Figure S4.

Comparison of sample dilution series measured by ELISA and FMIA. Samples were serially-diluted (1:100, 1:200, 1:400, and 1:800) and tested by ELISA and FMIA; each set of colored data points represents the dilution series for a given sample tested by both methods. There is no purified source of the target analyte (LPS-specific human IgM antibody), so creating a quantitative response curve by conventional sample spiking was not possible. Because the analyte concentrations for these patient samples are not known, the response curves for each sample dilution series were normalized to fall on a single response curve to estimate the linear response range from patient samples. Specifically, the linear portion of the dilution series for each patient sample (data points of a given color) was fit to a line, and the data points for that sample were scaled by the slope of the response curve to collapse the data on a common response curve. The normalization has no effect on the shape of individual response curves. Thus, the concentration scale is arbitrary, but it gives a qualitative indication of the linear range for the two formats. Each color represents the dilution series for an individual sample, and error bars are for two replicates performed on each sample at each dilution. Saturation at high signals is qualitatively indicated by the gray trace. The apparent linear range is similar for ELISA and FMIA.