If I take naproxen 500 mg twice daily for one week, how can I determine whether I have ankylosing spondylitis, and will my stiffness fully improve with naproxen?

Using Naproxen as a Diagnostic Test for Ankylosing Spondylitis

A one-week trial of naproxen 500 mg twice daily can help identify ankylosing spondylitis, but improvement in stiffness alone is not diagnostic—you must also assess the pattern and degree of response, with a positive response defined as ≥50% improvement in back pain or a 20 mm reduction on a 0-100 scale, combined with clinical features of inflammatory back pain. 1

Understanding the NSAID Response Test

• NSAIDs are first-line treatment for active ankylosing spondylitis and typically produce dramatic improvement in inflammatory back pain and stiffness when AS is present 1, 2
• The American College of Rheumatology defines NSAID failure as inadequate response after at least 2 different NSAIDs, each tried for at least 15 days at maximal recommended doses 2
• For diagnostic purposes, a therapeutic trial should last at least 3 months at maximal anti-inflammatory dose to definitively assess response, though initial improvement is typically seen within 1-2 weeks 1

What Constitutes a Positive Response Suggesting AS

• A responder is defined as achieving either 50% relative improvement OR an absolute 20 mm reduction (on a 0-100 mm scale) in the BASDAI score, which includes measures of back pain, stiffness, and fatigue 1
• Evaluation should occur between 6-12 weeks for formal assessment, though symptomatic improvement often begins within days to weeks 1
• Historical studies show that patients with AS correctly identified when they were taking naproxen versus placebo (8 of 10 patients, p=0.02), indicating the response is clinically meaningful and noticeable 3

Critical Clinical Features That Distinguish AS from Mechanical Back Pain

Look for these specific inflammatory back pain characteristics:

• Morning stiffness lasting >30 minutes that improves with exercise but not rest 1
• Pain that awakens you from sleep in the second half of the night (after-midnight pain), which responds dramatically to NSAIDs 4
• Age of onset <40 years with insidious onset over at least 3 months 1
• Improvement with exercise and worsening with rest—the opposite pattern of mechanical pain 1
• Alternating buttock pain suggesting sacroiliitis 1

What Naproxen Will NOT Do

• Naproxen will not improve all stiffness symptoms if they are purely mechanical or degenerative in origin 1
• Mechanical back pain from degenerative spondylosis shows minimal and inconsistent response to NSAIDs, particularly for radicular symptoms 5
• If your stiffness is primarily from osteoarthritis or degenerative disease, naproxen may provide modest pain relief but will not produce the dramatic improvement seen in inflammatory conditions 5

Additional Diagnostic Steps Beyond the NSAID Trial

After one week on naproxen 500 mg twice daily, assess:

• Degree of improvement in morning stiffness duration—patients with AS typically see reduction from hours to minutes 3
• Improvement in immobility stiffness (stiffness after sitting) 3
• Reduction in night pain and ability to sleep through the night 4
• Whether you can increase physical activities without discomfort—AS patients on effective NSAIDs report marked functional improvement 3

If you experience ≥50% improvement in these inflammatory symptoms, you should:

• Obtain HLA-B27 testing, as this genetic marker is present in 90-95% of AS patients 1
• Request imaging with MRI of the sacroiliac joints to detect early inflammatory changes (sacroiliitis) that may not yet be visible on plain radiographs 1
• Have inflammatory markers checked (ESR and CRP), though these may be normal in up to 50% of AS patients 1
• Undergo formal rheumatologic evaluation including measurement of spinal mobility (modified Schober test, chest expansion, occiput-to-wall distance) 1

Important Caveats

• A good response to NSAIDs is suggestive but not diagnostic of AS—other inflammatory conditions and even some mechanical pain can respond to NSAIDs 1
• Lack of response to one NSAID does not rule out AS—you must trial at least 2-3 different NSAIDs before concluding NSAID failure, as individual response varies 1, 2
• One week may be too short for definitive assessment—while initial improvement occurs quickly, formal evaluation should occur at 6-12 weeks 1
• Naproxen 500 mg twice daily (1000 mg total) is an appropriate dose and has been shown equivalent to other NSAIDs including indomethacin in AS 6, 4