[Laboratory diagnosis of monarthritis: how much, what for, when?]

Analysis of the synovial fluid is the major investigation of monoarthritis. Appearance, viscosity (low if inflamed), cell number and differential, presence of crystals or organisms are all relevant. If septic arthritis is suspected, culture of other sites such as blood, urine, sputum etc. is essential, and may alone yield the organism. If mycobacterium is possible, synovial membrane staining and culture is usually necessary. Gonococcal may be lost in culture if the specimen is not immediately processed. Partially treated sepsis may produce sterile culture, and early work suggests that P.C.R. may diagnose these cases. Other investigations such as erythrocyte sedimentation rate, C-reactive protein indicate inflammatory activity, though they are not specific. Antibodies such as antinuclear antibodies, rheumatoid factors lead towards an "autoimmune" disease diagnosis, that do not alone measure activity. Specific antibodies to virus e.g. parvovirus may be diagnostic. The monoarthritis must be seen in the total patient context, where often clues e.g., asymptomatic uveitis (in juvenile chronic arthritis) and psoriasis may give the diagnosis.