FFR, or fractional flow reserve, is a test that measures how well blood flows through a narrowed section of a coronary artery. It produces a simple number between 0 and 1 that tells your cardiologist whether a blockage is actually starving your heart muscle of blood, or whether it looks worse on imaging than it really is. The test is done during a cardiac catheterization and takes only a few minutes.
How FFR Works
When plaque builds up inside a coronary artery, it creates a narrowing called a stenosis. An angiogram can show that narrowing on screen, but the image alone doesn’t always reveal how much the blockage restricts blood flow. A narrowing that looks moderate on camera might be causing real problems, or it might be allowing plenty of blood through. FFR answers that question with a direct pressure measurement.
During the test, a cardiologist threads a thin wire with a tiny pressure sensor on its tip through a catheter and past the narrowed area of the artery. The wire records the blood pressure on both sides of the blockage: before it and after it. The FFR value is the ratio of the pressure downstream of the blockage to the pressure upstream. A perfectly healthy artery with no narrowing would have an FFR of 1.0, meaning pressure is equal on both sides. The lower the number drops, the more the blockage is choking off blood flow.
There’s one critical step that makes the measurement accurate. Your arteries naturally adjust their own resistance, so a resting measurement wouldn’t capture the worst-case scenario. To get a true reading, the cardiologist administers a drug (most commonly adenosine) that forces the tiny blood vessels downstream to relax and open as wide as possible. This creates a state of maximum blood flow, mimicking what happens during physical exertion. Only under these conditions does the pressure drop across the blockage reliably reflect how much flow is being lost.
What the Numbers Mean
The key cutoff is 0.80. An FFR value at or below 0.80 means the blockage is significant enough to limit blood flow to your heart muscle in a meaningful way. In clinical terms, this generally means revascularization (opening the artery with a stent or bypass surgery) is recommended. An FFR above 0.80 suggests the narrowing isn’t restricting flow enough to warrant a procedure, and medication alone is typically the safer path.
Between 0.75 and 0.80 sits what cardiologists call the “gray zone.” Earlier research used 0.75 as the threshold, while later landmark trials (known as the FAME and FAME-2 studies) established 0.80 as the standard. A large observational study of nearly 1,500 patients found that treating blockages even in this gray zone with modern stenting techniques led to fewer cardiac events over time compared to medication alone. So while some nuance exists in that narrow range, the 0.80 threshold is what current guidelines rely on.
What Happens During the Procedure
FFR is performed during a diagnostic cardiac catheterization, so if you’re already having a catheterization, the FFR adds only a few extra minutes. A guide catheter is positioned at the opening of the coronary artery, and the cardiologist records the baseline pressure there. Then the specialized pressure wire is advanced past the blockage, and the pressure beyond it is recorded.
Next comes the adenosine. It’s given either through an IV in your arm or directly into the coronary artery through the catheter. Once administered, the cardiologist monitors the pressure readings for about three to four minutes while maximum blood flow is achieved. You may feel brief chest discomfort, flushing, or shortness of breath while the adenosine is working. These sensations are temporary and fade quickly once the drug wears off, which happens within seconds to a couple of minutes.
The cardiologist can see the pressure sensor on the X-ray screen to confirm it’s in the right position. Once the FFR number is recorded, the decision about whether to place a stent can be made right then, during the same procedure. This is one of FFR’s biggest practical advantages: it turns a subjective visual assessment into an objective, numbers-based decision in real time.
Why FFR Matters for Treatment Decisions
Before FFR became standard, cardiologists relied heavily on how a blockage looked on the angiogram. The problem is that appearances can be deceiving. A narrowing that looks like it’s blocking 60 or 70 percent of an artery might not actually limit blood flow in a way that harms the heart. Stenting arteries that don’t truly need it exposes patients to procedural risks and long-term blood thinner requirements without a clear benefit.
The landmark clinical trials changed this. The DEFER trial showed that patients whose blockages had FFR values above 0.75 did just as well without a stent as with one. The FAME trials then demonstrated that using FFR to guide stenting decisions led to better outcomes and fewer unnecessary procedures compared to relying on angiography alone. Based on this evidence, current European Society of Cardiology guidelines recommend FFR testing before revascularizing any “intermediate” stenosis, meaning one that isn’t obviously severe or obviously harmless on imaging.
FFR vs. iFR: The Newer Alternative
A newer test called iFR (instantaneous wave-free ratio) works on a similar principle but skips the adenosine. Instead of measuring pressure during drug-induced maximum flow, iFR captures the pressure gradient during a specific portion of the heart’s natural resting cycle when the small downstream vessels are already at their most relaxed. The diagnostic accuracy is comparable to FFR.
The practical difference for patients is comfort. Because iFR doesn’t require adenosine, it avoids the chest tightness and flushing that many people experience during FFR. A large trial published in the New England Journal of Medicine found that significantly more patients in the FFR group reported chest discomfort compared to the iFR group. Both tests are now accepted in clinical guidelines as valid tools for the same purpose, and which one your cardiologist uses often comes down to institutional preference and the specific clinical scenario.
Limitations of FFR
FFR’s accuracy depends entirely on achieving true maximum blood flow. If the adenosine doesn’t fully dilate the downstream vessels, perhaps because of an inadequate dose or because the tiny vessels are already diseased, the pressure drop across the blockage will be underestimated. The FFR number will look more reassuring than it should. This is one reason the test can be less reliable in certain patient populations, including people with widespread small-vessel disease.
FFR also measures one blockage at a time. In patients with multiple narrowings along the same artery or in different arteries, interpreting the results becomes more complex because each blockage affects the pressure environment around the others. And because the test requires a catheterization, it carries the same baseline risks as any catheter-based procedure: a small chance of bleeding, vessel injury, or allergic reaction to contrast dye. These complications are uncommon, but they’re the reason FFR isn’t used as a screening tool. It’s reserved for situations where an angiogram has already identified a questionable blockage and a clear answer will change the treatment plan.