Monoarticular disease

Non-traumatic monoarthritis is a common presenting problem in both primary and secondary care. The differential diagnosis is broad, encompassing both inflammatory and non-inflammatory causes. A careful history and examination will allow the underlying cause to be elicited in many cases. However, particularly in the acute setting, the history and examination findings do not allow exclusion of the diagnosis of primary concern-septic arthritis. Arthrocentesis with Gram stain and culture of the aspirated fluid, alongside polarized light microscopy, is the key initial investigation in any patient with an acute monoarthritis. Additional laboratory and radiological investigations can supplement the diagnostic reasoning process, in cases where the diagnosis remains unclear.