It is sometimes difficult to distinguish infection from disease flare in febrile patients with systemic lupus erythematosus (SLE). Chill, leucocytosis, and increased C reactive protein (CRP) are known to be markers favouring infection.1 Procalcitonin (PCT) is the precursor of calcitonin and is synthesised in the parafollicular C cells of the thyroid. Serum PCT increases in severe bacterial or fungal infection but does not increase, or increases only slightly, in viral infections.2 ,3 The purpose of this study was to evaluate the usefulness of serum PCT in febrile episodes of patients with SLE to distinguish infection from disease flare.

We prospectively enrolled 19 patients with SLE with fever who were admitted to Seoul National University Hospital between October 1998 and April 1999. Fever was defined as an axillary temperature over 38°C. Eleven patients with inactive SLE were enrolled as controls. Blood of the febrile lupus patients was withdrawn three times: on the day of the hospital visit, and after 24 hours and 48 hours. Another sample was withdrawn two weeks after defervescence to control infection or because of a decrease in lupus activity. At the detection of fever, blood cultures and other necessary cultures were performed with complete blood count, Westergren erythrocyte sedimentation rate (ESR), CRP, serum anti-dsDNA, …