Poor Drainage After Furosemide: Does an Obstructive Curve Mean Surgery?

On MAG3, the amount of radioactively labeled tracer in the kidney over time is plotted as a curve. Normally the tracer rises in the kidney, peaks, and then falls as it flows through the ureter into the bladder. If emptying is slow, a diuretic (furosemide) is given and the time it takes for half of the tracer to leave the kidney is measured: this is T½.

Guidelines (EAU/ESPU 2024; the SNMMI/EANM pediatric renography guideline) clearly state that T½ is not an indication on its own. A long T½ increases the likelihood of surgery, but many technical and physiological factors can make drainage look worse than it actually is.

• Inadequate hydration: a kidney that has received little fluid responds poorly to furosemide, and the curve falls slowly.
• A full bladder: in an infant without a catheter, bladder pressure delays emptying of the upper system; this is why catheterization is recommended in infants.
• A very dilated pelvis: a large reservoir empties slowly even when not obstructed (the "reservoir effect"), giving a long T½.
• Low function: a poorly functioning kidney cannot produce enough urine, so the response to furosemide stays weak; this is a function problem, not a drainage problem.
• An infant under 6 weeks old: an immature kidney can give a misleading result in both function and drainage.
• Protocol differences: when furosemide is given (F+20, F-15, or F0) and whether the child is lying down or upright change the result; numbers cannot be compared one-to-one between centers.

An experienced reader does not look at the T½ number but at the whole shape of the curve. If a clear fall has begun after furosemide but has not reached half by 20 minutes, this is a different picture from a plateauing or rising curve. The latter is more suggestive of true obstruction.

The meaning of poor drainage is determined by the findings alongside it. In the combinations below, an "obstructive curve" stops being just a number and surgery is weighed more strongly.

• Poor drainage plus split function under 40%, or a drop of more than 10 points on serial measurement.
• Poor drainage plus a clear and progressive increase in AP diameter on ultrasound, or thinning of the parenchyma.
• Poor drainage plus symptoms: recurrent episodes of flank pain (Dietl's crisis) or febrile urinary tract infection.
• Poor drainage plus a curve that still has not emptied, and keeps rising, even after voiding and in an upright position.

In a child with good function, a stable ultrasound, and no symptoms, a poor-drainage report usually opens this path: reviewing the technical conditions, and if needed, a repeat MAG3 with a standard protocol (adequate fluids, catheter, appropriate furosemide timing, post-void imaging), along with serial ultrasound in the meantime.

• Drainage can also improve over time during follow-up; especially in infancy, emptying can speed up as a dilated pelvis shrinks. This is why the decision rests on the course over time, not a single snapshot. Follow-up is more frequent in a child with high-grade hydronephrosis; if fever occurs together with flank pain, you should present the same day without waiting for the scheduled visit.