The renal artery ratio (RAR) is calculated by dividing the peak systolic velocity in the renal artery by the peak systolic velocity in the abdominal aorta. A normal RAR falls below 3.5, and values at or above that threshold suggest significant narrowing of the renal artery. It’s one of the primary measurements used during Doppler ultrasound to screen for renal artery stenosis.
The RAR Formula
The calculation itself is straightforward:
RAR = Peak Systolic Velocity in the Renal Artery รท Peak Systolic Velocity in the Aorta
Both velocities are measured in centimeters per second (cm/s) using spectral Doppler ultrasound. For example, if the highest velocity recorded in the renal artery is 350 cm/s and the aortic velocity is 80 cm/s, the RAR would be 4.4, which falls well above the normal range and suggests significant stenosis.
Where to Measure Each Velocity
Getting an accurate RAR depends heavily on where you sample each vessel. The aortic measurement should be taken from a longitudinal view of the abdominal aorta at or slightly above the level where the renal arteries branch off. This gives you a representative baseline velocity for comparison.
The renal artery velocity requires sampling at multiple points: the origin of the artery, its middle segment, and the hilum where it enters the kidney. The highest peak systolic velocity recorded anywhere along the main renal artery is the number you use in the formula. Normal peak systolic velocity in the main renal artery runs between 60 and 100 cm/s. Values above 180 to 200 cm/s start raising concern for narrowing.
Regardless of transducer position, the Doppler angle of insonation must be kept at 60 degrees or less. Steeper angles introduce significant measurement error in velocity readings, which directly distorts the ratio.
What the Numbers Mean
An RAR below 3.5 is generally considered normal. Once the ratio climbs above that, the likelihood of hemodynamically significant renal artery stenosis increases substantially. Here’s how the numbers break down based on a study in the Journal of Vascular Surgery that compared Doppler findings against angiography:
- Normal arteries: average RAR of 2.2
- Less than 60% stenosis: average RAR of 2.9
- 60% or greater stenosis: average RAR of 4.5
The threshold with the best overall accuracy for detecting stenosis of 60% or greater was an RAR of 3.7, which yielded 69% sensitivity, 91% specificity, and 82% overall accuracy. That high specificity means a ratio above 3.7 is fairly reliable for confirming significant disease, though the moderate sensitivity means some cases of stenosis will produce lower ratios and be missed by the RAR alone.
How RAR Compares to PSV Alone
Peak systolic velocity in the renal artery can also be used on its own to screen for stenosis. A PSV above 198 cm/s provided 87% sensitivity and 92% specificity for detecting stenosis greater than 60% in one large study. The RAR showed similar specificity (92%) but notably lower sensitivity (76%). In practice, the two measurements are typically used together rather than in isolation.
Combining a PSV of 200 cm/s or higher with an RAR of 3.5 or higher produced 72% sensitivity, 83% specificity, and 78% overall accuracy. The RAR adds value because it accounts for the patient’s overall hemodynamic state. Someone with low cardiac output or aortic disease may have abnormally low aortic velocities, which would inflate the ratio even without true renal artery narrowing. Conversely, a patient with a high-flow state might have elevated renal artery velocities that look alarming on their own but produce a normal ratio when compared to equally elevated aortic flow.
Supporting Measurements
RAR and renal artery PSV are considered “direct” criteria because they measure flow at the site of potential narrowing. Two “indirect” criteria supplement them by looking at what happens downstream in the kidney itself.
The resistance index (RI) is measured at the renal hilum on each side. A difference in RI greater than 0.05 between the two kidneys suggests stenosis on the side with the lower value, because the narrowing dampens the normal pulsatile flow pattern reaching that kidney.
Acceleration time (AT) measures how long it takes for blood flow to rise from its lowest point in diastole to the peak systolic velocity. A prolonged acceleration time downstream from a stenosis reflects the “tardus parvus” waveform pattern, where the pulse arrives late and blunted. AT tends to have poor sensitivity (around 50%) but good specificity (around 95%) for stenosis under 80%, making it more useful for confirming disease than ruling it out.
Factors That Affect Accuracy
Several conditions can make the RAR less reliable. Aortic aneurysms or severe atherosclerotic disease in the aorta can alter aortic velocities unpredictably, throwing off the denominator of the ratio. Low cardiac output from heart failure reduces velocities in both vessels, potentially masking stenosis. High heart rates and variations in blood pressure also influence Doppler waveform shape and velocity readings.
Body habitus plays a practical role too. Bowel gas and abdominal fat can obscure the renal arteries, making it difficult to capture a clean spectral waveform at the correct angle. Fasting before the exam (typically 8 to 12 hours) helps reduce bowel gas and improves image quality, though the measurement principles remain the same regardless of preparation.
End-diastolic velocity is sometimes reported as an additional stenosis criterion, but it depends heavily on heart rate and kidney function. Patients with early kidney damage often have increased resistance in the small vessels of the kidney, which reduces end-diastolic flow and makes this measurement unreliable as a standalone indicator.