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A cross sectional study in a routine setting has found that Farr radioimmunoassay is more sensitive than a new automated fluorescence immunoassay for diagnosing systemic lupus erythematosus (SLE).
Farr immunoassay performed better at identifying antibodies to dsDNA in serum samples of patients with suspected SLE or lupus like disease (LLD). At specificities of 95% and 99% the corresponding sensitivities were 72% and 56%, respectively, but in the automated fluorescence immunoassay (ELIA dsDNA test) were only 44% and 17%, respectively. These values were obtained from receiver operating curves (ROC) for each test calculated from samples from SLE and non-SLE, non-LLD patients. Of all 440 patients, 115 (36%) were grouped as non-SLE, non-LLD; 77 (18%) LLD; and 248 (56%) as SLE according to American College of Rheumatology (ACR) criteria.
The samples were sent for routine hospital testing for dsDNA antibodies. Each sample was tested by the two methods according to the manufacturers’ instructions. A researcher who was blinded to the assay results grouped patients by the number of ACR criteria they displayed (≥4, SLE; 1–3, LLD; and zero or antinuclear antibody positive, non-SLE, non-LLD).
Farr immunoassay is commonly used to detect dsDNA antibodies—one of the four ACR criteria for diagnosing SLE. It is very specific and predictive for SLE. The ELIA dsDNA test has recently been claimed to be comparable in performance and cost, but quicker and more convenient. However, the comparisons relied on less appropriate control samples from a mixture of sources and less “realistic” (non-routine) test conditions.