Laboratory Evaluation, Imaging, and Diagnostic Procedures
Serology
ANA by immunofluorescence is a screening test only; low titers occur in healthy children, so order it when objective findings suggest connective tissue disease. Anti-dsDNA and anti-Sm are specific for SLE; anti-Ro/La for Sjögren and neonatal lupus; U1RNP for MCTD; Scl-70 and centromere for scleroderma; Jo-1 for myositis. RF is often negative in JIA and can be positive in healthy children; anti-CCP is more specific. ANCA supports vasculitis but does not replace biopsy or imaging.
Acute-phase and routine tests
CRP rises and falls faster than ESR; ESR is raised by anemia and IVIG and falls with hypofibrinogenemia. Obtain CBC, renal and liver panels, urinalysis, and urine protein-to-creatinine ratio at diagnosis. Synovial fluid leukocyte counts overlap between inflammatory and septic arthritis, so Gram stain and culture remain essential.
Imaging and procedures
Ultrasound detects synovitis and guides injections; MRI with STIR sequences detects early inflammation, osteomyelitis, and CRMO; radiographs show damage and growth disturbance. Slit-lamp biomicroscopy is mandatory for uveitis screening. Nailfold capillaroscopy and biopsy support dermatomyositis and vasculitis.
Traps
Do not order broad autoantibody panels without a hypothesis; incidental positives cause harm. A normal ESR does not exclude inflammation.
Rule of thumb
Order the ANA only when the history and examination already suggest connective tissue disease.