The detection of calcium pyrophosphate crystals in sequential synovial fluid examinations of patients with osteoarthritis: once positive, always positive

The aim of this study was to investigate whether calcium pyrophosphate dihydrate (CPPD) crystals are constantly detectable in sequential synovial fluid (SF) examinations of patients with initially CPPD-positive osteoarthritis (OA). For this purpose, we searched our SF database for CPPD-positive patients, who had two or more SF analyses between 2008 and 2012 to get sequential information. The database contains SF data determined by a standardised procedure. Of 1,878 samples, examined within the defined time period, 60 samples from sequential SF analyses of 23 patients were eligible for this study. The number of examinations ranged from 2 to 7 (median, 2), and the median interval between the first and last arthrocentesis was 12 months (2–43). CPPD crystals were detected in all of the sequentially examined samples according to the defined criterion of positivity. All of the subjects had OA of the knee, with a median Kellgren–Lawrence grade of 3 in the initially performed X-rays, and meniscal calcifications were found in six subjects. In conclusion, our results suggest that CPPD crystals are a regular feature of sequentially examined SF of initially CPPD-positive patients with symptomatic OA of the knee.