Interlaboratory reproducibility of SBA for the three serotypes, S. flexneri 2a (A and B), S. flexneri 3a (C and D), and S. sonnei (E and F), determined with 32 adult human sera. KIs from individual laboratories are compared against the average KI before (A, C, and E) and after (B, D, and F) normalization. The normalization used Korean QC19 as the reference serum, with assigned KIs of 28,000 for S. flexneri 2a, 19,000 for S. flexneri 3a, and 1,100 for S. sonnei. Average KI shows the geometric means of KIs from the four laboratories (marked with different colors). The dotted lines indicate 2-fold deviations from identity.