Supravital staining of cells in noninflammatory synovial fluids: analysis of the effect of crystals on cell populations.

Leukocyte differential counts are rarely performed on synovial fluids (SF) with low leukocyte (WBC) counts as they have been difficult to do with standard Wright stains. We performed SF analysis including supravital (SV) stains for differential counts on 100 consecutive relatively noninflammatory synovial fluids defined as any SF with less than 2,000 cells/mm3. SV stains gave clear cellular morphology. Monocytes were the most prominent cells in all noninflammatory synovial fluids. Thirty-eight fluids were found to have crystals (monosodium urate (MSU) in 15, calcium pyrophosphate dihydrate (CPPD) in 5, CPPD plus apatite-like crystals in 9, apatite-like clumps alone in 8 and lipid liquid in 1). WBC counts, percentages of polymorphonuclear (PMN) and absolute PMN counts were greatest in fluids with MSU or CPPD crystals. Fluids with apatite-like clumps alone tended to have the lowest WBC counts. The presence of WBC over 500 cells/mm3, over 20% PMN and absolute PMN counts over 250 cells/mm3 should encourage a careful search for crystals, especially MSU, in noninflammatory synovial fluids.