Figure 117

Farr assay for quantitative measurement of anti-double-e-stranded DNA (dsDNA) antibodies. The serum to be tested is added to a tube containing radiolabeled dsDNA. When antibodies to dsDNA are present, they bind to the dsDNA. Eventually, formed complexes are precipitated in 50% ammonium sulfate. By testing several dilutions of the serum and comparing them with a standard curve the results can be expressed in units per milliliter. Because high salt conditions are used, this assay detects only high avidity anti-dsDNA antibodies [4]. Positivity and titer in this Farr assay are correlated with renal disease in patients with systemic lupus erythematosus. This titer can be used to monitor lupus disease activity together with complement levels and clinical parameters. In 80% to 90% of cases, disease onset or flare-up is associated with increases in anti-dsDNA titers in the Farr assay [6]. (From Maddison [2]; with permission.)

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