A DTPA renal scan is a nuclear-medicine test that shows how each kidney receives blood, takes up tracer and drains urine over time. It is often used when ultrasound shows hydronephrosis and the urologist needs to know whether the dilatation represents functionally important obstruction, and how much each kidney contributes to total function. The report may include a renogram curve, relative/split renal function and drainage measurements before and after a diuretic such as furosemide. No single drainage half-time should be interpreted without hydration, bladder fullness, kidney function and the curve shape.

How the test works

A small amount of technetium-labelled tracer, commonly 99mTc-DTPA, is injected into a vein. A gamma camera records tracer arrival and clearance from each kidney. In a diuretic renogram, furosemide is given according to protocol to increase urine flow and test whether retained tracer washes out from a dilated collecting system.

What the renogram phases mean

Perfusion

The first seconds show blood flow to each kidney. Marked asymmetry may reflect vascular or severe functional differences but must be interpreted with the entire study.

Uptake/function phase

Tracer accumulation in renal tissue reflects filtration/function. DTPA is filtered by the glomeruli; in significantly impaired renal function, another tracer such as MAG3 may provide better drainage assessment in some settings.

Excretion/drainage phase

Tracer should move into the collecting system and then the bladder. A rising or persistently retained curve can suggest obstruction, but poor kidney function, dehydration, a very large compliant pelvis or a full bladder can also slow drainage.

Split or differential renal function

The report may state left 48% and right 52%, for example. This is relative function, not absolute eGFR. A kidney contributing 25% has reduced relative function compared with the opposite kidney, but management depends on cause, symptoms, trend and whether function is stable or potentially recoverable.

Drainage half-time (T1/2)

T1/2 estimates how quickly retained tracer falls after diuresis. Historical thresholds such as less than about 10 minutes being reassuring and more than about 20 minutes being concerning are only guides. Modern interpretation emphasises the entire curve, post-void images, gravity-assisted drainage, renal function and technical quality rather than a rigid cutoff.

Why a dilated kidney may drain slowly without true obstruction

A very large renal pelvis can act like a reservoir and empty slowly despite no damaging pressure obstruction. Conversely, a poorly functioning kidney may not generate enough tracer/urine flow for a reliable washout test. The nuclear-medicine physician integrates quantitative values with image sequences and clinical context.

Common reasons to order the scan

• PUJ obstruction/hydronephrosis.
• Comparison of function before deciding on reconstructive surgery or nephrectomy.
• Postoperative assessment when obstruction is uncertain.
• Selected congenital urinary tract abnormalities.
• Functional comparison when one kidney looks scarred or small.

Preparation and aftercare

Hydration is important unless medically restricted. Some protocols require bladder emptying before/during imaging or catheterisation in selected patients. Radiation exposure is relatively low. Patients should tell the department about pregnancy/breastfeeding and medicines or conditions that affect hydration/urine flow.

Why technique can change a DTPA result

Hydration, timing of furosemide, whether the bladder was full, patient position and renal function all affect washout. A full bladder can raise downstream pressure and make drainage look worse. Post-void or gravity-assisted images can show that retained tracer empties once the patient stands or empties the bladder.

For serial follow-up, it is ideal to compare studies performed with similar protocols. A small change in split function or T1/2 across different centres may be technical rather than true deterioration, so the actual curves and images are more useful than the summary line alone.

How to read function and drainage separately

A renal scan answers two different questions that are often mixed together. Split function asks how much each kidney contributes to total tracer uptake. Drainage asks what happens to tracer after it reaches the collecting system, often after a diuretic. A kidney can have preserved function but delayed drainage, or low function with little tracer available to assess drainage confidently.

T1/2 is therefore not a stand-alone obstruction test. Hydration, bladder fullness, renal function, collecting-system size, timing of furosemide and analysis method all affect the curve. The shape of the renogram and post-void images should agree with the clinical question before a borderline number changes management.

A worked way to think about a renal scan

Imagine a report showing right/left split function of 35/65 with a dilated right collecting system. If the right kidney shows good tracer uptake but delayed washout that improves after diuretic and after voiding, the dilatation may not represent fixed high-grade obstruction. If uptake is falling on serial scans and the curve remains persistently obstructive despite good technique, the finding carries more weight.

Conversely, a very poorly functioning kidney may produce an indeterminate drainage curve simply because little tracer reaches the collecting system. In that setting, anatomy, symptoms, infection history and sometimes a temporary drainage test may be more useful than forcing the T1/2 into ‘obstructed’ or ‘not obstructed’.

The best renal-scan interpretation therefore states the tracer, diuretic protocol, relative function, curve pattern and whether drainage improves after diuretic/voiding—not just one percentage and one half-time.

What to bring for consultation

• Renal-scan images/curves as well as the summary percentages.
• Ultrasound/CT showing hydronephrosis and anatomy.
• Creatinine/eGFR.
• Previous renal scan for trend comparison.
• History of pain, infections, stones or prior PUJ/ureteric surgery.

FAQs

Related reading

• Split Kidney Function: What Does It Mean?
• Hydronephrosis on Ultrasound
• Serum Creatinine Test Explained
• High Creatinine with Urinary Obstruction
• Urologist in Latur

References

• Taylor AT, Brandon DC, de Palma D, et al. SNMMI Procedure Standard/EANM Practice Guideline for Diuretic Renal Scintigraphy in Adults With Suspected Upper Urinary Tract Obstruction 1.0. Semin Nucl Med. 2018;48(4):377-390.
• RadiologyInfo.org (ACR/RSNA). Renal (Kidney) Scintigraphy. Last reviewed June 15, 2026 https://www.radiologyinfo.org/en/info/renal