A renal scan is a type of nuclear medicine imaging test that evaluates how well your kidneys are working. Unlike an ultrasound or CT scan, which mainly show the structure of your kidneys, a renal scan tracks a small amount of radioactive tracer as it moves through your kidneys in real time. This lets doctors measure how efficiently each kidney filters blood, drains urine, and receives blood flow.
How a Renal Scan Works
A technologist injects a radioactive tracer into a vein in your arm. The tracer travels through your bloodstream to your kidneys, where it behaves like the substances your kidneys naturally filter. A specialized camera positioned over your abdomen detects the low-level radiation the tracer emits and converts it into images. These images show the tracer arriving at each kidney, being processed, and draining into the bladder.
The camera doesn’t emit any radiation itself. It simply picks up signals from the tracer already inside you. Because the images are captured continuously over time, doctors can watch the entire filtering and drainage cycle unfold, not just a single snapshot.
Types of Renal Scans
There are several variations of the test, each using a different tracer designed to highlight a specific aspect of kidney function.
- DTPA scan: This tracer is filtered by the kidneys the same way they filter waste from blood, making it useful for measuring your overall filtration rate. It delivers the lowest radiation dose of the common renal tracers, around 0.01 mSv.
- MAG3 scan: This tracer is cleared by the kidney’s tubular cells rather than filtered, so it provides a clearer picture of blood flow through the kidneys. It produces sharper images in patients who already have reduced kidney function, making it the preferred choice in many clinical settings.
- DMSA scan: Rather than tracking drainage, this tracer binds directly to kidney tissue. It’s especially useful for detecting scarring or areas of infection in the kidney itself. DMSA scans are more sensitive than ultrasound for catching early kidney infections in children.
Your doctor chooses the tracer based on the specific question they need answered. Sometimes a medication is added to the test to stress the kidneys in a controlled way. A diuretic (a drug that increases urine production) can be given partway through the scan to help determine whether a dilated urinary tract is truly blocked or just naturally wide. A blood pressure medication called captopril can be given before the scan to reveal whether narrowed kidney arteries are causing high blood pressure.
Conditions a Renal Scan Can Diagnose
Renal scans are ordered when doctors need functional information that standard imaging can’t provide. Common reasons include:
- Kidney blockage (hydronephrosis): The scan can distinguish a true obstruction from a urinary tract that appears dilated on ultrasound but is actually draining normally.
- Narrowed renal arteries: In patients with hard-to-control high blood pressure, a captopril renal scan can reveal whether reduced blood flow to one kidney is the underlying cause.
- Kidney scarring: After infections or urine reflux, DMSA scans detect areas of damaged tissue that may not show up on ultrasound.
- Post-transplant monitoring: After a kidney transplant, renal scans track how well the new kidney is functioning and can help identify early signs of rejection or other complications.
- Vesicoureteral reflux: In children with recurrent urinary tract infections, scans help determine whether urine is flowing backward from the bladder toward the kidneys, a condition that affects 25 to 40 percent of children with UTIs.
What Happens During the Test
You’ll typically lie on your back on a flat table with the gamma camera positioned behind or above you. The tracer injection feels like a standard blood draw. Once the tracer is in, the camera begins recording immediately for dynamic scans (MAG3 or DTPA) or after a waiting period of a few hours for static scans (DMSA). You need to stay relatively still while the camera captures images, but the process is painless.
For dynamic scans, imaging usually runs about 20 to 40 minutes. If a diuretic is given to test for obstruction, the scan may be extended to observe how your kidneys respond. DMSA scans require a longer gap between injection and imaging (typically two to three hours) because the tracer needs time to bind to kidney tissue, but the actual imaging session is shorter. You may be asked to drink extra fluids before or during the test to stay well hydrated, since dehydration can slow drainage and affect the accuracy of results.
Understanding Your Results
One of the most important measurements from a renal scan is split (or differential) function: the percentage of total kidney work each kidney is doing. In a healthy person, each kidney handles roughly 45 to 55 percent of the total workload. If one kidney falls below that range, it suggests that kidney is impaired while the other is compensating.
For drainage scans, doctors look at how quickly the tracer clears from each kidney. The key metric is called the half-clearance time, which is how long it takes for half the tracer to leave the kidney after it peaks. A normal half-clearance time is about 8 to 9 minutes. Significantly prolonged drainage suggests an obstruction. The time-to-peak (how quickly the tracer accumulates in the kidney) is also measured, with a normal value around 3 minutes.
DMSA scans produce a different kind of result. Instead of drainage curves, they show a map of functioning kidney tissue. Cold spots, areas where the tracer didn’t bind, indicate scarring or active infection. Your doctor will compare these findings with any previous imaging and your symptoms to determine next steps.
Radiation Exposure
Renal scans involve very small amounts of radiation. The effective dose ranges from about 0.01 mSv for a DTPA scan to about 0.04 mSv for a DMSA scan. For perspective, the average person absorbs 1 to 3 mSv per year just from natural background radiation (cosmic rays, radon in soil, and similar sources). The tracer is eliminated from your body through urine within hours to a day, so the exposure is brief.
For pregnant patients, the fetal radiation dose from a renal scan is generally less than 5 mGy, which is well below the threshold associated with harm. The American College of Obstetricians and Gynecologists states that when a renal scan is clinically necessary during pregnancy, it should not be withheld. Breastfeeding also does not need to be interrupted after receiving the standard tracers used in kidney imaging.
Renal Scans in Children
Renal scans play a particularly important role in pediatric medicine. Urinary tract infections in young children, especially those under two, can cause lasting kidney damage if the underlying cause isn’t identified and treated. DMSA scans are the gold standard for detecting early kidney infections (pyelonephritis) in children, outperforming ultrasound in sensitivity while delivering less radiation than a CT scan.
In one study, 96 percent of infants under one year old who had both urinary tract dilation on ultrasound and vesicoureteral reflux showed abnormalities on DMSA scanning. Even among children with low-grade reflux, 72 percent had abnormal DMSA results. This makes the scan a critical tool for identifying which children need closer monitoring or treatment to protect their kidney function long-term.