Diff-Quik® staining method for detection and identification of monosodium urate and calcium pyrophosphate crystals in synovial fluids

The aim of this study was to evaluate whether DQ could prove useful to identify monosodium urate (MSU) and calcium pyrophosphate dehydrate (CPPD) crystals on permanent mounted stained slides. To this end, we studied 27 synovial fluid (SF) samples obtained from the knees of patients with the pseudogout (n=21) and acute gouty arthritis (n=6). Wet analysis for crystal detection and identification was performed within one hour of joint aspiration. In addition, we studied 16 inflammatory synovial effusions obtained from patients with knee arthritis not induced by crystals. For each SF, DQ stained slides were analyzed by 2 experienced doctors in SF analysis. The observers were blinded to the type of crystal present in the SF. Each slide was analyzed by compensated polarized and transmitted light microscopy. SF was considered positive if intracellular and/or extracellular crystals were clearly identified. In addition, the observers were asked to identify the type of the crystals using compensated polarized light microscopy. Sensitivity, specificity, accuracy, positive predictive value (PPV), and negative predictive value (NPV) of the DQ staining method were determined. 51 true positive and 28 true negative specimens were correctly classified (39 CPPD samples, 12 MSU samples, and 28 samples of crystals-unrelated arthropathies). All MSU specimens were correctly diagnosed.