What substances in urine sediment can be identified using polarizing microscopy to help diagnose or rule out diseases or disorders?

Substances in Urine Sediment Identifiable by Polarizing Microscopy

Polarizing microscopy is an essential diagnostic tool for identifying at least six different substances in urine sediment that can help diagnose or rule out specific diseases or disorders.

Key Substances Identifiable by Polarizing Microscopy

1. Calcium Oxalate Crystals

• Types and Appearance:
  • Whewellite (calcium oxalate monohydrate): Appears as white or pale yellow crystals with disorganized internal structure under polarized light 1
  • Weddellite (calcium oxalate dihydrate): Often appears as envelope-shaped or octahedral crystals 2
• Clinical Significance:
  • Finding >200 pure whewellite crystals per cubic millimeter is highly suggestive of Primary Hyperoxaluria (PH1), especially in young children 1
  • Crystal size ≥35 μm indicates simultaneous hypercalciuria and hyperoxaluria 2
  • Dodecahedral weddellite crystals suggest heavy hypercalciuria (>6 mmol/L) 2

2. Monosodium Urate Crystals

• Appearance: Needle-shaped crystals that strongly polarize light
• Clinical Significance:
  • Diagnostic for gout when identified in synovial fluid 1
  • The identification of monosodium urate (MSU) crystals using polarizing microscopy remains the gold standard for gout diagnosis with 100% specificity 1

3. Cystine Crystals

• Appearance: Hexagonal, flat plates that are highly birefringent under polarized light
• Clinical Significance:
  • Pathognomonic for cystinuria, an inherited disorder of amino acid transport 2, 3
  • Calculation of global crystal volume is clinically useful for monitoring patients with cystinuria 2

4. Struvite (Magnesium Ammonium Phosphate) Crystals

• Appearance: Coffin-lid shaped crystals that show birefringence
• Clinical Significance:
  • Indicates urinary tract infection with urease-producing bacteria 2
  • Associated with infection stones (struvite stones) 4
  • Often found in combination with apatite in approximately 7% of urinary calculi 5

5. 2,8-Dihydroxyadenine Crystals

• Appearance: Round, brown crystals with radial striations that are strongly birefringent
• Clinical Significance:
  • Diagnostic for adenine phosphoribosyltransferase deficiency, a rare inherited metabolic disorder 2, 3
  • Can lead to crystalluria, nephrolithiasis, and kidney failure if untreated

6. Drug-Induced Crystals

• Types:
  • Sulfonamides (sulfamethoxazole)
  • Certain antibiotics (amoxicillin, ceftriaxone)
  • Antiretrovirals (atazanavir)
• Clinical Significance:
  • Can cause acute kidney injury due to intrarenal crystal precipitation 3
  • Identification helps distinguish drug-induced nephropathy from other causes

Clinical Application and Technique

Proper Technique for Crystal Identification

• Use first morning urine or fresh fasting samples 2
• Examine within two hours of collection 2
• Store samples at 37°C or room temperature before examination 2
• Use a Malassez cell and polarized microscopy 2

Interpretation Criteria

1. Chemical nature of crystals (especially for abnormal crystals)
2. Crystalline phase of common chemical species
3. Crystal morphology
4. Crystal size
5. Crystal abundance
6. Crystal aggregation
7. Frequency of crystalluria on serial samples 2

Clinical Pitfalls and Considerations

• Crystal identification requires expertise and proper equipment, which may not be readily available in all clinical settings 1
• Certification for examination of urinary sediment may limit its use by some nephrologists 1
• For unusual crystals, infrared spectroscopy may be needed for definitive identification 3
• Presence of crystalluria in >50% of serial first morning urine samples is a reliable marker for detecting risk of stone recurrence in patients with history of nephrolithiasis 2

Monitoring Applications

• Crystalluria examination is valuable for:
  • Detecting and monitoring inherited and acquired diseases associated with stone formation 3
  • Monitoring patients with primary hyperoxaluria after kidney transplantation (goal: negative crystalluria or oxalate crystal volume <100 μm³/mm³) 1
  • Evaluating risk of recurrent stone formation 2
  • Assessing response to treatment in metabolic stone disorders 4

Polarizing microscopy of urine sediment remains an inexpensive yet powerful diagnostic tool that provides critical information for the diagnosis and management of various metabolic, inherited, and drug-induced disorders affecting the urinary system.