Abstract

A technique for the measurement of DNA binding activity of sera is described which uses primary internal reference standards in a Farr assay. This allows the DNA binding activity of test sera to be measured in relation to the reference standards and the results are expressed as units/ml. The use of this standardized assay has led to diminished interassay variation and close interlaboratory correlation, and has provided a clinically useful test. In survey using the standardized assay, normal sera were found to have binding activity which did not exceed 15 units/ml; 71% of patients with systemic lupus erythematosus (SLE) had levels greater than 25 units/ml; while only 8% of patients with diseases other than SLE had levels above 25 units/ml. Values between 15 and 25 units/ml appeared to have little diagnostic significance.