MKSAP: 72-year-old man with acute swelling and severe pain of the right knee

Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 72-year-old man is evaluated in the emergency department for acute swelling, severe pain, and warmth of the right knee that woke him from sleep. He does not recall any inciting injury to the knee. Three months ago, he had an acutely swollen great toe that improved within 3 days, for which he did not seek treatment. History is also significant for hypertension and diabetes mellitus. Medications are hydrochlorothiazide and metformin.

On physical examination, temperature is 37.8 °C (100.1 °F), blood pressure is 130/75 mm Hg, pulse rate is 90/min, and respiration rate is 12/min. BMI is 33. The right knee is warm and swollen without overlying erythema; tenderness to palpation and decreased range of motion due to pain are noted. There is no skin breakdown or abrasions over the right knee. Examination of the other joints is unremarkable.

Which of the following is the most appropriate next step in management?

A. Obtain a knee MRI
B. Obtain a serum urate level
C. Perform joint aspiration
D. Start empiric colchicine

MKSAP Answer and Critique

The correct answer is C. Perform joint aspiration.

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.

Key Point

  • Analysis of synovial fluid from joint aspiration is the gold standard to diagnose gout and exclude infection.

This content is excerpted from MKSAP 17 with permission from the American College of Physicians (ACP). Use is restricted in the same manner as that defined in the MKSAP 16 Digital license agreement. This material should never be used as a substitute for clinical judgment and does not represent an official position of ACP. All content is licensed to on an “AS IS” basis without any warranty of any nature. The publisher, ACP, shall not be liable for any damage or loss of any kind arising out of or resulting from use of content, regardless of whether such liability is based in tort, contract or otherwise.

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