Measurements of Endpoint Titers Based on the Fluorescence Intensity Trend in Anti-Nuclear Antibody Testing

Abstract

Background: Automated systems for antinuclear antibody (ANA) testing provide endpoint titers that are predicted based on the fluorescence intensity (FI) value at a screening dilution (single-well titration [SWT]) showing frequent titration errors (more than plus or minus 1 dilution).

Methods: Line slope titration (LST) was based on the trend of FI values on dilutions. Three dilutions per specimen were prepared considering a patient's previous titer or FI at the screening dilution. On the XY plot, with the reciprocal of dilution as the X-axis and FI value as the Y-axis, a fitted line was drawn to obtain the endpoint titers.

Results: The titration error rate (no. of errors/total no.) of LST using a regression line was lower than that of SWT (31/710 [4.4%] and 152/674 [22.6%], respectively; P < .000000001), with serial dilution as a reference. When comparing a regression line using 3 dilution points with a line using 2 dilution points, the error rate of the former was not significantly different from that of the latter (31/710 [4.4%] and 31/746 [4.2%], respectively; P = .842).

Conclusions: This LST method is useful as an accurate, cost-effective, and rapid approach to measure endpoint titers in routine ANA testing.

Keywords: anti-nuclear antibody testing; automated system; endpoint titer; fluorescence intensity; serial dilution; systemic autoimmune rheumatic disease.