Figure 3.

Flow chart of receiver operating characteristic (ROC) analysis. $S_{{\nu }_{\mathit{bm}}}^D$ and $S_{{\nu }_{\mathit{bm}}}^0$ are the SERS signals at the Raman shift of the biomarker peak,${\nu }_{\text{bm}}$, with and without antibiotic treatment, respectively. ${\nu }_{\text{bm}}$ is 730 cm⁻¹ for S. aureus and is 724 cm⁻¹ for E. coli. $r_{{\nu }_{\text{bm}}}$ is the ratio between $S_{{\nu }_{\text{bm}}}^{\text{D}}$ and $S_{{\nu }_{\text{bm}}}^0$. D1 indicates the lowest drug concentration, D2 indicates the second lowest drug concentration, and so on. $r_{{\nu }_{\text{bm}}}^{\ast }$ is the cutoff signal ratio. $r_{{\nu }_{\text{bm}},\text{Dj}}$ is the signal ratio at ${\nu }_{\text{bm}}$ of the bacterial sample treated with the antibiotic of concentration Dj. E. c. and S. a. are the abbreviations of E. coli and S. aureus, respectively. $n_{\text{TP}}$, $n_{\text{FP}}$, $n_{\text{TN}}$ and $n_{\text{FN}}$ are the numbers of true positive, false positive, true negative and false negative cases, respectively. M is the total number of samples. $max\left(r_{{\nu }_{\text{bm}}}\right)$ is the maximal measured $r_{{\nu }_{\text{bm}}}$ obtained from the M samples.