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 (diVerence NS). The percentages of birefringence for monosodium urate were 127% (CI 103, 151) for observer 1 and 107% (CI 100, 114) for observer 2 (diVerence 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 CPPD crystal identification. (Ann Rheum Dis 1999;58:582-584) The gold standard for the precise diagnosis of joint inflammation in gout and calcium pyrophosphate dihydrate (CPPD) crystal arthropathy requires finding either monosodium urate (MSU) or CPPD crystals in synovial fluid (SF) samples obtained from inflamed joints.1 The standard procedure for the identification of these crystals requires the use of a polarised microscope fitted with a first order red compensator. The diVerent shape of the crystals (MSU crystals being always acicular, and acicular, rhomboid or parallelelipedic in the case of CPPD) and the type of elongation (strong negative for MSU and weakly positive for CPPD) allows a proper identification.2 MSU crystals are strongly birefringent when seen with the microscope fitted with crossed polarised filters, and acquires a negative elongation when the first order red compensator is used. On the other hand, CPPD crystals are described as being only weakly birefringent and it has been noted that some CPPD crystals may lack birefringence. The process of crystal search in SF requires: firstly, to determine whether the fluid contains crystals, and then proper crystal identification. As the compensated polarised microscope is the standard for crystal identification, it is probably also widely used to determine whether a SF sample contains crystals or is free of them. As CPPD crystals are weakly birefringent, and on occasions may be nonbirefringent, these crystals may pass unnoticed if their search is conducted in this mann...

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