Fig 3. Classification of samples.

Shown are the estimated components of the mixture distribution using the parameter posterior medians (left-hand panel; blue: susceptible; purple: infected; red: infected with increased antibody concentration), and receiver operating characteristic of binary classifications taking these estimates as ground truth (right-hand panel). The maximal Youden index for classification of uninfected versus infected persons is 0.97 at antibody concentration -0.70 U/ml, with sensitivity 0.99 and specificity 0.98. This value corresponds well with the threshold for infection of -0.8 U/ml provided by the supplier of the assay. The maximal Youden index for classification of persons with increased antibody concentration is 0.71 at antibody concentration 1.81 U/ml, with sensitivity 0.84 and specificity 0.87.