Laboratory Findings in Juvenile Rheumatoid Arthritis

Hematologic abnormalities often reflect the degree of systemic or articular inflammation, with elevated white blood cell and platelet counts and decreased hemoglobin concentration and mean corpuscular volume. The ESR and CRP usually mirror these findings, along with elevated serum immunoglobulins. It is not unusual for the ESR to be normal in some children with chronic arthritis. Because platelets are an acute-phase reactant, a high ESR and neutropenia with a low platelet count may be a clue to leukemia as a cause of periarticular swelling and pain.

Elevated ANA titers are present in at least 40–85% of children with oligoarticular or polyarticular JRA, but are unusual in children with systemic-onset disease. ANA seropositivity is associated with increased risk for the development of chronic uveitis in a child with limited joint disease.

Rheumatoid-factor (RF) seropositivity may be associated with onset of polyarticular involvement in an older child (?8%) and the development of rheumatoid nodules, and with a poor overall prognosis with eventual functional disability. Both ANA and RF seropositivity occur in association with transient events during childhood, such as viral infections, particularly Epstein-Barr virus. Seropositivity for both ANA and RF must be defined at a specific titer in relation to accepted positive and negative controls and a laboratory-defined coefficient of variation.

Bone mineral metabolism and skeletal maturation are often abnormal in children with JRA with a history of active synovitis, relatively independent of onset type or course subtype, and predominantly affect appendicular cortical bone, with less effect on the normal age-related development of trabecular bone. Increased levels of cytokines such as IL-6 may decrease bone formation (reflected by decreased serum levels of osteocalcin and bone-specific alkaline phosphatase) to a greater extent than bone resorption (which may also be decreased, as reflected by decreased levels of tartrate-resistant acid phosphatase). Abnormalities of skeletal growth become most prominent during the pubertal growth spurt and in postpubertal children (Tanner stages IV–V) and lead to failure of the child to achieve acceptable peak bone mass (osteopenia).

Early radiographic changes of arthritis include soft tissue swelling, regional osteoporosis, and periosteal new-bone apposition about affected joints . Regional epiphyseal closure may be stimulated, and local bone growth decreased. In large joints, linear growth may be accelerated and limb length discrepancy, especially with involvement of a knee, becomes prominent. Continued active disease may lead to subchondral erosions and narrowing of cartilage space, especially in small tubular bones, with varying degrees of bony destruction and, potentially, fusion.

Characteristic radiographic changes in cervical spine, most frequently in the neural arch joints at C2-3 may progress to atlantoaxial subluxation.

MRI studies may be helpful to evaluate both joint and soft tissues and are more sensitive to early, minimal changes than is plain radiography