Split (Differential) Kidney Function and What the 40% Threshold Means

Split (differential) function is calculated by looking at what proportion of the intravenous radioactive tracer each kidney takes up in the first 1–3 minutes. The result is written as two numbers that add up to 100%, such as "right 48%, left 52%."

The EAU/ESPU guideline defines split function falling below 40% as "impaired function" and lists it among the indications for pyeloplasty. The logic here is that a widened kidney's function dropping notably below the lower limit of normal suggests the obstruction has left a mark on the tissue.

MAG3's split function measurement carries roughly ±5 points of variability. That means a kidney read at 38% could come back at 42% if re-scanned under the same conditions. For this reason, a borderline value does not decide surgery on its own.

Sometimes a value above normal, such as 58–60%, is reported for a widened kidney. This is called "supranormal" function and is most often a measurement artifact: material pooled in a large pelvis can be miscounted as though it belongs to the kidney tissue.

• A supranormal value does not mean the widened kidney is working better than the other one; when it comes to the decision, this number is interpreted simply as "preserved function," nothing more. There isn't full agreement in the literature on this point; your doctor evaluates it together with the ultrasound and drainage findings.

A value like 35% is a signal to "look carefully," not a signal for "immediate surgery." Your doctor first questions the conditions of the measurement: the baby's age, hydration, the catheter, the furosemide timing. Then the other components of the same report are examined: the drainage curve, the post-void image, and the AP diameter and parenchymal thickness on ultrasound.

• If these findings also point toward obstruction, or the low value is confirmed on a repeat measurement, the scale tips toward pyeloplasty. If the findings conflict, close-interval follow-up and a repeat MAG3 within 3 months is the reasonable path. The goal of the decision is to intervene at the right time before more function is lost — without rushing, but without letting it drag on either.