What is the most likely test to confirm the diagnosis in a patient with a 2-month history of knee swelling, pain, and inflammation, and 2 years of low back pain and morning stiffness?

Radiography of the Sacroiliac Joints (Answer C)

Radiography of the sacroiliac joints is the most appropriate test to confirm the diagnosis in this patient with suspected axial spondyloarthritis. 1, 2

Clinical Reasoning

This 25-year-old man presents with a classic constellation of features suggesting axial spondyloarthritis (axSpA):

• Chronic inflammatory back pain (2 years of low back pain with morning stiffness) beginning before age 45 1
• Peripheral joint involvement (2 months of knee swelling, pain, and inflammation) 1
• Age of onset consistent with axSpA (symptoms typically begin in young adulthood) 1

Why Radiography of the Sacroiliac Joints is the Answer

Conventional radiography of the sacroiliac joints is recommended as the first-line imaging modality for diagnosing sacroiliitis as part of axial spondyloarthritis. 1, 2 The American College of Radiology rates this as a "9" (usually appropriate) for initial evaluation of suspected axial spondyloarthropathy. 1

• If sacroiliitis on radiography meets the modified New York criteria, the diagnosis of axial spondyloarthritis can be confirmed 1
• Radiography demonstrates chronic erosions, sclerotic changes, joint space narrowing, and ankylosis that are sequelae of inflammatory sacroiliitis 2
• This test has a specificity of approximately 75-80% and provides objective evidence needed for diagnosis 1

Why the Other Options Are Incorrect

A. Uric Acid Level

• This would be appropriate for suspected gout, not for the inflammatory back pain and sacroiliitis pattern seen here [@General Medicine Knowledge]
• The clinical presentation does not suggest crystal arthropathy

B. Antinuclear Antibodies (ANA)

• ANA testing is used for systemic lupus erythematosus and other connective tissue diseases [@General Medicine Knowledge]
• Not indicated for axial spondyloarthritis diagnosis 1

D. Rheumatoid Factor

• Rheumatoid factor is used for diagnosing rheumatoid arthritis [@General Medicine Knowledge]
• Spondyloarthropathies are typically seronegative (RF-negative) conditions [@General Medicine Knowledge]

E. Synovial Fluid Analysis

• While this could evaluate the knee effusion, it would not confirm the underlying diagnosis of axial spondyloarthritis [@General Medicine Knowledge]
• The key to diagnosis lies in demonstrating sacroiliitis, not analyzing peripheral joint fluid [@1@, @5@]

Important Clinical Considerations

If radiographs are negative or equivocal but clinical suspicion remains high, MRI of the sacroiliac joints should be the next step. [1, @4@, 1, @6@] MRI has superior sensitivity (79%) and specificity (89%) for early disease and can detect inflammatory changes (bone marrow edema) before structural damage appears on radiographs. [@5@, 2]

Common Pitfalls to Avoid

• Do not rule out spondyloarthritis based on negative HLA-B27 alone - while HLA-B27 has 90% sensitivity, the condition occurs in HLA-B27 negative patients [@1@, @3@]
• Radiographs may be normal in early disease - structural changes can lag 3-7 years behind symptom onset [@2@, @6@]
• Normal inflammatory markers do not exclude the diagnosis - ESR/CRP have only 50% sensitivity in axial spondyloarthritis [@1@, @3