THE INTERNAL REFERENCE TECHNIQUE IN MICRODIALYSIS - A PRACTICAL APPROACH TO MONITORING DIALYSIS EFFICIENCY AND TO CALCULATING TISSUE CONCENTRATION FROM DIALYSATE SAMPLES

In microdialysis experiments, 'recovery' estimations are required to calculate extracellular concentrations of the compounds determined. Generally, relative recovery (RR) is determined in vitro as: RR = c(d)/c(s), with (c(d)) being the concentration of a compound in a dialysate fraction and (c(s)) its known concentration within a sample solution. To determine recovery(in vivo), relative loss (RL) was defined RL = (c(p) - c(d))/c(p) with (c(p) - c(d)) being the loss of a compound from the perfusate and (c(p)) its perfusate concentration. RL was determined in vitro and in vivo by adding an 'internal reference compound' to the perfusate. Here, C-14-labelled lactate was used as the compound of interest. Comparing RL and RR in vitro, we found both to be similar. In vivo, however, RL was 34% of RL(in vitro) (CSF) and 46% of RL(in vitro) in agar-containing CSF. During ischaemia, RL of lactate even decreased to only 35% of the pre-ischaemic control level. We conclude that RL and RR represent inverse measurements of 'recovery.' Whereas RR can only be determined in vitro, RL can be determined in vivo. We found recovery(in vivo) to be different from recovery(in vitro). Moreover, recovery(in vivo) decreased during ischaemia. By means of the measured recovery(in vivo) extracellular lactate concentrations prior and during ischaemia were calculated. The results, therefore, validate the 'internal reference technique' as a practical method for estimating recovery(in vivo) and for controlling dialysis efficacy in vivo even continuously.