Arthrocentesis of asymptomatic knees and first metatarsophalangeal joints and synovial fluid analysis are simple procedures that facilitate the diagnosis of gout during intercritical periods.

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...

Objective-To determine the precision and agreement of synovial fluid (SF) cell counts done manually and with automated counters, and to determine the degree of variability of the counts in SF samples, kept in the tubes used for routine white blood cell (WBC) counts-which use liquid EDTA as anticoagulant-at 24 and 48 hours at 4°C, and at room temperature. Methods-To determine precision, cell counts were repeated 10 times-both manually and by an automated counter-in a SF sample of low, medium, and high cellularity. The variances were calculated to determine the interobserver variation in two manual (M1,M2) and two automated cell counts (C1,C2). The agreement between a manual (M1) and automated counter (C1) results, was analysed by the Bland and Altman method and the diVerence against the mean of the two methods was plotted. Then, the mean diVerence between the two methods was estimated and the standard deviation of the diVerence. To determine the eVects of storage, SF samples were kept in a refrigerator at 4°C, and at room temperature; cell counts were done manually (M1) and automatically (C1) at 24 and 48 hours and the changes analysed by the Bland and Altman method. The variances were compared using an F test. Results-(1) Precision. With the manual technique, the coeYcients of variation were 27.9%, 14%, and 10.7% when used for counting the SF with low (270), medium (6200), and high cellularities (25 000). With the automated technique the coeYcients of variation were 20%, 3.4%, and 2.9% in the same SF samples. In the fluids of medium and high cellularity, the variances of the automated cell counts were significatively lower (F test, p<0.002) than those of the manual counts. (Ann Rheum Dis 1997;56:622-626) The white blood cell (WBC) count in synovial fluid provides information about the amount of inflammation present in the joints, and is the basis for the classification of synovial fluids (SF) into the categories of non-inflammatory, inflammatory, and septic.1-3 The WBC count is generally performed manually, using saline as a diluent to avoid the formation of a mucin clot, which would result from the use of haematological diluents containing acetic acid. 4 DiVerent problems have been found in relation to the performance of the WBC counts in the SF; a poor reproducibility of manual WBC counts, 5 as well as contradictory results in relation to the stability of the SF have been reported, emphasising the need of performing the procedure without delay. 6 Counting by means of automatic counters has occasionally been done, but problems such as the error produced by fat globules have been found when the counts are performed by such a method. 7Our aim is to determine the precision and reproducibility of WBC counts of SF

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