Discrepancies between two procedures for ds-DNA antibody detection: Farr test and indirect immunofluorescence on Crithidia luciliae.

Of the 54,911 sera routinely tested for anti-double stranded-DNA antibodies (Ab), 2,297 gave a positive reaction with both indirect immunofluorescence (IIF) on Crithidia luciliae (CL) and the Farr test, or with only one of the two tests. Of the sera giving positive reactions, only 1,499 (65.3% of the positive sera) were positive with both reactions. Among the remaining 798 sera (34.7% of positive sera), 48.25% gave a positive reaction with the Farr test and 51.75% with the IIF reaction. Of the discrepant Farr test(+), IIF-CL(-) sera, slightly fewer than half corresponded to a false positive reaction of the Farr test due to the presence in the serum of proteins other than Ab which were able to bind to the labelled ds-DNA to form a complex precipitable by 50% saturated ammonium sulfate solution. Slightly more than half of the other Farr test(+) IIF-C'(-) sera corresponded to a defect in the IIF-CL reaction. Among the discrepant Farr test(-) IIF-CL(+) sera, 1/3 corresponded to false positive reactions IIF-CL and 2/3 of the remaining sera contained weakly avid anti-ds-DNA antibodies undetectable using the Farr test.