Validation of urea as an endogenous reference compound for the in vivo calibration of microdialysis probes

Microdialysis is a widely used experimental technique, which offers the opportunity to measure drug and metabolite concentrations in the interstitial space fluid in animals and humans. However, microdialysis probes need to be calibrated in vivo to obtain a recovery factor, which describes the relative drug transfer across the membrane. Recently, novel time-saving calibration techniques, based on the use of reference compounds, have been developed. In particular, the use of endogenous urea levels has been advocated. In the present study we set out to validate the use of the urea reference technique for microdialysis probe calibration in healthy volunteers, employing glucose and paracetamol as model analytes. Urea calibration was compared with the results of two standard calibration techniques, i.e. the no net flux technique and the retrodialysis technique. For glucose, recovery values, calculated by the urea reference technique differed significantly from those values, which were assessed by the no net flux technique (p < 0.05), whereas for paracetamol recovery values did not differ significantly, albeit a high intramethod variability was observed (CV=66%). As a conclusion, we could not confirm the hypothesis that recovery values calculated by the urea reference technique provide equivalent results compared with standard calibration techniques.