Most calcium pyrophosphate crystals appear as non-birefringent

Objective-To determine the proportion of calcium pyrophosphate dihydrate (CPPD) crystals that appear as nonbirefringent when observed under the polarised Light microscope. Methods-Two observers examined independently 10 synovial fluid samples obtained during an episode of arthritis attributable to CPPD crystals. Ten synovial fluid samples from patients with acute gout were used as a reference. The examination was performed after placing a fluid sample in a Niebauer haemocytometric chamber; a crystal count was done first under ordinary light, then in the area corresponding to a 0.1 ml, under polarised light Results-The percentages of birefringence appreciated for CPPD were 18% (confidence intervals (CI) 12, 24) for observer 1, and 17% (CI 10, 24) for observer 2 (difference NS). The percentages of birefringence for monosodium mate were 127% (CI 103, 151) for observer 1 and 107% (CI 100, 114) for observer 2 (difference NS). Percentages above 100% indicate that crystals missed under ordinary light became apparent under polarised Light. Conclusion-Only about one fifth of all CPPD crystals identified by bright field microscopy show birefringence when the same synovial fluid sample is observed under polarised Light. If a search for CPPD crystals is conducted under polarised Light, the majority of the crystals will be missed. Ordinary light allows a better rate of CPPD crystal detection but observation under polarised light of crystals showing birefringence is required for definitive C-PPD crystal identification.