Captopril Renography⁹⁹ᵐTc-MAG3 (or DTPA)

Quick Answer

Captopril (ACE-inhibitor) renography detects the functional significance of renal artery stenosis in suspected renovascular hypertension. In a kidney whose perfusion pressure depends on angiotensin-II-mediated efferent arteriolar constriction, an ACE inhibitor unmasks a fall in GFR, producing a characteristic change on the post-captopril renogram compared with baseline. It assesses physiology, not anatomy.

Principle

In hemodynamically significant renal artery stenosis, glomerular filtration is maintained by angiotensin-II constriction of the efferent arteriole. ACE inhibition removes that support, so the affected kidney shows reduced/ delayed tracer handling after captopril relative to the baseline study.

Interpretation

• Positive: post-captopril deterioration in the renogram (delayed time-to-peak, cortical retention, reduced function) in one kidney → suggests functionally significant, potentially treatable RAS.
• Normal baseline and post-captopril → low probability of renovascular hypertension.
• Best performance is in patients with normal or near-normal renal function; it is less reliable in significant CKD or bilateral disease.

Preparation

Hold ACE inhibitors/ARBs before the study per protocol; ensure hydration; monitor blood pressure (captopril can cause hypotension).

The ACE inhibitor unmasks the stenosis

Captopril renography detects the functional significance of renal artery stenosis, not its anatomy. In a kidney whose GFR is propped up by angiotensin-II efferent constriction, an ACE inhibitor removes that support and unmasks a fall in filtration — the affected kidney shows delayed time-to-peak, cortical retention, reduced function on the post-captopril study versus baseline. The change between the two studies is the signal. Yield is highest with preserved renal function and poor in advanced CKD or bilateral disease.

High-Yield Pearls

• The test evaluates functional significance, complementing anatomic imaging (CT/MR angiography, Doppler).
• Highest yield in preserved renal function; poor in advanced CKD or bilateral RAS.
• A change between baseline and post-captopril studies is the diagnostic signal.

Common Pitfalls

• Continuing ACE inhibitors/ARBs, dehydration, or hypotension confounding results.
• Applying it to advanced CKD where sensitivity/specificity fall.

Related Pages

• Tracer: Tc-99m-MAG3; protocol: Renal transplant, Diuretic renography.
• Interactive: Renogram Explorer — drag a curve and see the baseline-vs-post-ACEI change and the consensus positivity criteria live.

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