Laboratory Workup for Psoriatic Arthritis
Order ESR and CRP as mandatory baseline tests, then add RF and anti-CCP antibodies to exclude rheumatoid arthritis, particularly in polyarticular presentations. 1
Core Laboratory Tests
Mandatory Baseline Assessment
- ESR and CRP are the only laboratory tests specifically recommended by OMERACT 8 consensus guidelines for baseline PsA evaluation 1
- These acute phase reactants assess inflammation severity, though they are elevated in only approximately 50% of PsA patients 2
- Despite limited sensitivity, elevated ESR is one of the best predictors of radiographic damage progression and is associated with increased mortality when >15 mm/h 2
Differential Diagnosis Testing
- RF and anti-CCP antibodies should be ordered to exclude rheumatoid arthritis, especially when patients present with polyarticular disease that may be clinically indistinguishable from RA 1
- RF is present in 5-13% of PsA patients, and anti-CCP antibodies occur in similar percentages, so their presence does not absolutely exclude PsA but warrants careful diagnostic scrutiny 2, 3
- The presence of RF or anti-CCP does not preclude PsA diagnosis but should prompt careful review of clinical features 3
Critical Clinical Context
No Diagnostic Markers Exist
- There are no true diagnostic laboratory markers for PsA—diagnosis relies primarily on clinical criteria, specifically the CASPAR classification system 1
- Laboratory tests serve to assess inflammation severity and exclude other diagnoses rather than confirm PsA 2
CASPAR Criteria Framework
- The diagnostic algorithm should start with confirmation of inflammatory musculoskeletal disease through clinical examination, followed by CASPAR scoring (requiring ≥3 points), then obtaining RF and ordering hand/foot radiographs as needed 4
- CASPAR criteria demonstrate 98.7% specificity and 91.4% sensitivity for established disease 4
Additional Baseline Assessments
Imaging
- Plain radiographs of hands and feet should be performed at baseline for all patients with suspected inflammatory arthritis to identify juxta-articular new bone formation (worth 1 CASPAR point) and establish baseline for damage progression 4, 3
Comprehensive Disease Assessment
- Baseline evaluation should include 68 tender joint count, 66 swollen joint count, patient global assessment, pain scores (VAS or category scales), physical function (HAQ), and quality of life measures (SF-36 or PsAQoL) 5
- Fatigue assessment through patient self-report or FACIT instrument 5
Common Pitfalls to Avoid
- Do not rely on normal ESR/CRP to exclude active PsA, as approximately half of patients have normal acute phase reactants despite active disease 2
- Do not dismiss PsA diagnosis based solely on positive RF or anti-CCP, as these occur in 5-13% of PsA patients 2
- Recognize that up to 20% of patients develop arthritis before psoriasis appears, sometimes years before skin changes 3
- Synovial fluid analysis, when available, may show elevated leukocyte counts indicating inflammation even when serum acute phase reactants are normal 2