

What are the three crystals commonly found in normal alkaline urine?

: Triple phosphate, amorphous phosphates, ammonium biurate.

Describe triple phosphate crystals.

: "Coffin-lid" prisms; may be seen in UTIs or kidney stones, often clinically insignificant.

How do amorphous phosphates differ from amorphous urates?

: Colorless granules; form white precipitate, dissolve in acetic acid but not with heat.

What do ammonium biurate crystals look like and what do they indicate?

: "Thorny apple" spheres; suggest old or unpreserved urine.

Name three abnormal crystals of metabolic origin.

: Cystine, tyrosine, leucine.

Describe cystine crystals and their clinical relevance.

: Hexagonal plates; indicate cystinuria, a rare inherited disorder.

What do tyrosine crystals look like and when are they seen?

: Fine needles in clusters; seen in liver disease with positive bilirubin.

Describe leucine crystals and their associations.

: Yellow-brown spheres with concentric rings; seen in liver disease and maple syrup urine disease.

What do cholesterol crystals look like and what condition are they linked to?

: Flat plates with notched corners; associated with nephrotic syndrome.

What are bilirubin crystals and when do they appear?

: Yellow to reddish-brown granules or needles; present in liver disorders with excess bilirubin.

What is hemosiderin and when is it found in urine?

: Stored iron pigment; seen in intravascular hemolysis.

Name three abnormal crystals of iatrogenic origin.

: Radiographic dye, sulfonamide, ampicillin.

What is the major component of urinary casts?

: Uromodulin (Tamm-Horsfall protein), secreted by renal tubular epithelial cells.

Where are casts formed in the nephron?

: Distal convoluted tubules and collecting ducts.

What factors promote cast formation?

: Low urine flow, high solute concentration, and acidic pH.

What is the sequence of cast degeneration?

: Cellular → granular → waxy.

Describe hyaline casts and their significance.

: Colorless, homogeneous; seen in dehydration, fever, or renal disease.

What do RBC casts indicate?

: Glomerular damage; correlate with hematuria and proteinuria.

What do WBC casts suggest?

: Pyelonephritis; correlate with WBCs, bacteria, and leukocyte esterase.

What do epithelial (RTE) casts indicate?

: Tubular necrosis or chronic renal disease.

Describe fatty casts and their clinical relevance.

: Fat droplets in matrix; seen in nephrotic syndrome and diabetes.

What are granular casts and what do they signify?

: Coarse or fine granules; indicate prolonged renal disease.

Describe waxy casts and their associations.

: Refractive with jagged ends; seen in chronic renal failure or transplant rejection.

What chemical finding correlates with all cast types?

: Positive protein.

How do you differentiate WBC casts from epithelial casts?

: WBCs have granular nuclei; RTEs are larger with round nuclei.

How do you distinguish RBC casts from fatty casts?

: RBC casts are uniformly colored; fatty casts show Maltese crosses under polarized light.

How do you tell bacteria from amorphous material?

: Bacteria are motile and rod-shaped; amorphous material is static and granular.

How do amorphous urates differ from amorphous phosphates?

: Urates are pink and dissolve with heat; phosphates are white and dissolve in acid.

How do uric acid crystals differ from cystine crystals?

: Uric acid polarizes and forms rosettes; cystine is hexagonal and non-polarizing.