To show that hydronephrosis (HN) can be tracked by the quantitative reproducible hydronephrosis index (HI) and that HI is useful for serial ultrasound (US) studies to determine whether HN is improving or deteriorating.

We found 60 hydronephrotic kidneys in 46 study patients. The other 32 kidneys were normal or absent. Serial US studies were performed more than 1 month apart over a 3-year period. Hydration was maximized with oral fluids. Cases ranged in age from 2 days to 13 years. We determined HI as follows: Operators outlined the perimeters of the kidney and dilated renal pelvis in the maximal longitudinal view. Respective areas were automatically calculated. We obtained HI by outlining the area of the kidney and separately outlining the area of the dilated renal pelvis within the kidney. We calculated HI percentage as 100 × (Total area of kidney minus area of dilated pelvis and calices)/(Total area). This percentage calculation represents the renal area determined reproducibly in a standardized fashion as if the calices were not there and is recorded as a dimensionless number.

Hydronephrosis for 30 of 60 kidneys (50%) showed decreasing HN, and for 17 of 60 kidneys (28%) showed increasing HN. In 13 of 60 (22%) HN was unchanged. Statistical analysis showed that HI was determined with an objectivity of 99.8%.

Hydronephrosis is the most common abnormality detected with ultrasonography. The quantitative method for HI provides a reproducible measure of HN. With longtitudinal studies, the quantitative HI shows whether HN is improving or deteriorating.