Comparison of measles-specific IgG (OD values) in serum and oral fluid of 283 adolescents. Horizontal lines indicate cutoff values of the serum assay (negative, <0.1 OD; positive, >0.2 OD; greyzone, 0.1 to 0.2 OD). IgG-positive (▴), -negative (▵), and -equivocal (□) oral fluid samples. In contrast to the serum assay, greyzone intervals of the oral fluid assay are defined with respect to negative controls on the same test plate and not in absolute OD values.