Laboratory Testing for Rheumatoid Arthritis
For any patient with suspected rheumatoid arthritis presenting with synovitis, order rheumatoid factor (RF), anti-cyclic citrullinated peptide (anti-CCP) antibodies, ESR or CRP, complete blood count, liver transaminases, renal function tests, and urinalysis as your initial laboratory panel. 1
Essential Diagnostic Tests
Autoantibody Testing
- Test rheumatoid factor (RF) in all patients with suspected RA who have synovitis, even though 30-40% of RA patients may be RF-negative 1, 2
- Measure anti-CCP antibodies particularly when RF is negative or when combination therapy is being considered 1, 2
- Anti-CCP has higher specificity (90%) than RF and provides important prognostic information about persistent and erosive disease 3
- High-positive RF or anti-CCP carries more diagnostic weight than low-positive results in classification criteria 3
Inflammatory Markers
- Measure ESR or CRP to assess disease activity and prognosis; CRP is preferred as it is more reliable and not age-dependent 1, 3, 4
- These acute phase reactants are incorporated into composite disease activity measures (DAS28-ESR, DAS28-CRP, SDAI) that guide treatment decisions 4
Baseline Laboratory Panel
- Complete blood count with differential to assess for cytopenias and calculate inflammatory ratios 1, 4
- Liver transaminases (AST, ALT) for baseline hepatic function 1
- Renal function tests (creatinine, BUN) for baseline kidney assessment 1, 4
- Urinalysis to assess for renal involvement 1, 4
Additional Baseline Tests
- Antinuclear antibodies (ANA) to exclude other rheumatic diseases 1
- Comprehensive metabolic panel including glucose and uric acid levels 4
Critical Clinical Caveat
Refer patients urgently for specialist evaluation even when RF is negative or acute-phase reactants are normal, particularly if small joints of hands or feet are affected, more than one joint is involved, or there has been a delay of three months or longer between symptom onset and seeking medical advice 1, 2, 3. This is crucial because 30-40% of RA patients are seronegative, and 40% have normal ESR or CRP at presentation 5.
Pre-Treatment Screening (When Biologics Considered)
- Tuberculosis screening with tuberculin skin test or interferon-gamma release assay (IGRA preferred if prior BCG vaccination) 4
- Hepatitis B surface antigen, surface antibody, core antibody, and hepatitis C antibody before starting biologic therapy 4
- Pregnancy testing before starting teratogenic medications like methotrexate or leflunomide 4
Prognostic Laboratory Assessment
Once RA is suspected, measure the following to predict persistent and erosive disease: number of swollen and tender joints, ESR or CRP levels, RF and anti-CCP antibody levels, and obtain baseline radiographs to assess for erosions 1. Dual positivity for both RF and anti-CCP indicates higher risk of aggressive disease 2, 3.