Alizarin red S staining of synovial fluid in inflammatory joint disorders.

In 153 patients with mainly inflammatory joint disorders alizarin red S staining was used to detect calcium-containing crystals in synovial fluid (SF). The reproducibility of the results of this staining technique in 207 SF samples proved to be fairly good (65% concordant results after two observations). Electron-microscope studies confirmed the presence of apatite or calcium pyrophosphate dihydrate (CPPD) crystals in 92% of the alizarin red S positive samples, and in 27% of the weakly positive samples. Based on these figures it can be estimated that approximately 16% of SF samples obtained from patients with inflammatory joint disorders contain apatite and/or CPPD crystals. In contrast with other studies, mainly involving patients with degenerative joint disorders, no correlation was found between alizarin red S staining results and either radiological joint destruction (n = 130) or age.