The significance of discrepant Farr and Crithidia luciliae tests

The most specific serological test for systemic lupus erythematosus (SLE) is the detection of anti-dsDNA antibodies by the Farr or Crithidia luciliae (CL) assay. Serological interpretation is difficult when these assays give discrepant results (i.e., one is positive and the other negative). In the present study 34 patients with discrepant results (18 CL positive Farr negative; 16 CL negative Farr positive) were reviewed to determine whether they fulfilled ARA criteria for SLE at presentation or at follow up 1-50 months (mean 15.8 months) later. Only 2 patients had SLE, both of whom fulfilled ARA criteria at presentation. Discrepant CL and Farr assays are associated with SLE uncommonly and rarely precede the development of clinical lupus.