To evaluate whether the Diff Quik (DQ) staining method might prove useful in identifying monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals on permanent mounted stained slides.
27 synovial fluid (SF) samples obtained from the knees of 21 patients with acute CPPD disease and 6 with acute gout were studied. Wet analysis for crystal detection and identification was performed within one hour of joint aspiration. In addition, 16 inflammatory synovial effusions obtained from patients with knee arthritis induced by non-crystalline inflammatory diseases were studied. For each SF, a DQ stained slide was analysed by two of the authors trained in SF analysis. The observers were blinded to the type of crystals present in the SF. Each slide was analysed by compensated polarised as well as transmitted light microscopy. An SF was considered positive if intracellular and/or extracellular crystals were clearly identified. In addition, the observer was asked to identify the type of the crystals using compensated polarised 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 cases were correctly classified (39 CPPD samples, 12 MSU samples, 28 samples of crystal unrelated arthropathies). Overall, four false positive and three false negative cases were reported. In all the false positive cases, extracellular CPPD crystals were erroneously identified, whereas CPPD crystals present in the SF were not identified in the three false negative cases. All MSU specimens were correctly diagnosed. The overall specificity, sensitivity, and accuracy using DQ stained slides for crystal confirmation were respectively 87.5%, 94.4%, and 91.9%. The PPV was 92.7% and the NPV 90.3%. In particular, the specificity, sensitivity, and accuracy for CPPD detection were 90.9%, 92.9%, and 91.9%, with a PPV of 90.7 and an NPV of 93.0%. All the MSU specimens were correctly identified, providing 100% sensitivity, specificity, accuracy, PPV, and NPV.
Stained preparations of SF, including DQ stained smears, could provide a useful tool for delayed SF analysis suitable for quality controls, including cytological examination and crystals detection and identification.