Normalized clearance-to-uptake slope ratio: A method to minimize false- positive diuretic renograms

The success of diuretic renography in the evaluation of children with renal outflow tract obstruction has been hampered by a high incidence of false-positive and indeterminate results. The main causes of interpretative errors are collecting system dilatation and impaired renal function. In this prospective study, we introduce a parameter with the potential to overcome these two problems: the normalized slope ratio (SRn). This ratio is calculated from the division of the maximum renal clearance slope by the maximum renal uptake slope, both normalized to peak renal activity. Fifty- one children (94 renal units) were evaluated. Traditional parameters such as diuretic half-time and percentage of retention at 30 min showed specificities of 77% and 85%, respectively. However, the normalized clearance slope increased specificity to 94% and SRn further improved it to 98%. The SRn takes into account the renal uptake curve, whose slope is dependent on glomerular function, and thus may correct for renal dysfunction. It also differentiates obstructed from patent kidneys even in cases with significant collecting system dilatation. As shown by regression analysis, it is independent of collecting system size. We recommend the routine use of this parameter to reduce false-positive and indeterminate results of diuretic renograms.