Concise report
Most calcium pyrophosphate crystals appear as non-birefringent
José Ivorra, José Rosas, Eliseo Pascual
Sección de
Reumatología, Hospital General Universitario de Alicante, and
Universidad Miguel Hernández, Alicante, Spain
Correspondence to: Dr E Pascual, Hospital General Universitario de Alicante, c/ Maestro Alonso, 109, 03010 Alicante, Spain.
Accepted for publication 22 April 1999
OBJECTIVE
To determine
the proportion of calcium pyrophosphate dihydrate (CPPD) crystals that
appear as non-birefringent when observed under the polarised light microscope.
METHODSTwo 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
RESULTSThe
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 urate 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.
CONCLUSIONOnly 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.
© 1999 by Annals of the Rheumatic Diseases
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