What is the likely diagnosis and management for an adult patient with intermittent knee swelling and pain that resolves without intervention, a normal uric acid level, and a normal X-ray?

Intermittent Knee Swelling with Normal Uric Acid: Likely NOT Gout

This presentation with a normal uric acid level of 4.1 mg/dL makes gout unlikely, and you should pursue alternative diagnoses including osteochondritis dissecans, meniscal pathology, or other mechanical knee disorders. 1

Why Gout is Unlikely Here

Normal uric acid significantly reduces gout probability. While 63% of acute gout attacks can present with normal serum uric acid levels during the acute phase 2, this patient has:

• Intermittent symptoms that resolve spontaneously - atypical for gout's characteristic rapid onset of severe pain reaching maximum within 6-12 hours 1
• Knee involvement without mention of erythema, podagra, or rapid severe pain - the classic gout presentation has a likelihood ratio of only 1.27 for "painful swollen joint" alone 1
• Uric acid of 4.1 mg/dL is well below the saturation threshold of 6.0-6.8 mg/dL where monosodium urate crystals form 3, 4

The combination of hyperuricemia (>mean+2SD) with joint inflammation has a likelihood ratio of 9.74 for gout 1, but this patient has the opposite - a low-normal uric acid.

Recommended Diagnostic Approach

Obtain knee MRI without contrast as the next step to evaluate for:

• Osteochondritis dissecans (OCD) - presents with intermittent pain, swelling, and mechanical symptoms in patients with normal radiographs 1
• Meniscal tears - can cause intermittent swelling and pain that may temporarily resolve 1
• Articular cartilage injury - another cause of intermittent symptoms 1

MRI is appropriate when radiographs are normal but clinical suspicion for internal derangement exists 1.

If You Still Suspect Gout Despite Low Probability

Joint aspiration with polarized light microscopy is mandatory if gout remains in your differential 5. This has:

• Likelihood ratio of 566.60 - making it the definitive diagnostic test 1
• Should be performed in any undiagnosed inflammatory arthritis regardless of how atypical the presentation 5
• Synovial fluid to serum uric acid ratio ≥1.01 has 89.6% sensitivity and 66.3% specificity for gout when crystal analysis is unavailable 6

Common Pitfalls to Avoid

• Do not exclude gout solely based on normal uric acid - but in this case, the entire clinical picture (intermittent, self-resolving, knee location, normal X-ray) makes it unlikely 2
• Do not assume all monoarthritis is gout - the positive predictive value of clinical diagnosis alone is only 64% 4
• Do not order bone scan - it has low specificity and poor anatomic resolution compared to MRI 1

Alternative Diagnoses to Consider

Mechanical knee disorders are more consistent with this presentation:

• OCD lesions - often present with pain, swelling, and intermittent symptoms in younger patients 1
• Meniscal pathology - can cause intermittent effusions and pain 1
• Patellofemoral disorders - may present with intermittent symptoms and normal initial radiographs 1