What are the recommended diagnostic studies for a patient suspected of having renal artery stenosis, considering their kidney function and overall clinical presentation?

Diagnostic Studies for Renal Artery Stenosis

First-Line Imaging

Duplex ultrasound (DUS) is the recommended first-line diagnostic study for suspected renal artery stenosis, with magnetic resonance angiography (MRA) or computed tomography angiography (CTA) reserved for cases where DUS is inconclusive or technically limited. 1

• DUS should be performed as the initial screening modality (Class I, Level B recommendation) 1
• The examination requires patients to be NPO (nothing by mouth) to minimize bowel gas that obscures visualization of the renal arteries 1, 2
• Peak systolic velocity (PSV) ≥200 cm/s in the main renal artery indicates ≥60% stenosis, with sensitivity of 73-91% and specificity of 75-96% 1, 3
• Renal-aortic ratio (RAR) ≥3.5 improves diagnostic specificity by differentiating true stenosis from generalized elevated velocities due to hypertension 3, 4

Secondary Imaging When DUS is Inconclusive

When duplex ultrasound is technically inadequate, shows equivocal findings, or suggests significant stenosis requiring confirmation, proceed to cross-sectional imaging 1, 3:

• CTA demonstrates sensitivity of 92-98% and specificity of 92-98% 3
• MRA shows sensitivity of 94-97% and specificity of 85-93% 3
• Both modalities are equally recommended (Class I, Level B) when DUS is insufficient 1

Choosing Between CTA and MRA

In patients with normal renal function, either CTA or MRA is appropriate, but in patients with impaired renal function (GFR <30 mL/min), non-contrast MRA techniques should be preferred over gadolinium-enhanced MRA or CTA. 3, 2

• CTA provides higher spatial resolution and may be more readily available but requires iodinated contrast 1
• Gadolinium-enhanced MRA carries risk of nephrogenic systemic fibrosis in dialysis patients (1-6% incidence) 1
• Non-contrast MRA techniques achieve sensitivity of 74%, specificity of 93%, and accuracy of 90% in severe renal dysfunction 2

Additional Diagnostic Parameters

Beyond PSV measurements, several supportive findings increase diagnostic confidence 1, 3:

• Acceleration time >70 milliseconds indicates significant proximal stenosis 3
• Parvus-tardus waveform (small peak with slow upstroke) in intrarenal arteries is highly suggestive of proximal stenosis 3
• Renal resistance index >0.8 suggests poor kidney viability and predicts unfavorable response to revascularization 1, 4

Invasive Confirmation

Digital subtraction angiography (DSA) should be reserved for pre-intervention confirmation rather than initial screening 1, 3:

• DSA allows direct pressure measurement across the lesion 3
• Systolic pressure gradient >20 mmHg or mean pressure gradient >10 mmHg confirms hemodynamically significant stenosis 1, 3
• Consider DSA when non-invasive tests are equivocal despite high clinical suspicion, or when already scheduled for another angiographic procedure (e.g., coronary angiography) 1

Assessment of Kidney Viability

Before considering any intervention, kidney viability must be assessed (Class I, Level B recommendation), as revascularization of non-viable kidneys provides no benefit. 1, 4

Signs of Viable Kidney:

• Renal size >8 cm 4
• Distinct cortex >0.5 cm with preserved corticomedullary differentiation 4
• Albumin-creatinine ratio <20 mg/mmol 1, 4
• Renal resistance index <0.8 1, 4

Signs of Non-Viable Kidney:

• Renal size <7 cm 4
• Loss of corticomedullary differentiation 4
• Albumin-creatinine ratio >30 mg/mmol 1, 4
• Renal resistance index >0.8 1, 4

Clinical Indications for Testing

Testing should be pursued only in specific high-risk clinical scenarios 1, 4:

• Rapidly progressive or treatment-resistant hypertension (diastolic BP >110 mmHg on >3 medications) 1, 4
• New-onset hypertension in patients <35 years or >50 years of age 1, 4
• Rapidly declining renal function, particularly after starting ACE inhibitors or ARBs 1, 4
• Flash pulmonary edema with relatively preserved left ventricular function 1, 4
• Abdominal bruit on physical examination 1, 4
• Generalized atherosclerotic disease with hypertension 1

Common Pitfalls to Avoid

• Do not screen asymptomatic patients or those with well-controlled hypertension, as routine revascularization of unilateral atherosclerotic RAS is not recommended (Class III, Level A) 4
• Do not rely solely on anatomic stenosis severity—hemodynamic significance and kidney viability are equally important for determining intervention benefit 4
• Do not assume negative DUS rules out stenosis in technically challenging patients (large body habitus, bowel gas), as false-negative results can occur even with severe stenosis 2
• Do not proceed with DUS in non-fasted patients, as the likelihood of a nondiagnostic study is substantially increased 2
• Higher velocity thresholds are required for in-stent restenosis: PSV ≥395 cm/s or RAR ≥5.1 for detecting ≥70% in-stent restenosis 3

Special Considerations for Fibromuscular Dysplasia

• MRA may be more effective than DUS for detecting subtle beading and distal branch involvement characteristic of fibromuscular dysplasia 2
• These patients typically present with severe hypertension at young age (<35 years) and have better response to revascularization than atherosclerotic disease 1