Calculating the precision of slope-intercept glomerular filtration rate measurements

Glomerular filtration rate (GFR) can be measured by observing the clearance of a suitable tracer from blood plasma after an intravenous injection. Slope-intercept GFR is an estimate of GFR calculated from an exponential fit to multiple blood sample measurements. The precision of the result depends on the uncertainties on all the measured quantities, but the most important factor is the error on the slope and intercept of the fit. However, these two errors are not independent as an increase in slope tends to produce an increase in intercept as well. This study derives the appropriate equations for calculating the precision of the result and shows that, if this dependence is not taken into account, the uncertainty in GFR can be significantly overestimated. The advantage of using Chi-square as a quality control measure is also introduced.