Recovery of renal retention function by differentiating the Patlak-Rutland plot.

INTRODUCTION

It has been suggested that renal retention function can be estimated by differentiation of the Patlak-Rutland plot.

AIM

To evaluate the error resulting from this method.

METHODS

A total of 5800 renograms were generated by convolving real input functions with artificial retention functions. Ten bi-exponential plasma disappearance curves of Tc mercaptoacetyltriglycine (Tc-MAG3) with varying renal clearances served as input functions and 580 retention functions with mean transit time between 3 and 60 min, and variable ratios of minimal to mean transit time served as original retention functions. The retention function was estimated by differentiation of the Patlak-Rutland plot of each renogram. A variant of this retention function was calculated by setting negative values to zero.

RESULTS

Minimal transit time was estimated correctly in all cases but the recovered retention function systematically underestimated the original, and negative values were observed. Mean transit time was underestimated with a difference ranging from -22.05 to -0.06 min. By setting the negative values to zero, a less important underestimation was observed which ranged from -13.37 to 0.00 min. Maximal transit time was underestimated systematically with a difference ranging from -22.05 to 0.00 min. All differences were influenced by mean transit time, renal clearance and ratio of minimal to mean transit time.

CONCLUSION

Estimation of the renal retention function by differentiation of the Patlak-Rutland plot results in systematic and sometimes important underestimations. By setting the negative values of the recovered retention function equal to zero, important but still partial improvement can be obtained.