The leukocyte count helps determine if the joint effusion reflects an inflammatory or noninflammatory process. Synovial fluid is usually heparinized before the cell count to reduce clumping.

Leukocyte counts greater than 2,000/mm³ usually result in loss of transparency in the fluid, but confirmation that this is due to leukocytosis is important. A classification of joint effusions based on leukocyte counts has been developed. Counts over 75,000/mm³ suggest the presence of infection, but very high counts (including those that are neutrophil predominant) can occur in psoriatic and crystal-induced arthritis and less commonly in rheumatoid arthritis.

Noninflammatory effusions usually contain three or fewer leukocytes per high-power field. Not all noninflammatory effusions are due to osteoarthritis. Some other causes include traumatic arthritis, acromegaly, hemochromatosis, hyperparathyroidism, ochronosis, Paget disease of bone, aseptic necrosis, amyloidosis, hypertrophic pulmonary osteoarthropathy, pancreatitis, and apatite-associated arthritis.

The wet preparations that had been examined for crystals can be used to provide a rough estimate of the leukocyte count. Other findings may also be noted (see Plate 5-15). For example, fat droplets, which usually indicate trauma, a result of marrow fat leaking into the synovial space, can also occur in pancreatic disease with synovial fat necrosis. Crystals of apatite deposition disease, seen in calcific tendonitis or in patients with Milwaukee shoulder/knee syndrome, create irregular, shiny, often nonbirefringent intracellular or extracellular chunks (of 2 to 20 µm) visible on wet preparations; individual crystals can be seen only on electron microscopy.

Single drops of joint fluid can be placed on a glass slide and smeared out into a thin preparation as for a blood smear. Air-drying preserves the cells for staining later in the day. If infection is being considered, smears should be stained with Gram’s stain. Identification of pathologic organisms can guide the choice of initial antibiotic therapy, but failure to find bacteria on a Gram-stained preparation does not exclude infectious arthritis because positive stains for bacteria are uncommon, even with staphylococcal infection.

A differential count with more than 95% polymorphonuclear neutrophils is consistent with infection or crystal-induced disease (see Plates 5-14, 5-38 to 5-40) even if the leukocyte count is not very high. An inflamed joint space produces an ideal medium for the development of LE cells, which are seen almost exclusively in systemic lupus erythematosus (see Plate 5-51). Mononuclear cells that have phagocytized necrotic neutrophils may occur in reactive arthritis (see Plate 5-33), other seronegative spondyloarthropathies, or (occasionally) gout or pseudogout (see Plates 5-38 and 5-39). Large cells seen in blood smears include synovial lining cells, most common in noninflammatory disorders, and activated lymphocytes, which are common in rheumatoid arthritis and should not be confused with the rare tumor cell found in joints.