MEASUREMENT OF LOW AVIDITY ANTI-DSDNA BY THE CRITHIDIA-LUCILIAE TEST AND THE PEG ASSAY

  • 1 January 1982
    • journal article
    • research article
    • Vol. 49  (3) , 603-610
Abstract
With the immunofluorescence technique (IFT) using C. luciliae as a substrate, 14,417 sera of patients with systemic lupus erythematosus used for routine anti-dsDNA [double stranded DNA] determination, were screened for the presence of antibodies to dsDNA. The 1260 sera that were IFT positive were then assayed with the Farr radioimmunoassay, in which 3H-labeled PM2-DNA is used as antigen. Only 470 sera (37%) were Farr positive. This discrepancy is, at least partially, caused by the fact that the Farr assay does not detect anti-DNA of low avidity, whereas the Crithidia-IFT does. Of the IFT-positive/Farr negative sera 68% were positive with the PEG [polyethylene glycol] assay, a radioimmunoassay that also employs double stranded PM2-DNA as antigen, and that detects anti-dsDNA of low avidity. The IFT performed on IFT positive/Farr negative sera was rather irreproducible. This was due to local increases of the salt concentration resulting from the way the assay was performed. The problem could be overcome by careful control of the assay conditions, i.e. never letting Crithidia slides dry up after washing with PBS. In the PEG assay, these sera sometimes showed a DNA binding that decreased with time. This is caused by a parallel increase in pH during the incubation as a result of CO2 evaporation from the serum.