[Algorithm for workup of acute swollen knee without previous trauma].

Acute non-traumatic swelling of the knee is a common clinical challenge. The reason for the swelling has to be explored in order to give the correct treatment. This article gives an overview of possible diagnoses and suggests an algorithm for an approach to non-traumatic knee arthritis. Fundamental in the examination is a thorough patient anamnesis, which will help to differentiate between inflammatory and non-inflammatory arthritis. If there is not an obvious degenerative reason for the knee effusion, a joint aspiration followed by synovial fluid white cell count (WCC), microscopy and/or culture will add valuable information. Also, a normal C-reactive protein value together with synovial fluid WCC below 2,000 per cubic mm will make a septic arthritis diagnosis unlikely.