Sensitivity and specificity of IgA and IgG for estimation of optimal cut-off levels. Sensitivity and specificity of different IgA (figure 2a) and IgG (figure 2b) antibody levels (U/ml) against B. pertussis from the convent population during the pertussis epidemic. Sensitivities are calculated in the 83 serum samples obtained in week 9 and 13 of the epidemic, from the pertussis cases (n = 46) who had been coughing for between 1 and 87 days of their total period of clinical pertussis. Pertussis cases had a laboratory confirmed B. pertussis infection and a clinical pertussis (see methods). Specificities are calculated in the 68 serum samples obtained in week 9 and 13 of the epidemic from the internal controls (n = 35). Specificities of IgA and IgG levels calculated in the sera (n = 4275) of external controls (n = 4275) are indicated as well. The sensitivity en specificity mark in pertussis cases and internal controls of the IgA level of 24 U/ml and the IgG level of 27 U/ml with a specificity of 90% in external controls are indicated in black symbols.