During retrodialysis, the microdialysis probe is perfused with an analyte-containing solution and the disappearance of drug from the probe is monitored. The recovery for this method can be computed as the ratio of drug lost during passage (C−C) and drug entering the microdialysis probe (C). In principle, retrodialysis can be performed using either the analyte itself (retrodialysis by drug) or a reference compound (retrodialysis by calibrator) that closely resembles both the physiochemical and the biological properties of the analyte. Despite the fact that retrodialysis by drug cannot be used for endogenous compounds as it requires absence of analyte from the sampling site, this calibration method is most commonly used for exogenous compounds in clinical settings.