Juvenile arthritis (JA) may be difficult to diagnose. Many children may not experience identifiable symptoms, and the symptoms they do have may be caused by other conditions, such as lupus, Lyme disease or bone disorders. Also, joint swelling may not be immediately obvious or may not be the first symptom to show in a child.

Parents may notice the child limping or not using a limb because of pain and stiffness. Diagnosis typically begins with the child's pediatrician. A pediatrician who suspects some form of JA may refer the case to a pediatric rheumatologist, a physician who specializes in arthritis and other inflammatory diseases in children.

A physical examination includes checking the joints for swelling, warmth, tenderness and range of motion. The physician notes descriptions and incidences of pain and its duration. The physician may try to move various joints to see if motion causes pain. The diagnosis will also include a medical history, including any family history of arthritis or autoimmune conditions, previous injuries or surgeries and general use of the joints. The physician will also note any fevers, rashes or infections.

Many laboratory tests are used to screen for specific kinds of arthritis or other diseases. Certain blood tests may produce characteristic results for some types of JA. However, none of these tests definitively diagnose JA. Some of the blood tests used include:

• Complete blood count (CBC). Measures the levels of different types of blood cells. Children with certain types of JA may show a low count of red blood cells (anemia) and higher counts of white blood cells.
  
  
• Erythrocyte sedimentation rate (ESR or sed rate). When elevated, indicates inflammation, which may be related to many forms of infection.
  
  
• Antinuclear antibody (ANA) test. Tests for certain autoimmune disorders such as lupus. Children with certain types of juvenile rheumatoid arthritis (JRA) test positive and are more likely to develop eye inflammations. Children with other types of JRA usually test negative for ANA.
  
  
• Rheumatoid factor test. Identifies the presence of the rheumatoid factor antibody. Absence or presence of the antibody may help a physician define a specific type of JA. For example, the antibody is common in certain forms of JRA but absent for others.
  
  
• HLA-B27 test. Presence of this genetic marker in the white blood cells may indicate many conditions, including ankylosing spondylitis, some types of JRA and Reiter's syndrome.

Another test that may be used is a synovial fluid analysis. Fluid from an affected joint is withdrawn with a needle (a procedure called arthrocentesis). Examining this fluid may help determine whether inflammation or infection is present, as well as what is causing the inflammation.

A diagnostic examination where JA is suspected may also include x-rays. Images are taken of the affected joints and may help rule out other causes. Other tests may also be used to rule out other potential bone problems such as congenital deformities or bone cancer. These include a CAT scan (computed axial tomography), where multiple x-ray images are taken, and MRI (magnetic resonance imaging), which uses powerful magnets to form an image of the bone and tissues.

For many forms of JA, other specialists may be involved to address conditions that affect other parts of the body. An ophthalmologist (physician who specializes in eye diseases) may examine JA patients who are susceptible to eye inflammations, which may not cause symptoms. The ophthalmologist may use a device called a slit lamp, which has high-intensity light that can be focused in a narrow area. It is used to examine some of the structures of the eye, such as the iris and other parts of the uveal tract that are subject to inflammation.

A dermatologist (physician who specializes in skin diseases) may be consulted for conditions that affect the skin, such as psoriatic arthritis or juvenile dermatomyositis.