MAG3 Diuretic Renography (Kidney Scan): How It's Done, and How to Prepare

MAG3 is a low-dose radioactive tracer given through a vein, which is filtered by the kidneys and passed out in the urine. A gamma camera records, over several minutes, how the tracer arrives at the kidneys, is filtered out, and travels down through the ureters to the bladder.

Families often mix up two different scans. DMSA is a static scan that shows damage and scarring in the kidney tissue, and it is used mainly to assess kidney reflux (vesicoureteral reflux, VUR) and damage after infection. MAG3 is a dynamic test: it measures flow and drainage over time.

• In UPJ obstruction (ureteropelvic junction obstruction; kidney outlet narrowing), the question being asked is "is this kidney obstructed, and how well is it functioning" — so the standard test is MAG3 diuretic renography. If your doctor requests both tests together, know that each answers a different question.

Kidneys mature over the first few weeks after birth; a scan done too early can show falsely low function and falsely slow drainage. For this reason, MAG3 is usually planned after the 4th–6th week of life.

The exact technique varies slightly by center, but a well-performed diuretic renogram follows these basic steps:

• Fluid loading: The child is given adequate fluid, intravenously (sometimes by mouth), before and during the test. A dehydrated kidney empties urine slowly and can skew the result.
• IV line: A thin intravenous line is placed; the tracer and the diuretic are given through it.
• Bladder catheter: In infants, and in children where a full bladder might hinder emptying or reflux is suspected, a thin catheter is placed in the bladder. A full bladder can artificially slow the kidney's drainage.
• Imaging: The child lies on their back; the camera sits below or above, and continuous recording is taken for roughly half an hour. Staying still matters; infants are usually kept calm by feeding and swaddling, older children by distraction with a parent present, and sedation is rarely needed.
• Diuretic: The diuretic furosemide is given intravenously. The timing varies by protocol: 20 minutes after the tracer (F+20), 15 minutes before (F-15), or at the same time (F0). Because the protocol used affects how the result is interpreted, it is noted in the report.
• Delayed image: After the child is stood up or sat upright and voids, an additional image is taken. Gravity and post-void emptying show that many kidneys that look "stuck" are actually draining well.

• The dose of the tracer and the diuretic is determined by the nuclear medicine team based on the child's weight; the information here is for background only, and the procedure itself is the responsibility of the performing center.

The instructions given by the center on the day of the test take priority; in general, the following helps:

• Fasting is not usually required; feed your baby normally and give plenty of fluids unless the center says otherwise.
• Bring diapers, a change of clothes, a pacifier, and a familiar toy; bringing the baby close to nap time can help them stay calm.
• Bring your previous ultrasound and scan reports; they're needed for comparison.
• The radiation dose is low; even so, since the tracer is excreted in urine, it's advised to wash hands carefully when changing diapers that day.
• Normal activities can resume the same day after the test; if a catheter was placed, there may be mild burning once or twice.

• If fever, inability to urinate, or marked distress occurs after the test, contact your doctor. What the split function and T½ values mean is explained under "how to read the MAG3 result."