Our daughter may have an infected hip. The doctor is running tests to see if it’s a septic hip or synovitis. What’s the difference between these two things and how can they tell which it is?

Septic arthritis of the hip in children is a painful joint condition caused by a bacterial infection. Synovitis is an inflammation of the synovial fluid protecting the joint.

Either condition can occur in children. Synovitis tends to go away on its own without damaging the hip joint. Septic hip arthritis can leave permanent damage to the cartilage, growth plate, and even cause death of the bone. Early treatment is very important in a septic condition.

After observing the child and conducting a physical exam, the doctor will order blood tests and possible X-rays. The lab tests will measure white blood cells, the sed rate, and possibly C-reactive protein levels. If all three tests are positive, chances are your child has a septic hip rather than synovitis.

The presence of fever (99 degrees or higher) is also a red flag. Children rarely have a fever with synovitis but almost always have one with a septic hip. If the tests still aren’t conclusive, then the doctor can remove some fluid from the hip and test it for bacteria. This is called joint fluid aspiration. The child must be anesthetized and a needle inserted into the joint to remove the fluid. The test is 100 percent conclusive but invasive so doctors try to make the diagnosis without this step.