Synovial fluid tests

Infection, crystal arthropathies, osteoarthritis, trauma and a variety of systemic diseases can create a painful, swollen peripheral joint of which septic arthritis is the most serious cause. Synovial fluid (SF) analysis is widely used to aid the diagnosis and management of both acute and chronic arthritis, and can be diagnostic in patients with bacterial infections or crystal-induced synovitis. Most native joints can be aspirated by any physician competent to do so, but where there is difficulty, joints can be aspirated under ultrasound guidance. In cases of suspected infected prosthetic joints, these must be referred to the orthopaedic surgeons for aspiration in theatre under strict asepsis. The SF should be sent to the laboratory promptly for microscopy, culture and crystal search using polarized light microscopy. Ordinarily SF is viscous, straw-coloured and essentially acellular, but in diseased states components of the SF can vary in characteristic ways. Although a positive Gram stain and culture can clinch the diagnosis of septic arthritis, the absence of organisms on Gram stain or a negative subsequent synovial fluid culture does not exclude a diagnosis of septic arthritis. A recent systematic review of the literature has shown that, although no investigation had sufficient sensitivity and specificity to confirm the diagnosis of septic arthritis in all cases, the single most useful investigation is synovial fluid microscopy and culture. All doctors in training should gain experience in joint aspiration to save delays in diagnosis.