Red cells may indicate recent haemarthrosis or a traumatic aspiration.
Bacterial arthritis is associated with turbid or purulent synovial fluid, white cell counts > 50,000 x 106L (mainly neutrophils), low glucose (compared to concurrent plasma levels) and high protein.
These findings can also be present in other inflammatory joint conditions, eg, rheumatoid arthritis, rheumatic fever, and crystal arthropathy.
Cell counts > 100,000 x 106 /L suggest septic arthritis. Not all cases of bacterial infection are associated with an elevated white cell count.
In viral arthritis the synovial fluid white cell count is increased with predominantly mononuclear cells, although rubella has been associated with predominance of neutrophils.
Sodium urate and calcium pyrophosphate crystals are diagnostic of gout and pseudogout, respectively.
A fibrin clot is present in inflammatory or septic fluid and the mucin clot is friable.
Complement components C3 and C4 are low in rheumatoid arthritis.