Measurement of low avidity anti-dsDNA by the Crithidia luciliae test and the PEG assay.

With the immunofluorescence technique (IFT) using Crithidia luciliae as a substrate, 14,417 sera sent to our laboratory for routine anti-dsDNA determination, were screened for the presence of antibodies to dsDNA. The 1,260 sera that were found IFT positive were then assayed with the Farr radioimmunoassay, in which 3H-labelled PM2-DNA is used as antigen. Only 470 sera (37%) were found to be Farr positive.

Specificity in systemic lupus erythematosus of antibodies to double-stranded DNA measured with the polyethylene glycol precipitation assay.

Recently, a new radioimmunoassay--the polyethylene glycol (PEG) assay--was introduced to measure antibodies to double-stranded (ds) DNA. In this method, polyethylene glycol precipitation of formed 3H-DNA/antiDNA complexes is used instead of the ammonium sulfate precipitation used in the Farr assay. In contrast to the Farr assay, with which only high-avidity antibodies to dsDNA are detected, the PEG assay also reportedly measures anti-dsDNA of relatively low avidity.

Avidity of antibodies to dsDNA: comparison of IFT on Crithidia luciliae, Farr assay, and PEG assay.

The Farr assay is thought to detect only antibodies to DNA of relative high avidity. This is due to the high salt concentration of the employed ammonium sulfate precipitation, which dissociates DNA-anti-DNA complexes of low avidity. A recently introduced method to detect anti-DNA, the PEG assay, circumvents these dissociating reaction conditions by using polyethylene glycol instead of ammonium sulfate to precipitate the complexes; we therefore thought to measure antibodies to DNA of low avidity as well.