Polarised Microscopy in Medical Diagnostics
Primary Clinical Applications
Polarised microscopy is essential for identifying birefringent crystals in synovial fluid (for gout and pseudogout diagnosis) and urinary specimens (for kidney stone analysis and crystalluria assessment), where it enhances detection of monosodium urate, calcium pyrophosphate, and calcium oxalate crystals through their characteristic optical properties. 1, 2, 3
Kidney Stone and Crystalluria Analysis
Crystal Identification in Urine
Finding >200 pure whewellite (calcium oxalate monohydrate) crystals per cubic millimeter in urinary sediment is highly suggestive of primary hyperoxaluria type 1, especially in young children. 1
Crystalluria assessment using polarised microscopy enables rapid, non-invasive, and inexpensive evaluation of urinary calcium oxalate crystal volume and morphological analysis. 1
The combined approach of crystal morphology, birefringence features under polarised light, and urine pH successfully identifies "typical" crystals in 98.3% of cases (793 of 807 crystalluria samples). 4
Most common crystals identified include calcium oxalate (75.9%), uric acid (25.9%), and amorphous urates (7.9%), with rare findings of brushite, ammonium biurate, and cystine (0.1%-0.7%). 4
Post-Transplant Monitoring
Oxalate crystal volume measurement via polarised microscopy is useful for post-transplantation monitoring in primary hyperoxaluria patients, as positive crystalluria indicates risk of calcium oxalate deposits on the graft. 1
The therapeutic goal after kidney transplantation is achieving negative crystalluria or an oxalate crystal volume <100 μm³/mm³. 1
Stone Composition Analysis
Calcium oxalate monohydrate stones from primary hyperoxaluria type 1 patients have a peculiar morphology (white or pale yellow with disorganized internal structure) that reflects their rapid formation speed. 1
Stones from primary hyperoxaluria types 2 and 3 frequently contain mixtures of calcium oxalate and calcium phosphate, making them indistinguishable from idiopathic stone formers by composition alone. 1
Synovial Fluid Crystal Analysis
Gout Diagnosis
Polarised light microscopy is the gold standard for identifying monosodium urate crystals in synovial fluid aspirated from affected joints. 2, 3
Monosodium urate crystals demonstrate strong birefringence, with 107-127% of crystals visible under polarised light (percentages >100% indicate crystals missed under ordinary light become apparent under polarisation). 5
Pseudogout (CPPD) Detection - Critical Limitation
Only approximately 17-18% of calcium pyrophosphate dihydrate crystals show birefringence under polarised light microscopy. 5
If searching for CPPD crystals exclusively under polarised light, the majority (>80%) will be missed. 5
Ordinary bright field microscopy allows better CPPD crystal detection rates, but polarised light observation of birefringent crystals is required for definitive identification. 5
Technical Methodology
Standard Examination Protocol
Crystal examination should begin under ordinary light for initial detection, followed by polarised light examination in the corresponding area for definitive identification. 5
Phase contrast microscopy optimally visualizes deformed red blood cells and red blood cell casts, which indicate glomerular sources of hematuria. 1
When Polarised Microscopy Is Insufficient
When the combined approach (morphology, birefringence, pH) is inconclusive for "atypical" crystals (occurring in approximately 1.7% of cases), Fourier transform infrared microspectroscopy is required for definitive identification. 4
Scanning electron microscopy with energy-dispersive spectroscopy provides comprehensive information on stone morphology, crystal structure, elemental composition, and phase relationships when detailed analysis beyond polarised microscopy is needed. 6
Clinical Integration
Hematuria Workup Context
Crystalluria assessment via polarised microscopy rapidly excludes other crystal species not normally found in urine, such as cystine. 1
This examination is part of the initial urinalysis evaluation that includes dipstick testing, microscopic examination for red blood cell casts, and assessment of proteinuria. 1
Therapeutic Monitoring
Serial crystalluria measurements using polarised microscopy assess therapeutic efficacy in stone formers and patients on RNA interference therapy for primary hyperoxaluria. 1
Patients with primary hyperoxaluria and eGFR >30 mL/min/1.73m² should have urinary oxalate and other parameters monitored every 3-6 months during the first year of therapy. 1