Figure 4.

Dilution of serum in SAB assays does not result in a predictable or quantitative reduction in bead MFI. Serial dilutions were performed on dithiothreitol-treated serum tested on SAB in 10 consecutive patients with DSA. The expected change in MFI from baseline over dilution if neat serum MFI quantitatively represented antibody amount is shown with the dashed lines. MFIs of the highest bead(s) contributing to DSA in each of the sera are shown at each dilution as a percentage of the neat serum MFI (by definition, 100%). The MFIs vary widely through dilution; although antibody concentration is decreasing by a known amount through serial dilution, MFI changes are not proportional. Neat serum MFI does not, therefore, reliably represent antibody amount.