Changing the technique of background subtraction alters calculated renal function on pediatric...

Although many operative decisions are based on apparent changes in function detected by serial diuretic renography, the reliability of relative renal function data has been questioned. We investigated the effect of background subtraction on calculated renal function.

We reviewed 23 pediatric mercaptoacetyltriglycine renograms. Regions of interest for background subtraction were redrawn and relative renal function values were recalculated.

In patients with left hydronephrosis left differential renal function was consistently underestimated by 7.3% compared to the superior and 4.7% compared to the average background subtraction technique. In patients with right hydronephrosis, when considering lower pole background activity only, right differential renal function was consistently overestimated by 15.8% compared to the superior and 2.4% compared to the average background subtraction technique. In patients younger than 1 year at the time of study these differences were exaggerated.

Because of overlapping hepatic background activity, the position of regions of interest for background subtraction may greatly influence calculated renal function. We recommend that the preferred method of calculating background subtraction to minimize error be based on a region of interest surrounding the whole kidney. This method is especially pertinent in patients with right hydronephrosis and in those younger than 1 year. Reported relative renal function data should be interpreted with caution.