Validation of an endogenous reference technique for the calibration of microdialysis catheters

In order to validate the use of urea as an internal reference for the calibration of microdialysis catheters, both subcutaneous and muscle measurements were taken in 11 healthy volunteers. The equilibration calibration of urea, glucose and lactate, and the internal reference (3H-glycerol retrodialysis) calibration of glycerol were performed in both the muscle and fat. These data were then compared with those taken from the arterial plasma. Based on the average arterial-interstitial fluid concentration difference, the urea microdialysis relative recovery was estimated from dialysate and plasma content of urea. Interstitial urea was then used as an endogenous reference for the calculation of the relative recovery of each substance. This was carried out after defining the relative recovery ratios (RR) that were obtained from in vitro measurements on crystalline solutions with the assumption that RR in vivo = RR in vitro. The data demonstrate that the interstitial fluid concentrations of glucose, lactate and glycerol can be calculated from the knowledge of plasma and microdialysate content of urea together with the RR in vitro. Also, the data that were obtained using urea as an endogenous reference showed a significant correlation with those that were achieved using either equilibration or internal reference (retrodialysis) techniques for calibrating the microdialysis devices.