What is the diagnostic workup and treatment for an 18-year-old female with polyarticular joint pain?

Diagnostic Workup for Polyarticular Joint Pain in an 18-Year-Old Female

Begin with plain radiographs of all affected joints, followed by inflammatory markers (ESR and CRP), complete blood count, comprehensive metabolic panel, and rheumatoid factor/anti-CCP antibodies to distinguish inflammatory from non-inflammatory arthritis and guide further evaluation. 1, 2

Initial Clinical Assessment

Key Historical Features to Elicit

• Inflammatory vs. non-inflammatory pattern: Morning stiffness lasting >30-60 minutes, improvement with activity, and systemic symptoms (fever, weight loss, fatigue) suggest inflammatory arthritis 2
• Chronology: Acute onset (<6 weeks) suggests viral infection, crystal arthropathy, or reactive arthritis; chronic symptoms (>6 weeks) suggest rheumatoid arthritis, psoriatic arthritis, or systemic lupus erythematosus 3, 2
• Distribution pattern: Symmetric small joint involvement (hands, wrists, feet) suggests rheumatoid arthritis; asymmetric large joint involvement with axial symptoms suggests spondyloarthropathy 2
• Extra-articular manifestations: Skin rashes (psoriasis, malar rash), oral ulcers, uveitis, inflammatory back pain, enthesitis, or dactylitis narrow the differential 2

Physical Examination Findings

• Palpable synovitis (joint warmth, swelling, effusion) indicates inflammatory arthritis rather than osteoarthritis, which typically shows bony hypertrophy and crepitus 2
• Pattern recognition: Examine all joints systematically including temporomandibular, sternoclavicular, spine, and peripheral joints to identify distribution pattern 1
• Skin examination: Look for psoriatic plaques, nail pitting, malar rash, or nodules 2

Laboratory Evaluation

First-Line Laboratory Tests

• ESR and CRP: The combination provides the best sensitivity and specificity for inflammatory arthritis; both elevated strongly suggests inflammatory process 4, 1
• Complete blood count: Anemia and thrombocytosis suggest chronic inflammation; leukopenia may indicate systemic lupus erythematosus 2
• Comprehensive metabolic panel: Assess renal function and liver enzymes; provides baseline before initiating therapy 2
• Urinalysis: May reveal proteinuria or hematuria suggesting lupus nephritis 2

Second-Line Serologic Testing (Based on Clinical Suspicion)

• Rheumatoid factor and anti-CCP antibodies: Order when rheumatoid arthritis suspected; anti-CCP is more specific (>95%) than RF 1, 2
• Antinuclear antibodies (ANA): Order if systemic lupus erythematosus suspected based on extra-articular features 2
• HLA-B27: Consider if axial symptoms, enthesitis, or asymmetric arthritis suggest spondyloarthropathy 4

When to Perform Joint Aspiration

• Any acute monoarticular presentation to exclude septic arthritis, even if polyarticular pain present 4, 5
• Unexplained effusion with elevated inflammatory markers 4
• Synovial fluid analysis: Cell count with differential, Gram stain, culture (aerobic/anaerobic), and crystal analysis 4, 5

Imaging Strategy

Initial Imaging

• Plain radiographs of affected joints: First-line imaging for all patients; may show erosions, joint space narrowing, periarticular osteopenia (rheumatoid arthritis), or sacroiliitis (spondyloarthropathy) 4, 1
• Chest radiograph: Consider if systemic symptoms present to evaluate for sarcoidosis or tuberculosis 2

Advanced Imaging (When Indicated)

• MRI: Most sensitive for early erosive changes and bone marrow edema; reserve for cases where diagnosis remains unclear after initial workup or to assess disease activity 4, 1
• Musculoskeletal ultrasound: Can detect synovitis and erosions earlier than radiographs; useful adjunct when available 4, 1
• Whole-body MRI: Consider if chronic non-bacterial osteitis suspected (rare in this age group but can present with polyarticular pain) 4

Differential Diagnosis Considerations for This Age Group

Most Likely Inflammatory Causes

• Rheumatoid arthritis: Symmetric polyarthritis affecting small joints, positive RF/anti-CCP, elevated inflammatory markers 3, 1
• Psoriatic arthritis: Asymmetric arthritis, dactylitis, enthesitis, nail changes, family history of psoriasis 4, 2
• Reactive arthritis: Recent infection (GI or GU), asymmetric oligoarthritis, enthesitis 2, 5
• Systemic lupus erythematosus: Non-erosive arthritis, malar rash, photosensitivity, oral ulcers, positive ANA 2

Acute Self-Limited Causes

• Viral arthritis: Recent viral illness, acute onset, self-limited course over weeks 2, 5
• Post-infectious arthritis: Following COVID-19, Zika, chikungunya, or parvovirus B19 5

Less Common but Important

• Chronic non-bacterial osteitis: Bone pain with MRI showing bone marrow edema, sterile inflammation, may involve mandible, spine, or chest wall 4
• Inflammatory bowel disease-associated arthritis: Type I peripheral arthritis correlates with bowel disease activity 4

Critical Red Flags Requiring Urgent Evaluation

• Fever with acute monoarticular swelling: Septic arthritis until proven otherwise; requires immediate arthrocentesis 4, 5
• Rapidly progressive symptoms: May indicate aggressive rheumatoid arthritis or vasculitis 1
• Neurologic symptoms: Consider spinal involvement or systemic vasculitis 2

Common Pitfalls to Avoid

• Assuming osteoarthritis in young patients: Osteoarthritis is uncommon at age 18; inflammatory arthritis must be excluded 1, 2
• Relying solely on serology: RF and ANA can be positive in healthy individuals; diagnosis requires clinical correlation 2
• Delaying rheumatology referral: Early aggressive treatment of inflammatory arthritis prevents joint damage; refer within 6 weeks of symptom onset if inflammatory arthritis suspected 1
• Missing septic arthritis: Any acute joint with effusion requires aspiration to exclude infection, even in polyarticular presentation 4, 5