Diagnosis of Juvenile Idiopathic Arthritis
To diagnose JIA, the doctor will take ask you questions about your child’s symptoms, find out whether other family members have had similar problems, and do a thorough physical examination. The doctor may order X-rays or blood tests to rule out other conditions or infections, such as Lyme disease, that may cause similar symptoms or occur along with the arthritis.

Other tests may include:
CBC (complete blood count), a common blood test that checks all the basic cell types in blood, including red blood cells, white blood cells, and platelets. Knowing the amount and appearance of each cell type in a person’s blood can help doctors identify many medical conditions.
Blood culture, a test to detect bacteria that cause infections in the bloodstream.

Bone marrow biopsy, a test that allows doctors to look at blood where it's formed (in the bone marrow) to look for conditions such as leukemia.
Erythrocyte sedimentation rate, which checks how rapidly red blood cells settle to the bottom of a test tube. This rate often increases in people when inflammation is occurring in the body.

C-reactive protein (CRP) is a protein that the liver releases into the blood at the start of infection or inflammation; this level can rise if a person is having an arthritis flare-up.

A test for rheumatoid factor (RF) and cyclic citrullinated peptide antibody (CCP), substances made in the blood of children with some forms of JIA. But it's found more often in adults with rheumatoid arthritis.

ANA (antinuclear antibody), a blood test to detect autoimmune diseases. It's also useful in predicting which kids are likely to have eye disease with JIA.

X-rays of the affected joints, and sometimes an MRI, to detect changes in bone and joints to evaluate the causes of unexplained bone and joint pain. In some cases, doctors may do a test called a bone scan.