[Gonarthritis: diagnosis and therapy]

The patient presenting with an acutely swollen and tender knee joint is the most frequent rheumatologic emergency in clinical practice. Most important is the exclusion of an infectious arthritis, which requires immediate antibiotic treatment. Synovial fluid analysis for color, clarity, viscosity and cell count is the initial evaluation. In the presence of an opalescent or purulent fluid, cultures and stains with gram and acid-fast methods and polarizing microscopy for crystals should be performed. A noninflammatory, clear (reading test) and highly viscous ("Fadentest') fluid and a white blood cell count of < 2000/mm3 make an infectious arthritis extremely unlikely. The most frequent cause of a noninflammatory fluid is inflammatory osteoarthritis, which usually responds to NSAID treatment. Typical changes of the axial skeleton, tendons, skin and mucosa point to a spondylarthropathy.