Background The diagnosis of calcium pyrophosphate crystal (CPP) deposition disease (CPPD) is mainly based on the synovial fluid analysis and Xrays. US has demonstrated high sensitivity and specificity values for diagnosing CPPD compared to synovial fluid analysis as the gold standard (1), but less is known about sensitivity and specificity of synovial fluid analysis itself. Objectives to compare ultrasonography and synovial fluid analysis performances in the diagnosis of CPPD using a real gold standard. Methods We enrolled in our study all patients waiting to undergo knee replacement surgery due to severe osteoarthritis. Each patient underwent US examination of the knee, focusing on the menisci and the hyaline cartilage, the day prior to surgery, scoring each site according to the presence/absence of CPP as defined previously (1). The day of the surgery, synovial fluid of the knee (if present) was aspirated by the surgeon. After surgery, the menisci, condyles and the synovial fluid were retrieved and examined microscopically. Synovial fluid analysis was performed on wet preparations. For the meniscus and cartilage microscopic analysis, six samples were collected, either from the surface and from the internal of the structure trying to cover a large part of it. All slides were observed under transmitted light microscopy and by compensated polarised microscopy. A dichotomous score was given for the presence/absence of CPP. US and microscopic analysis were performed by different operators, blind to each other’s findings. Sensitivity and specificity of US and synovial fluid were calculated using microscopic findings of the menisci and cartilage as the gold standard. Results we enrolled in the study 15 patients (3 males), mean age of 72 years old (±6). Synovial fluid has been collected from 12 patients. If we consider all the structures examined with US (both menisci and cartilage of both condyles), were positive for CPP 11 patients while synovial fluid analysis was positive for 6 patients. At microscopic examination of the speciments, 11 patients were positive for CPP in at least one of the structures examined. US demonstrated a sensitivity of 91,6% and specificity of 100% while respective values for synovial fluid microscopic analysis were 60% and 100%. Furthermore, US demonstrated a higher sensitivity in identifying CPP in the medial meniscus than in the lateral meniscus or the hyaline cartilage (table 1). Specificity was of 100% in all cases as no false positives were found with either technique. Conclusions US demonstrated higher sensitivity and specificity values for identifying CPP deposits in the knee joint than synovial fluid analysis. We believe that US, thanks to its intrinsic characteristics and by virtue of the results presented above (even if preliminary), should be the first choise exam when CPPD is suspected. References Frediani B, Filippou G, Falsetti P, Lorenzini S, Baldi F, Acciai C, et al. Diagnosis of calcium pyrophosphate dihydrate crystal deposition disease: ultrasonographic criteria proposed. Ann Rheum Dis. 2005 Apr;64(4):638-40 Disclosure of Interest None Declared

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   |texte=   FRI0501 Ultrasound versus synovial fluid analysis for the diagnosis of calcium pyrophosphate dihydrate deposition disease: preliminary results.
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