Split kidney function—also called differential or relative renal function—shows what percentage of total measured renal tracer uptake comes from each kidney. A result such as left 45% and right 55% is usually close to symmetric. A result such as 25% and 75% means one kidney contributes less relative function, but it does not mean the patient has only “25% total kidney function.” Overall kidney function is assessed separately with creatinine/eGFR. Split function is most useful for one-sided obstruction, scarring or a small kidney and for comparing function over time.

How the percentage is generated

During a renal nuclear scan, software analyses tracer uptake by each kidney during a defined functional phase. The two sides are expressed as percentages adding to approximately 100%. The value is technique-dependent and can be influenced by background subtraction, kidney depth, poor function and motion.

What range is usually considered symmetric?

Many reports regard approximately 45-55% per kidney as reasonably symmetric, although a small difference outside that range may still be clinically unimportant. Trend and image quality matter more than chasing one percentage point.

What does 30% function mean?

A kidney contributing 30% has reduced relative function compared with the other kidney, but it may still provide substantial useful filtration. Treatment is not based on an automatic “cutoff for removal.” The cause, symptoms, infections, drainage, anatomy and likelihood of recovery or future deterioration all matter.

Split function versus eGFR

eGFR estimates combined filtration from both kidneys. A person can have a normal eGFR because one kidney compensates even when the other has low split function. Conversely, a 50/50 split can occur when both kidneys are equally impaired and total eGFR is poor. The tests answer different questions.

How obstruction affects split function

Long-standing PUJ or ureteric obstruction can reduce relative uptake. After relief of obstruction, function may stabilise or sometimes improve, particularly when damage is not advanced. In severe chronic damage, recovery may be limited. Serial scans should ideally use comparable technique.

Can split function vary between scans?

Yes. A difference of several percentage points can reflect technical and physiological variability. Larger consistent changes are more meaningful. The urologist should review the curves and images rather than comparing only the headline percentages.

When split function influences treatment

• Deciding whether reconstructive surgery is worthwhile in an obstructed kidney.
• Assessing a poorly functioning kidney with pain or recurrent infection.
• Planning surgery when one kidney is small/scarred.
• Following function after PUJ/ureteric reconstruction.
• Selected donor or congenital evaluations using different dedicated protocols.

Relative function should not be quoted as “kidney survival”

Patients are sometimes told that a kidney has “30% life left.” That is misleading. The percentage is a snapshot of relative tracer function at the time of the scan. A 30% kidney may remain stable for many years, improve after reconstruction or decline if obstruction continues. Prognosis comes from trend, anatomy, drainage and clinical events—not from converting the percentage into years or a countdown.

How to interpret change over time

Suppose a kidney is reported as 35% on one scan and 33% on another. That small difference may be within measurement variability. A repeated fall from 40% to 30% to 20% with persistent obstruction is far more meaningful and may influence intervention.

The decision is also different in a child or young adult with salvageable obstructed function compared with an older patient with a chronically scarred, asymptomatic kidney. Split function is a decision aid, not a stand-alone rule for reconstruction or nephrectomy.

The number that matters most is the trend when treatment is being considered

A single split-function value has measurement variability. When the decision is whether an obstructed kidney is deteriorating, comparing properly performed scans over time can be more informative than arguing over a 2-3 percentage-point difference. A convincing fall should be interpreted alongside symptoms, drainage pattern and ultrasound/CT anatomy.

Low relative function also does not automatically mean nephrectomy. The decision depends on symptoms, infection, salvageability, opposite-kidney health and whether the poorly functioning unit is causing harm. In selected obstructed kidneys, drainage or reconstruction may still be appropriate.

What to bring for consultation

• Full renal-scan report with curves.
• Previous split-function scans for comparison.
• Ultrasound/CT/MRI anatomy.
• Creatinine/eGFR and urine tests.
• History of obstruction, infection, stones and previous surgery.

FAQs

Related reading

• DTPA / Renal Scan Explained
• 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
• Kidney Disease: Improving Global Outcomes (KDIGO). 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(Suppl 4S):S117-S314 https://kdigo.org/guidelines/ckd-evaluation-and-management/