Due to high rates of renal blood flow, small arteriovenous differences, and analytical variation, physiologically impossible results such as negative or extremely high fractional extractions (FXs) are not uncommonly observed. These phenomena are not unique to renal experiments, but they can be observed also in splanchnic balance experiments (2).

Some investigators, when obtaining physiologically impossible negative renal glucose FXs, have chosen to consider them as zero; other investigators have accepted these data at face value, while others have repeated what they have considered to be erroneous measurements. The first approach seems reasonable, although it would introduce some bias favoring increased renal glucose uptake, which would lead to an increase in the calculation of renal glucose release (RGR). The second approach is very conservative but would have the greatest variance, thereby decreasing the power to detect small changes in RGR.