We took our kids to the northeast to see the fall colors this year. We knew Lyme disease is a problem there but we thought by now the ticks would be all dead. No such luck. Both our kids were bitten but only one developed any symptoms. They both tested positive on the special Lyme blood test. Now they are going through all the diagnostic tests. Should we ask for an MRI? Would that be helpful?

In cases where the diagnosis is already known, further testing such as CT scans or MRIs may not be helpful and they add quite a bit of expense. These kinds of advanced imaging are more likely to be ordered when the physician is trying to differentiate between various possible causes of joint pain and swelling.

For example, MRIs are helpful when trying to determine whether the cause of the symptoms is synovitis (inflammation of the synovial fluid in the joint), septic (infectious) arthritis, or Lyme disease. Patients with Lyme disease are more likely to have muscle inflammation and swollen lymph glands (as seen on MRIs) whereas patients with septic arthritis have fluid (edema) just under the skin.

There are other tests available to help make a differential diagnosis. Drawing some synovial fluid out from inside the swollen joint may be helpful. Physicians must keep in mind results of reported studies that showed a wide range of results even in known cases of Lyme disease. The test is still important to help rule out (or rule in) bacterial arthritis.

A positive ELISA blood test for Lyme disease helps make the final diagnosis. In rare cases, a positive Lyme test can be a false positive but generally, this is a very helpful test. Inflammatory markers in the blood (high white blood cell count or elevated Sedimentation rate) don’t specifically rule out Lyme disease but do help identify septic arthritis.