[Diagnostic Work-Up for Monoarthritis - Step by Step].

The work-up of acute monoarthritis is challenging due to the abundance of differential diagnoses. In addition to a bacterial septic arthritis, which can, if not treated promptly, cause rapid irreversible joint damage, many diseases have to be considered: inflammatory rheumatic diseases, activated osteoarthritis, other infectious arthritis, cristal induced arthritis, and rare tumorous diseases. In cases with high urgency, and/or when medical history, physical examination and laboratory parameters remain without a specific etiologic clue, septic arthritis has to be excluded by immediate diagnostic joint aspiration. In many patients the cause of monoarthritis can already be determined by ordering a leucocyte count of the synovial fluid sample, a microscopy for crystals, and gram staining and culture for bacterial pathogens.