Validation of the 'internal reference technique' for calibrating microdialysis catheters in situ

In vivo calibration of microdialysis catheters with [3H]glucose as internal reference was done in rat (n = 17) and human (n = 12) subcutaneous tissue. The estimated interstitial glucose level was compared with the glucose concentration in venous plasma which, in turn, has been shown to be identical to the interstitial glucose concentration. In subcutaneous tissue of anaesthetized male Sprague-Dawley rats, interstitial glucose was significantly overestimated (43%, P < 0.005, n = 8, and 19%, P < 0.005, n = 9, in normoglycaemic and hyperglycaemic animals, respectively). Furthermore, fractional outflux of [3H]glucose decreased continuously during prolonged perfusion of the microdialysis catheter. In contrast, in human subcutaneous tissue microdialysed with two catheters, correct measurements of interstitial glucose could be achieved and the precision was comparable to that obtained with equilibration calibration in vivo. The average relative error of the mean result of two catheters was 8.9% at a perfusate flow rate of 1 microL min-1. It may be suggested that calibration in vivo of microdialysis catheters with internal references may be used in human subcutaneous tissue. However, it is necessary to validate the calibration technique in each different tissue under reproducible experimental conditions since accumulation of the reference substance in the tissue may create artefactual results.