Fever of unknown origin (FUO) due to systemic lupus erythematosus (SLE) presenting as pericarditis.

Fevers of unknown origin (FUOs) are classified according to the underlying disorder. The 4 main clinical categories of FUOs are infectious, malignant, rheumatic/inflammatory, and miscellaneous disorders. Although malignancy remains the most common cause of FUOs, rheumatic/inflammatory disorders remain important diagnostically and therapeutically. Rheumatic/inflammatory disorders, for example, systemic lupus erythematosus (SLE) presenting as FUO, have become uncommon in recent years because of better serologic diagnostic tests. However, SLE remains a rare but important cause of FUO in adults. SLE may be a difficult FUO diagnosis when a patient presents with fever without joint manifestations as the only symptoms of SLE. During the workup of the patient described in this article, the other causes of pericarditis were ruled out and SLE pericarditis was diagnosed. This is a rare case of an adult FUO with pericarditis as the only manifestation of SLE.