Precise quantitation of antinuclear antibodies on HEp-2 cells without the need for serial dilution.

Using HEp-2 cells as a substrate, we developed a method to quantitate antinuclear antibodies (ANA) by comparing the green fluorescence intensity of unknown samples with that of calibrated standards. Intensity was then converted to international units per milliliter by reference to a standard curve. This method is accurate and precise around the cutoff for positively (5 to 10 IU/ml) and therefore provides a reliable screening test for active, untreated systemic lupus erythematosus. Furthermore, the method can identify sera likely to contain autoantibodies commonly detected in ANA-positive sera (SS-A, SS-B, Sm, small nuclear ribonucleoprotein, Scl-70, and double-stranded DNA).