Fig. 4. Linear regression plots of the concentrations measured by electrochemical detection and mass spectrometry. Each plot for (A ) lidocaine (n = 16), (B ) racemic bupivacaine (bupivacaine) (n = 12), and (C ) levobupivacaine (n = 12). Solid lines represent the regression lines. P < 0.001 for significance of the Spearman rank order correlation for lidocaine, bupivacaine, and levobupivacaine.