Most calcium pyrophosphate crystals appear as non-birefringent

Objective-To determine the proportion of calcium pyrophosphate dihydrate (C PPD) crystals that appear as nonbirefringent when observed under the polari sed Light microscope. Methods-Two observers examined independently 10 synovial fluid samples obta ined during an episode of arthritis attributable to CPPD crystals. Ten syno vial fluid samples from patients with acute gout were used as a reference. The examination was performed after placing a fluid sample in a Niebauer ha emocytometric 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% (con fidence intervals (CI) 12, 24) for observer 1, and 17% (CI 10, 24) for obse rver 2 (difference NS). The percentages of birefringence for monosodium mat e were 127% (CI 103, 151) for observer 1 and 107% (CI 100, 114) for observe r 2 (difference NS). Percentages above 100% indicate that crystals missed u nder ordinary light became apparent under polarised Light. Conclusion-Only about one fifth of all CPPD crystals identified by bright f ield microscopy show birefringence when the same synovial fluid sample is o bserved under polarised Light. If a search for CPPD crystals is conducted u nder 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 definiti ve C-PPD crystal identification.