This patient has inflammatory arthritis and tests negative for Borrelia burgdorferi, so she requires no additional evaluation or treatment for Lyme disease. Arthritis is a late manifestation of Lyme disease, typically occurring months to years after infection. This typically presents as inflammatory arthritis, involving a single or limited number of large joints, with the knee most commonly affected. Even without treatment, symptoms wax and wane, with spontaneous remission of inflammation often followed several months later by involvement of the same or a different joint. Because the clinical presentation of Lyme arthritis is nonspecific, laboratory confirmation of infection is required using a two-tiered serologic testing strategy. The initial test, an enzyme immunoassay (EIA), is exquisitely sensitive but not specific. If the result of this test is negative, no further evaluation is necessary. When the EIA finding is positive or equivocal, a more specific Western blot test is recommended for confirmation. A negative Western blot result is interpreted as a negative serologic result. A positive result must be further interpreted with respect to acuity of symptoms. When symptoms are present for less than 1 month, an isolated positive IgM Western blot result may be diagnostic of acute infection. However, symptoms present for more than 1 month provide ample time for IgG seroconversion. Many laboratories perform the IgM and IgG Western blots reflexively on any sample with a positive EIA finding. When this is the case, a positive Western blot IgM result without associated positive IgG result in a patient with more than 30 days of symptoms should be interpreted as a false-positive result. Therefore, results of Lyme serologic testing for this patient would be interpreted as negative, and the diagnosis of Lyme arthritis excluded. Further evaluation or treatment for Lyme disease is not necessary at this time, and other causes for this patient's inflammatory arthritis should be evaluated.

Detection of Borrelia bacteria in synovial fluid or tissue is not necessary to diagnose Lyme arthritis and would be indicated only in selected scenarios, such as new-onset arthritis in a patient previously known to be seropositive for Lyme disease.

If the Western blot IgG had returned positive, the recommended treatment would be a 28-day course of doxycycline or amoxicillin. Parenteral therapy with ceftriaxone would be indicated for patients with Lyme arthritis who do not experience a clinical response to oral antibiotics.