If Kidney Function Drops During Follow-up: A 10-Point Loss and the Surgery Decision

For a kidney under conservative follow-up, the first MAG3 is a starting point. What actually matters is where that point goes over time. If function stays stable, the kidney is protecting itself no matter how pronounced the swelling looks; if it drops, the obstruction has begun leaving a mark on the tissue.

The EAU/ESPU guideline lists a drop of more than 10 points in split function on serial measurement among the indications for pyeloplasty. For example, a kidney that falls from 48% to 36% has both dropped below the threshold and lost more than 10 points; these two findings together weigh the scale noticeably toward surgery.

The first question a family asks on hearing "function has dropped" is "is this surgery"; your doctor's first question should be "is this drop real." The following situations can make the number misleading; the drop is accepted only after these are ruled out.

• The first measurement was done in an infant under 6 weeks old (an immature kidney may give a genuinely low or high first reading).
• Hydration, bladder catheterization, or furosemide timing differed between the two scans.
• Background correction was done differently in a very dilated pelvis; a "supranormal" first value later returning to normal.
• Motion during the scan, a difference in position, or a different device/center.
• A change in the opposite kidney: since split function is a share, growth of the other kidney can make this side's share look lower.

Once technical causes have been ruled out and the drop confirmed, the picture is no longer "hydronephrosis being watched" but "a kidney losing function." At this point the guideline's recommendation is pyeloplasty; especially if the swelling on ultrasound is also increasing and drainage is poor, the decision is clear.

Loss of function is often silent; the child feels fine. This is why keeping to the follow-up schedule, and presenting without waiting for the scheduled visit when the changes below occur, matters.

• Being told at a check-up ultrasound that the AP diameter has clearly increased or that the parenchyma has thinned.
• New episodes of flank pain, especially after drinking a lot of fluid and accompanied by vomiting.
• Febrile urinary tract infection; same-day evaluation if fever (38°C/100.4°F or above) occurs together with flank pain.
• In a solitary kidney or bilateral hydronephrosis, decreased urine output, swelling, or fatigue.