Fig. 1.

Comparison of serum antibody concentrations (IU/ml) for the DIPNET EQA panel (n = 141) as measured by the diphtheria toxin (Dtoxin) MIA and ToBI (A), the diphtheria toxin MIA and Vero cell NT (B), or the competitive diphtheria toxin MIA and ToBI (C). The regression line is indicated as a solid line, the line of identity is dotted, and 3-fold deviations are indicated as interrupted lines. Vertical and horizontal dotted lines indicate the cutoffs to determine negative (<0.01 IU/ml), intermediate (0.01 to 0.09 IU/ml), positive (≥0.1 IU/ml) sera.