When your doctor orders a resistive index, a numerical value derived from Doppler ultrasound that measures resistance to blood flow in arteries. Also known as the resistance index, it's a simple but powerful tool used to spot problems like narrowed arteries or poor organ perfusion. You won’t find it on a routine blood test, but if you’ve had an ultrasound of your kidneys, liver, or brain, chances are this number was calculated behind the scenes.
The Doppler ultrasound, a non-invasive imaging technique that uses sound waves to track blood movement measures how fast blood flows during heartbeats and between beats. The resistive index takes those two speeds and turns them into a single number—usually between 0.5 and 0.8. A value above 0.7 in the kidney arteries might signal renal artery stenosis, a narrowing that reduces blood flow to the kidneys and can raise blood pressure. In the brain, a high resistive index could mean swelling or increased pressure. In pregnant women, it helps check if the placenta is delivering enough oxygen to the baby.
It’s not a diagnosis on its own, but it’s a red flag that tells doctors where to look next. A rising resistive index over time might mean a transplant kidney is failing. A sudden spike after surgery could point to a clot. And in older patients with high blood pressure, a persistently high index often means the small vessels in the kidneys are stiffening—early damage you can’t see without this test.
What makes the resistive index so useful is that it’s quick, cheap, and doesn’t need contrast dye or radiation. It’s used daily in hospitals—from the ER checking for kidney blockages, to the NICU monitoring newborns with low blood pressure, to oncology units tracking tumor blood flow after treatment. But it’s not perfect. Factors like age, body temperature, and even how hard your heart is working can shift the number. That’s why doctors don’t act on a single value—they watch trends, compare sides, and combine it with other tests like creatinine levels or CT scans.
Below, you’ll find real-world examples of how this number guides treatment decisions. From spotting hidden kidney disease in diabetics to adjusting meds for patients on dialysis, these posts show how a simple ultrasound number can change the course of care.
Renal ultrasound is the safest, first-line test for evaluating kidney size and urinary obstruction. It detects hydronephrosis, measures kidney dimensions, and uses resistive index to identify blockages-all without radiation or contrast.