What Is a Renal Scintigraphy Scan?
A kidney scan using DTPA (diethylenetriamine pentaacetic acid) or EC (ethylene dicysteine) is a nuclear medicine test that evaluates how your kidneys are working. Unlike an ultrasound or CT scan that shows the structure of the kidneys, a renal scintigraphy scan focuses on function: how efficiently each kidney filters waste from the blood and how well urine drains from the kidney into the bladder.
These scans go by several names, including renal scintigraphy, renogram, or dynamic kidney scan. The specific tracer used, whether DTPA or EC, depends on what information the doctor needs and what is available at the imaging centre.
DTPA vs EC: What Each Tracer Measures
Tc-99m DTPA
DTPA is filtered by the glomeruli, the tiny filtering units inside each kidney. Because it passes through the kidney purely by filtration (it is not secreted or reabsorbed by the kidney tubules), it provides a direct measurement of the glomerular filtration rate (GFR). GFR is the single most important number for assessing overall kidney function.
DTPA scans are particularly useful when the doctor needs an accurate GFR measurement for each kidney individually, known as split renal function. A blood test can tell you total GFR for both kidneys combined, but it cannot tell you how much work each kidney is doing on its own. DTPA scintigraphy can.
Tc-99m EC
EC is handled differently by the kidneys. It is taken up by the tubular cells and secreted into the urine. This means it clears from the blood faster than DTPA and produces higher-quality images with better kidney-to-background contrast. EC scans are often preferred when the primary question involves urinary drainage, as the images show the flow of urine through the collecting system more clearly.
EC also gives an estimate of effective renal plasma flow (ERPF), another measure of kidney function. In many centres, EC has largely replaced DTPA for routine renal scintigraphy because of its superior image quality, though DTPA remains the standard when a precise GFR measurement is the goal.
When Is a Kidney Scan Needed?
Your doctor may order a DTPA or EC kidney scan in several clinical situations:
Before Kidney Surgery
If surgery is planned on one kidney, whether for a tumour, stone, or other condition, the surgeon needs to know how much function the other kidney provides. A split function scan confirms that the remaining kidney can carry the full workload after surgery.
Suspected Obstruction
When urine flow from the kidney to the bladder is blocked (hydronephrosis), the kidney scan can determine whether the blockage is causing functional damage. A diuretic (usually furosemide) is given during the scan to challenge the drainage system. If the kidney drains normally after the diuretic, the obstruction is likely not clinically significant. If drainage remains delayed, intervention may be needed.
Kidney Transplant Evaluation
Both living donors and transplant recipients undergo renal scintigraphy. For donors, the scan confirms that both kidneys function well and helps the surgical team decide which kidney to take. For recipients, post-transplant scans monitor how the new kidney is performing and can detect early signs of rejection or complications.
Renovascular Hypertension
In patients with high blood pressure suspected to be caused by narrowed renal arteries, a kidney scan performed before and after an ACE inhibitor (captopril renogram) can help identify whether the narrowing is functionally significant and potentially treatable.
Monitoring Chronic Kidney Disease
In some cases, serial kidney scans over time help track how each kidney responds to treatment or how disease progression affects function in one kidney compared to the other.
The Procedure: What to Expect
Preparation: You will be asked to drink water before the scan to ensure you are well hydrated. Good hydration is important because it helps the kidneys function normally during the test and produces clearer images. Your doctor may ask you to stop certain medications temporarily.
Injection: A small amount of the radioactive tracer (Tc-99m DTPA or Tc-99m EC) is injected into a vein, usually in the arm. The injection feels like a standard blood draw.
Imaging: You lie on your back on the camera table with a gamma camera positioned behind you (over the kidneys). Imaging begins immediately after injection and continues for about 20 to 30 minutes. The camera captures dynamic images showing the tracer as it enters the kidneys, passes through them, and drains into the ureters and bladder.
Diuretic phase (if needed): If the doctor suspects obstruction, a diuretic injection is given partway through the scan, and imaging continues for another 15 to 20 minutes to see how the kidneys respond.
Blood samples (for GFR): If a precise GFR calculation is needed, blood samples are taken at set intervals after the tracer injection, usually at two and four hours. These samples measure how quickly the tracer clears from the blood.
Total time: Plan for one to two hours at the imaging centre, depending on whether the diuretic phase or blood sampling is included.
What the Results Show
The nuclear medicine physician analyses the images and generates time-activity curves for each kidney. These curves show how quickly each kidney takes up the tracer and how quickly it washes out. From these curves, several measurements are derived:
- Split renal function: The percentage of total kidney function contributed by each kidney. A normal split is roughly 50/50, though small variations (45/55, for example) are common and usually not concerning.
- GFR or ERPF: Quantitative measures of filtering and blood flow, reported in millilitres per minute.
- Drainage: Whether urine flow from each kidney is prompt, delayed, or obstructed.
- Response to diuretic: If a diuretic was given, the scan shows whether drainage improved, suggesting the obstruction is not severe, or remained poor, suggesting a significant blockage.
Is the Scan Safe?
The radiation dose from a DTPA or EC kidney scan is low, comparable to the natural background radiation you receive over a few months. The tracer is eliminated from the body through urine within hours. Allergic reactions are extremely rare. The scan is non-invasive aside from the injection and, in some cases, the blood draws.
Pregnant women should inform their doctor, as nuclear medicine scans are generally avoided during pregnancy unless the clinical need is urgent.
Making Sense of Your Results
A kidney scan is one piece of a larger puzzle. The results are interpreted alongside blood tests, ultrasound findings, CT or MRI images, and your clinical history. Your urologist, nephrologist, or surgeon will use the functional data from the DTPA or EC kidney scan to guide decisions about surgery, observation, or medical management.
If you have been scheduled for a renal scintigraphy scan, ask your doctor what specific question the scan is meant to answer. Understanding the purpose of the test makes it easier to understand the results and the next steps in your care.