A simple, rapid screening method using alizarin red S stain and ordinary light microscopy to detect microcrystalline or noncrystalline calcium phosphate salts was used on wet drop preparations of synovial fluids. This proved to be helpful in detecting apatite crystal clumps and small calcium pyrophosphate dihydrate (CPPD) crystals missed by polarized light. The staining was positive in 100% of synovial fluids from patients later proven to have apatite and/or CPPD deposition diseases. Apatite and CPPD crystals were commonly found together in the same fluids. In addition, some synovial fluids from patients with osteoarthritis, renal failure dialysis, rheumatoid arthritis, and gout also exhibited positive staining. The correlation of positive alizarin red S staining with radiologic evidence of osteoarthritis suggests that apatite crystals might be related to articular cartilage degeneration in different rheumatic diseases. (1,2).Recently, apatite crystals have been found in synovial fluid from patients with osteoarthritis (OA) and, more rarely, in patients with otherwise unexplained arthritis (3,4). However, individual apatite crystals are not identifiable under ordinary or polarized light because of their minute size (750-2,500 A in length) (5). Their precise identification requires electron microscopic techniques or x-ray diffraction analysis. A semiquantitative radioisotopic technique using 14C ethane-1 hydroxy 1, I diphosphonate (EHDP) binding to detect hydroxyapatite crystals in synovial fluid has been introduced by Halverson and McCarty (6,7). Although large amounts of apatite crystals tend to form aggregates which may appear as shiny but not birefringent clumps in synovial fluid, these shiny clumps are not always observed. At least some similar clumps might be due to protein OJ cell debris (4).These observations had led us to search for simpler methods of apatite identification in joint effusions. This study reports the use of a calcium stain, alizarin red S, on wet drop preparations of synovial fluid as a screening test to detect apatite crystals. The alizarin tests were performed on synovial fluid from patients with a variety of different joint diseases. The presence of alizarin stained clumps correlated with the findings of apatite crystals by transmission electron microscopy (TEM) and with the radiologic grade of OA. MATERIALS AND METHODSAlizarin red S staining techniques. Alizarin red S stain (Harleco, Gibbstown, NJ), 2% solution in distilled water,

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