How to Read a MAG3 Result: Split Function, T½, and the Drainage Curve

A MAG3 report generally has three parts: images of the kidneys over time, a time-activity (renogram) curve drawn for each kidney, and numerical results. In the numerical section, two values stand out: the split function percentage and the T½ time.

• These values complete the AP diameter and SFU grade from the ultrasound. Ultrasound answers "how wide," MAG3 answers "how well it's working and draining." No decision about UPJ obstruction (ureteropelvic junction obstruction; kidney outlet narrowing) is made without reading the two together.

Split function is calculated from how much of the tracer each kidney takes up in the first few minutes, with the two kidneys' totals treated as 100%. In two healthy kidneys, the share is roughly 45–55%.

T½ (the half-emptying time) is how long it takes, after the diuretic is given, for half of the activity in the kidney to drain out. A commonly used framework for interpreting it: under 10 minutes is usually normal, 10–20 minutes is equivocal, and over 20 minutes is called an "obstructive pattern."

T½ alone is not a reliable measure; the following situations can make drainage look falsely slow or falsely fast:

• Insufficient fluid: a dehydrated kidney responds poorly to the diuretic, and T½ lengthens.
• A full bladder or the absence of a catheter: bladder pressure slows flow from the ureter.
• A very large pelvis (a "reservoir" effect): a large renal pelvis can drain slowly even without obstruction.
• Low-function kidney: a poorly functioning kidney cannot produce enough urine, and T½ can lengthen in a way that is not meaningful.
• An infant younger than six weeks: an immature kidney responds slowly.
• The timing of the diuretic (F+20, F-15, F0) and the child's position: T½ values from different protocols cannot be compared directly.

• This is why an "obstructive pattern" in the report does not prove the kidney is truly obstructed or that surgery is needed. When poor drainage means surgery is discussed on its own page.

A good MAG3 interpretation places four pieces of information side by side: split function; the drainage curve and T½; the post-void image; and the ultrasound grade and its course over time. For example, a kidney with preserved function, a long T½, but good emptying on the delayed image is usually followed; a kidney with function below 40%, a plateau curve, and an increasing AP diameter tips the balance toward surgery.