THE ROBUSTNESS OF THE PATLAK-RUTLAND SLOPE FOR THE DETERMINATION OF SPLIT RENAL-FUNCTION

The aim of this study was to evaluate the variability of the Patlak-Ru tland slope as applied to the renal clearance method. We analysed the Tc-99(m)-mercaptoacetyltriglycine renograms of 17 non-selected patient s (both children and adults, number of kidneys = 34) with a single kid ney glomerular filtration rate of 6-73 ml min(-1) 1.73 m(-2). The acqu isition time for the renograms was 20 min with a frame duration of 20 s. First, correction for background activity was introduced using the perirenal area and the size ratio between the kidney and the perirenal area. The Patlak plot was then applied. The first two points were sys tematically rejected and a series of linear fits calculated from the e xperimental points, starting from the third point (i.e. 1 min). The fi nal point of the fit never exceeded the time of the peak of the renogr am minus one minute (T-max - 1) and was always less than 5 min. Althou gh simple visual inspection of the slopes would suggest that the exper imental points were distributed accurately along a straight line, it a ppeared that the slope was strongly dependent on the number of points used for the determination of the fit. As a second step, we restricted the number of points for fitting to the time interval between 1 min a nd 2 min 40 s. This procedure appeared to show that the Patlak methodo logy is reliable, giving rise to only small variations in slope, depen ding on the number of points chosen. However, even in this narrow time interval, significant errors can be made, either because of the very early escape of the radionuclide or because of statistical noise (e.g. in renal failure the signal-to-noise ratio is unfavourable). Therefor e, it is advisable, when using this methodology, to restrict the fitti ng procedure to the second minute of the study and to check visually t hat this fixed time interval gives rise to a slope that is well adapte d to the Patlak plot.