[Clinical reasoning and decision-making in practice. A man with inexplicable joint pain and subsequent poor clinical condition].

A 52-year-old man presented with polyarthritis and was negative for rheumatoid factor, anti-CCP and ANA. He was treated with low-dose methotrexate, the drug of first choice in rheumatoid arthritis. The arthritis disappeared, but the patient developed fever, progressive dyspnoea, appetite loss and weight loss. Upon hospital admission his medication was stopped and community-acquired pneumonia was diagnosed. The fever persisted despite antibiotic treatment. The tentative diagnosis of rheumatoid arthritis was changed to systemic lupus erythematosus, based on the change in clinical condition that could not be explained by polyarthritis and seroconversion to ANA- and anti-dsDNA-positive. The patient was treated with high-dose steroids and azathioprine and remained in remission for more than 1 year after treatment. The ANA test remained strongly positive, whereas anti-dsDNA was no longer detectable. This case stresses the limited value of classification criteria for the diagnosis of rheumatoid arthritis. To differentiate between rheumatoid arthritis and systemic lupus erythematosus, tests for autoantibodies against citrullinated peptides can be used. To differentiate between systemic lupus erythematosus and infection, tests for anti-dsDNA antibodies, antinuclear antibodies, C-reactive protein and complement can be used.