The Crithidia luciliae and Farr assays for the detection of systemic lupus erythematosus

The specificity and sensitivity of two commercial systems for the detection of anti-native deoxyribonucleic acid (DNA) antibodies in the diagnosis of active systemic lupus erythematosus were evaluated. Sera containing antinuclear antibodies from 64 patients were tested by an indirect immunofluorescent antibody technique employing the protozoan Crithidia luciliae as substrate and immunoradioassay based on the Farr technique. Twenty one patients had active systemic lupus erythematosus, 21 had other collagen vascular diseases, and 22 had miscellaneous non-collagen vascular diseases. The immunofluorescence test was 100 percent specific and 66.7 percent sensitive for systemic lupus erythematosus, whereas the radioimmunoassay was 90.7 percent specific and 80.9 percent sensitive. There was agreement between methods in 57 of the 64 (89.1 percent) sera tested.