Aspiration of the right knee is the most appropriate next step in management. This patient is likely to have gout based on his risk factors (older man, hypertension, diabetes mellitus, obesity), the description of the symptoms (sudden onset at night with severe pain), and the recent episode of great toe swelling consistent with podagra. The gold standard for diagnosing gout is identification of monosodium needle-shaped urate crystals within leukocytes via synovial fluid analysis. Furthermore, infectious arthritis must be excluded in a patient with monoarticular arthritis. This patient is at increased risk for joint infection given his age and presence of diabetes. Thus, joint aspiration should be performed and synovial fluid sent for Gram stain, cultures, leukocyte count, and crystal analysis. Although uncommon, it is important to note that gout and an infected joint can coexist.
MRI may be useful for a patient with a history of trauma or other reason to suspect a mechanical cause for knee pain. Although MRI may demonstrate inflammation, it does not typically distinguish between infectious and noninfectious causes. Therefore, MRI is not currently indicated in this patient with warmth over the joint as well as fever, which suggests an inflammatory process.
Obtaining a serum urate level may assist in the diagnosis of this patient because an elevated level (>6.8 mg/dL [0.40 mmol/L]) would help support a diagnosis of gout. However, this test is not definitive for the diagnosis. An elevated level does not prove that the patient has gout because asymptomatic hyperuricemia is common in the general population. A relatively low serum urate level also does not exclude gout because serum urate levels can be paradoxically low during acute gout attacks.
Starting colchicine would be an option for the treatment of acute gout in this patient, and resolution of inflammation with colchicine may in fact support a diagnosis of crystal-induced arthritis. However, the diagnosis must first be established and infectious arthritis excluded.