What is the likely diagnosis and first‑line treatment for a 57‑year‑old male (BMI 24) with intermittent sacroiliac pain, stiffness after rest, a swollen right proximal interphalangeal joint after golf, human leukocyte antigen B27 (HLA‑B27) positivity, normal C‑reactive protein (CRP) and erythrocyte sedimentation rate (ESR), and mildly elevated rheumatoid factor (RF) of 23 IU/mL?

Likely Diagnosis: Seronegative Spondyloarthropathy with Peripheral Arthritis

You most likely have seronegative spondyloarthropathy (SpA) with both axial (sacroiliac) and peripheral joint involvement, and should begin treatment with continuous NSAIDs plus structured physiotherapy immediately, with rheumatology referral for consideration of disease-modifying therapy if symptoms persist beyond 4-6 weeks. 1

Diagnostic Reasoning

Your clinical presentation strongly suggests SpA rather than rheumatoid arthritis, based on several key features:

• HLA-B27 positivity provides a 9-fold increased likelihood of axial SpA, with 90% sensitivity for the diagnosis 2
• Sacroiliac region pain with post-rest stiffness represents classic inflammatory back pain, which has 96% sensitivity for SpA diagnosis 3
• Asymmetric peripheral joint involvement (single PIP joint after activity) fits the pattern of SpA-associated peripheral arthritis rather than the symmetric polyarthritis typical of rheumatoid arthritis 2
• Mildly elevated RF (23 IU/mL) is a low-positive result that can occur in up to 15% of SpA patients and does not establish RA diagnosis 2
• Normal inflammatory markers do not exclude SpA—up to 50% of patients with active axial SpA have normal CRP at baseline, and ESR/CRP have only 50% sensitivity in early disease 2, 1

The combination of HLA-B27 positivity, inflammatory symptoms, and sacroiliac involvement yields a post-test probability of approximately 32% for axial SpA 2.

Immediate First-Line Treatment

Start continuous NSAIDs at full anti-inflammatory doses immediately—not "as needed"—combined with structured physiotherapy. 1

• NSAIDs should be prescribed continuously for 4-6 weeks to suppress inflammation, as response to NSAIDs is itself a diagnostic feature of inflammatory back pain 1
• Choose either a non-selective NSAID with gastroprotective agent or a selective COX-2 inhibitor given your age 1
• Physiotherapy must begin immediately, not after "failing" medications—it is as important as pharmacologic treatment for preventing long-term disability 2, 1
• Prescribe daily home exercises including spinal extension and range-of-motion exercises, plus supervised group physical therapy sessions 1

Additional Diagnostic Workup Needed

You require MRI of the sacroiliac joints to confirm inflammatory sacroiliitis, as plain radiographs miss most early disease and lag behind symptoms by 7+ years 2:

• MRI should include coronal/oblique sacroiliac joint images with T1 and STIR sequences 2
• MRI evidence of active inflammation (bone marrow edema) can justify treatment escalation even with normal CRP 1, 3
• However, MRI is not needed for routine monitoring once diagnosis is established 1

Consider plain radiographs of the right hand to differentiate inflammatory arthritis from osteoarthritis:

• In osteoarthritis, DIP involvement shows Heberden's nodes (bone spurs), whereas SpA shows joint inflammation without erosive changes 2
• Post-activity swelling after golf suggests inflammatory arthritis rather than mechanical osteoarthritis 2

Treatment Escalation Criteria

If inadequate response to continuous NSAIDs after 4-6 weeks, initiate methotrexate or consider anti-TNF therapy 1:

• Methotrexate is indicated for persistent peripheral arthritis in SpA 2
• For axial symptoms, sulfasalazine and methotrexate are generally ineffective—early progression to anti-TNF agents is often necessary 2
• Short-term low-dose prednisone can serve as bridge therapy while awaiting DMARD effect 1

Monitoring Strategy

Calculate disease activity scores at 4-6 weeks and every 1-3 months until remission 1:

• Use the Ankylosing Spondylitis Disease Activity Score (ASDAS) and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) 1
• Repeat ESR and CRP together every 1-3 months during active disease, as discordance between these markers is common in SpA 1
• Once remission/low disease activity is achieved, monitor every 3-6 months 1

Critical Pitfalls to Avoid

Do not dismiss inflammatory disease based on normal CRP alone—your elevated RF and inflammatory symptoms indicate active disease despite normal acute phase reactants 1:

• In axial SpA, up to 50% of patients with normal baseline CRP will show elevated CRP on repeat testing within 4-16 weeks 1
• ESR and CRP have only 50% sensitivity in AS and early AS 2

Do not use NSAIDs "as needed" for symptom relief—they must be used continuously to suppress inflammation and assess therapeutic response 1

Do not delay physiotherapy—it should begin immediately alongside pharmacologic treatment to prevent long-term disability 2, 1

Serial radiographs are not useful for monitoring—structural changes in the spine occur slowly, and radiographs do not assess disease activity 1

Rheumatology Referral

Refer to rheumatology at 3 months if symptoms persist despite continuous NSAIDs and physiotherapy, or earlier if 2, 1:

• Moderate-to-high disease activity persists after 4-6 weeks of optimal NSAID therapy
• Evidence of progressive peripheral arthritis
• Consideration needed for biologic therapy (anti-TNF agents)

The rheumatologist can provide specialized assessment, confirm diagnosis with appropriate imaging, and initiate disease-modifying therapy to prevent long-term disability 2.