Recommended Laboratory Workup for Inflammatory Arthritis
The essential laboratory workup for inflammatory arthritis should include erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), complete blood count, liver and kidney function tests, and urinalysis, along with baseline radiographs of affected joints. 1, 2
Core Laboratory Tests
Inflammatory Markers
- ESR and CRP: Should be performed at baseline for diagnosis and prognosis of inflammatory arthritis and repeated when clinically relevant 1
- ESR has diagnostic value for development of rheumatoid arthritis (RA)
- CRP may predict RA, especially when levels are higher
- Both help monitor disease activity and treatment response
Autoantibody Testing
- RF and ACPA: Essential tests for evaluation of inflammatory arthritis 1, 2
- Strong predictors of RA diagnosis and prognosis
- Presence increases probability of developing persistent synovitis and worse radiographic outcomes
- Negative tests do not exclude progression to RA (>30% of RA patients may have negative RF or ACPA) 3
Basic Laboratory Panel
- Complete blood count: Evaluates for anemia of chronic disease and monitors for medication toxicity 2, 4
- Liver function tests: Establishes baseline before starting medications like methotrexate 2
- Renal function tests: Assesses kidney function before starting certain medications 2
- Urinalysis: Screens for renal involvement and helps differentiate between types of inflammatory arthritis 1, 2
Imaging Studies
- X-rays of affected joints: Should be performed at baseline 1
- X-rays of hands, wrists, and feet: Recommended even if not clinically involved, as presence of erosions predicts development of RA and disease persistence 1
- Follow-up X-rays: Should be repeated within 1 year if disease persists 1
Additional Testing Based on Clinical Suspicion
Additional autoantibody tests if connective tissue disease/systemic inflammatory disorder is suspected: 1
- Antinuclear antibodies (ANA)
- Antibodies to extractable nuclear antigens (ENA)
- Anti-double-stranded DNA antibodies
HLA typing (particularly HLA-B27): Consider when spondyloarthritis is suspected, especially in RF and ACPA-negative patients 1
Pelvic/sacroiliac joint X-rays: Consider in RF and ACPA-negative patients or if spondyloarthritis is suspected 1
Diagnostic Value and Limitations
- Combined testing of RF and ACPA improves diagnostic accuracy (90.2% sensitivity, 83.3% specificity) compared to individual tests 5
- ACPA has higher specificity (90.4%) than RF (74.4%) for RA diagnosis 5
- Despite their utility, 30-40% of RA patients may have normal ESR or CRP, and >30% may have negative RF or ACPA tests 3
Pitfalls to Avoid
- Relying solely on laboratory tests for diagnosis - clinical evaluation remains essential
- Overlooking inflammatory arthritis in patients with only one or few affected joints initially 2
- Assuming that hand stiffness in older adults is always due to osteoarthritis 2
- Failing to repeat inflammatory markers when clinically indicated
- Not obtaining baseline X-rays, which are crucial for future comparison
The laboratory workup should be interpreted in conjunction with a thorough clinical assessment, including evaluation of joint patterns, duration of morning stiffness, and presence of extra-articular manifestations to guide diagnosis and treatment decisions.