Diagnostic Bloodwork for Juvenile Idiopathic Arthritis
No specific blood test is diagnostic for JIA, and the diagnosis remains primarily clinical based on persistent arthritis lasting more than 6 weeks in a child under 16 years after excluding other conditions. 1, 2
Essential Initial Laboratory Tests
The following blood tests should be ordered when JIA is suspected:
Complete blood count (CBC) with differential to assess for anemia of chronic disease, thrombocytosis (common in active inflammation), and leukocytosis, which are supportive but non-specific findings in JIA 1, 2
Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) to document inflammatory activity, though normal values do not exclude JIA, particularly in oligoarticular forms 2, 3
Antinuclear antibody (ANA) testing is indicated primarily for risk stratification of uveitis development (positive in 40-85% of oligoarticular JIA), not for diagnosis, as ANA has poor specificity and is positive in 5-15% of healthy children 1, 2
Rheumatoid factor (RF) should be tested to classify polyarticular JIA subtypes, as RF-positive polyarticular JIA (rare in children, <5% of cases) carries a worse prognosis, but RF is not diagnostic and has very low sensitivity in pediatric populations 1, 3
Additional Tests Based on Clinical Presentation
HLA-B27 testing is indicated only when enthesitis-related arthritis is suspected based on clinical features (enthesitis, sacroiliitis, family history), not as a screening test, as it has limited positive predictive value 1, 2
Liver function tests (LFTs) and renal function should be obtained as baseline before initiating disease-modifying antirheumatic drugs (DMARDs), as these medications require monitoring 4
Critical Diagnostic Caveats
None of these laboratory tests should be used as rheumatological "screens" in the absence of clinical arthritis, as no individual test is diagnostic and inappropriate testing leads to false-positive results, unnecessary anxiety, and misdiagnosis 1. The positive predictive value of ANA, RF, and HLA-B27 is extremely low when ordered without strong clinical suspicion for a specific JIA subtype 1.
Systemic JIA-Specific Testing
For suspected systemic JIA (characterized by quotidian fever, evanescent rash, and systemic inflammation):
Ferritin levels should be measured, as markedly elevated ferritin (often >500 ng/mL) with low glycosylated ferritin fraction suggests systemic JIA and helps differentiate from infection 4
IL-18, CXCL9, and S100A12 are emerging biomarkers with high sensitivity and specificity for distinguishing macrophage activation syndrome (MAS) from active systemic JIA, though these are not yet widely available for routine clinical use 4