What are the typical laboratory findings in RA patients?
• Complete blood count: Anemia of chronic disease and thrombocytosis correlate with active disease. The WBC count and the differential should be normal unless the patient has Felty’s syndrome or another disease.
• Chemistries: Normal renal, hepatic, and uric acid tests. Albumin may be low in active disease because it is a negative acute-phase reactant.
• Urinalysis: Normal.
• Erythrocyte sedimentation rate (ESR): Usually elevated. Can be normal in patients with early limited disease. ESR can be elevated as a result of inflammation and hypergammaglobulinemia.
• C-reactive protein (CRP): Usually elevated. May be more ideal than ESR in following disease activity because it is not influenced by hypergammaglobulinemia.
• RF: Positive in 60% to 80%, with a specificity of 80% to 86% for RA. RF positivity is associated with extraarticular manifestations including subcutaneous nodules. Note that several diseases with arthritis can have a positive RF such as hepatitis C (40%–75%), SLE (20%), Sjögren’s syndrome (SS, 70%), tuberculosis (60%), and subacute bacterial endocarditis (45%–68%).
• ACPA: The most common clinically available ACPA is called antibody to cyclic citrullinated peptide (anti-CCP). Several versions of anti-CCP testing are available, with sensitivities ranging from 57% to 67% and specificity ranging from 93% to 99%. Anti-CCP is positive in 10% to 15% of RF−RA patients. Both RF and anti-CCP are associated with disease severity, erosions, and increased mortality. Higher titer antibodies (in the case of ACPA and RF) have a higher specificity for RA. However, antibody titers do not correlate with disease activity, so serial testing is unnecessary.
• Antinuclear antibodies (ANAs): Positive in 30% to 50%. Not typically directed against any specific antigens (e.g., SS-A, SS-B, ribonucleoprotein, Smith, double-stranded DNA).
• Anti-neutrophil cytoplasmic antibodies: Usually negative. If positive, it should not have specificity against proteinase 3 or myeloperoxidase.
• Complement (C3, C4, CH50): Normal or elevated. If it is low, consider a disease other than RA.
• Novel autoantibodies: There are several autoantibodies in testing and development that may be of significance to RA diagnosis and management. These include antibodies to carbamylated proteins.
• Novel inflammatory panels: There are a growing number of inflammatory markers such as cytokines, chemokines, adipokines, and other products of joint inflammation such as 14-3-3 eta which are being evaluated in the diagnosis and management of RA
