How to Read the Results of a Nuclear Stress Test

A nuclear stress test report compares blood flow through your heart muscle at rest and under stress, then layers on measurements of how well your heart pumps. The core of the report is a set of color-coded images showing whether every region of your heart received adequate blood during peak demand. Understanding those images, the numbers that accompany them, and the red flags your cardiologist looks for can turn an intimidating document into something that actually makes sense.

Rest Versus Stress Images

The fundamental comparison in any nuclear stress test is between two sets of images: one taken while your heart is at rest and another taken during or shortly after stress (either exercise on a treadmill or a drug that mimics the effect of exercise). A small amount of radioactive tracer is injected into your bloodstream, and a camera detects where that tracer lands in the heart muscle. Regions that receive good blood flow light up brightly; regions that don’t receive enough appear dimmer or absent.

Your report will typically display these images side by side, sliced through the heart in three standard orientations so the doctor can inspect every wall segment. Most software divides the heart into 17 segments arranged in a bull’s-eye (or “polar map”) display, where each wedge of the target represents a specific part of the heart wall. The brightness or color of each wedge reflects how much tracer was taken up by that segment relative to the others.

Reversible Defects, Fixed Defects, and Normal Scans

The language your report uses to describe what the images show boils down to three possibilities for each segment of the heart:

Normal perfusion: The segment looks equally bright on both the rest and stress images. Blood flow was adequate under both conditions.
Reversible defect: The segment appears dim or absent during stress but fills in (normalizes) at rest. This pattern suggests ischemia, meaning a coronary artery supplying that region can deliver enough blood at rest but cannot keep up when the heart works harder.
Fixed defect: The segment appears dim or absent on both the stress and rest images. This typically indicates scar tissue from a prior heart attack, where the muscle is no longer alive enough to take up tracer under any condition.

Reversible defects are the findings that most often lead to further testing or treatment, because they signal muscle that is still alive but at risk. In one study tracking patients who underwent revascularization (a bypass or stent procedure), regions with reversible defects recovered normal contraction about 79% of the time, compared with only 30% of regions that had mild-to-moderate fixed defects.¹ That gap is why cardiologists care so much about the distinction: a reversible defect points to muscle that can be saved.

Fixed defects are not always hopeless, though. Small areas of fixed uptake reduction may still contain some viable tissue, especially if the defect is mild rather than severe. The practical upshot: fixed does not automatically mean “nothing can be done,” but reversible is a much stronger predictor of recovery if blood flow is restored.

Summed Scores and What the Numbers Mean

Many reports include a set of numerical scores that quantify the severity of any perfusion abnormality. The most common are the summed stress score (SSS), summed rest score (SRS), and summed difference score (SDS). Each of the 17 heart segments gets a score from 0 (normal uptake) to 4 (no uptake at all), and those individual scores are added up.

SSS: Reflects the total perfusion abnormality during stress. A higher number means more of the heart showed reduced blood flow under demand.
SRS: Reflects the total perfusion abnormality at rest. A higher number means more scar.
SDS: The difference between SSS and SRS. This isolates the amount of ischemia, the muscle that is underperfused during stress but looks fine at rest.

A completely normal scan produces scores near zero across the board. An SDS above roughly 2 to 4 starts to raise concern about meaningful ischemia. However, these scores describe relative differences in tracer uptake across the heart’s segments, not absolute blood flow. Research comparing the SDS to directly measured blood flow found that while a high SDS reliably corresponds to reduced flow in the affected territory, a normal SDS does not always rule out problems, particularly in cases where all regions of the heart are equally underperfused.² That limitation matters and is discussed further in the section on balanced ischemia below.

Ejection Fraction and Ventricular Volumes

Modern nuclear stress tests don’t just measure blood flow. Because the camera captures the heart at multiple points in its beating cycle (a technique called gated imaging), the software can also calculate your left ventricular ejection fraction (EF), which is the percentage of blood the heart pumps out with each beat, along with the size of the heart chamber at its fullest and emptiest points.

The ejection fraction is one of the most prognostically powerful numbers on the report. A landmark study of over 2,600 patients found that those with a post-stress EF of 45% or higher had annual cardiac death rates below 1%, even when their perfusion images showed severe abnormalities. Patients whose EF fell below 45%, on the other hand, had high annual mortality rates even when their perfusion defects were only mild or moderate.³ In practical terms, the ejection fraction can override the perfusion results when it comes to predicting risk: a normal EF is reassuring even with some perfusion abnormality, while a low EF is concerning even if the perfusion images look relatively clean.

Your report may also note end-diastolic volume (EDV) and end-systolic volume (ESV). These describe the physical size of the left ventricle when it is fully filled and fully squeezed, respectively. An ESV above roughly 70 mL or an EDV above about 120 mL was identified in the same study as a threshold suggesting elevated risk. Large volumes can signal that the heart is dilated and struggling to pump efficiently. Gated imaging also reduces false-positive results overall, because the wall motion data help distinguish a genuine perfusion defect from an imaging artifact.⁴

High-Risk Markers Your Report May Flag

Beyond perfusion patterns and ejection fraction, a handful of additional findings on a nuclear stress test can point toward more serious or widespread coronary artery disease.

Transient ischemic dilation (TID) is one of them. If the heart’s left ventricle appears noticeably larger on the stress images than on the rest images, it suggests the inner layer of muscle is being so starved of blood during stress that the cavity looks bigger. TID is associated with more severe and extensive coronary disease.⁵ A meta-analysis of TID’s diagnostic performance found it had a pooled specificity of about 88% for detecting extensive coronary disease, though its sensitivity was only around 44%, meaning it misses many cases but is highly suspicious when present.⁶

Increased lung uptake of the tracer is another warning sign. Normally, the lungs should not hold much of the tracer after injection. If the lung fields appear unusually bright, it can indicate that blood is backing up from a heart that cannot handle the stress load, often pointing to left ventricular dysfunction or severe ischemia.

Exercise Responses That Appear in the Report

If your stress test involved actual treadmill exercise rather than a drug, the report will typically include data from the exercise portion itself: how long you exercised, the peak heart rate you reached, your blood pressure response, and any ECG (electrocardiogram) changes observed during exertion.

Two exercise findings carry particular weight. The first is an abnormal blood pressure response, where systolic blood pressure fails to rise appropriately or actually drops during exercise. The second is significant ST-segment depression on the ECG, a pattern that suggests the heart muscle is not getting enough oxygen. A classic study found that an abnormal blood pressure response during exercise had a specificity of about 87% for three-vessel or left main coronary disease, and marked ST-segment depression had a specificity near 90% for the same.⁷ When either shows up alongside an abnormal perfusion scan, the combination is especially telling.

It is worth noting that ST-segment changes can occasionally appear in people without significant coronary disease, including some physically active older adults.⁸ Context always matters: the exercise data and the perfusion images are interpreted together, not in isolation.

Why Some Results Are Wrong: Artifacts and False Positives

Not every abnormality on a nuclear stress test reflects real heart disease. Soft tissue between the camera and the heart can absorb some of the radiation before it reaches the detector, creating the appearance of reduced blood flow in a segment that is actually fine. Breast tissue is a frequent culprit in women, and the diaphragm can cause similar problems in both sexes. One study found that over half of apparent perfusion defects were actually caused by these attenuation artifacts.⁹

Labs use several strategies to deal with this. CT-based attenuation correction applies a low-dose CT scan to map the density of the tissue overlying the heart, then mathematically adjusts the perfusion images. Prone imaging, where you lie face-down for an additional set of images, shifts the position of soft tissue relative to the heart. Both approaches improved specificity without sacrificing sensitivity in that same study. If your report mentions attenuation correction or prone imaging, it means the lab took extra steps to reduce the chance of a false alarm.

Pharmacological stress agents can also influence false-positive rates. One comparison found that patients stressed with regadenoson (a drug commonly used when patients can’t exercise) had a false-positive rate of about 35%, compared with 24% for those who exercised on a treadmill.¹⁰ This doesn’t mean drug-based stress tests are unreliable, but it is one reason doctors prefer treadmill exercise when you’re physically able to do it.

The Balanced Ischemia Blind Spot

One of the most important limitations of nuclear stress testing is something called balanced ischemia. Because standard SPECT imaging works by comparing how bright each segment is relative to the others, a heart with equally reduced blood flow everywhere can look deceptively normal. If all 17 segments are equally underperfused, there is no relative difference for the software to flag.

This matters most in patients with severe disease in all three major coronary arteries. Case reports document patients with strongly positive ECG changes during exercise whose nuclear perfusion images came back completely normal, only to be found to have critical three-vessel disease on angiography.¹¹ Balanced ischemia is one reason a negative stress test is not always truly negative.¹²

The high-risk markers discussed earlier, particularly TID and increased lung uptake, can sometimes tip off the reader to balanced ischemia even when the perfusion images look normal. PET scanning, which can measure absolute blood flow in milliliters per minute per gram of tissue rather than just relative brightness, largely sidesteps this problem.¹³

SPECT Versus PET and Why It Matters for Your Results

Most nuclear stress tests in the United States use SPECT (single-photon emission computed tomography), but PET (positron emission tomography) is becoming more widely available. The type of scan affects both the accuracy of your results and what kind of information is reported.

A systematic review found that SPECT had an average sensitivity of about 82% and specificity of about 76% for detecting coronary artery disease, while PET achieved roughly 91% sensitivity and 89% specificity.¹⁴ A separate meta-analysis of nearly 12,000 patients confirmed PET’s higher sensitivity, though it did not find a clear difference in specificity between the two.¹⁵

Beyond raw accuracy, PET produces clearer images with fewer uncertain readings. In a head-to-head comparison, expert readers rated 96% of PET studies as definitively normal or abnormal, compared with only 82% of SPECT studies; the remaining 18% of SPECT studies fell into an equivocal gray zone. PET also identified disease in a second or third coronary vessel 71% of the time, versus 48% with SPECT.¹⁶ If your report comes from a PET scan, the findings carry somewhat more weight, and the risk of an equivocal result is lower.

How Caffeine and Preparation Affect Accuracy

If your stress test uses a vasodilator drug like regadenoson or adenosine instead of treadmill exercise, caffeine can interfere with the results. These drugs work by dilating coronary arteries so the camera can see which vessels respond normally and which don’t. Caffeine blocks the same receptors these drugs target, blunting their effect.

A prospective multicenter trial found that consuming 200 mg or 400 mg of caffeine (roughly two to four cups of coffee) before a regadenoson stress test significantly reduced the number of reversible defects detected on imaging, compared with placebo.¹⁷ A systematic review and meta-analysis reached the same conclusion: caffeine before a pharmacological stress test may reduce diagnostic accuracy for detecting ischemia.¹⁸ This is why you are told to avoid coffee, tea, chocolate, and certain medications for at least 12 to 24 hours before a pharmacological stress test. If you accidentally consumed caffeine beforehand and your results were normal, it is worth mentioning to your doctor, since a true abnormality may have been masked.

Radiation Dose and Safety

A question many patients have after reading their results is how much radiation they received. Traditional two-day SPECT protocols deliver a meaningful dose, but newer approaches have brought exposure down dramatically. A study using a high-efficiency camera and a stress-first protocol found that patients who had normal stress images and did not need the rest portion of the test received an average effective dose of just about 1 mSv, roughly comparable to a mammogram. Even patients who needed the full stress-plus-rest protocol averaged about 2.2 mSv.¹⁹ Not every lab uses these ultra-low-dose protocols, so your actual exposure may vary, but the trend across the field is toward lower doses.

What Happens After an Abnormal Result

A nuclear stress test does not, by itself, decide whether you need a stent, bypass surgery, or medication changes. It provides one layer of information that your cardiologist combines with your symptoms, risk factors, and other test results. When the clinical suspicion for significant disease is high, symptoms are hard to control, or a change in treatment is expected, invasive coronary angiography, where a catheter is threaded into the heart’s arteries to look for blockages directly, often becomes the next step.²⁰ Conversely, a normal result in a patient with a low pre-test probability of disease is powerful reassurance that further invasive testing is unlikely to be needed.²¹

It is worth asking your cardiologist not just “is it normal?” but which specific findings drove the interpretation. The combination of perfusion pattern, ejection fraction, ventricular size, presence or absence of TID, exercise tolerance, and ECG response paints a more nuanced picture than a single normal-or-abnormal verdict.

Incidental Findings on the CT Portion

If your nuclear stress test included a CT scan for attenuation correction, there is a chance the report mentions something outside the heart entirely. These low-dose CT images capture a slice of your chest, and radiologists or nuclear medicine physicians reviewing them occasionally spot lung nodules, thyroid abnormalities, enlarged lymph nodes, or other findings unrelated to the original reason for the test.

These incidental findings are common, and many turn out to be benign. However, some are clinically significant. A study examining CT attenuation correction images found that incidental findings had a relatively high positive predictive value, particularly for malignancies, and recommended that these images be routinely reviewed and reported.²² Additional research in a veteran population confirmed that such findings are frequent.²³ Current guidance supports reviewing the CT portion of hybrid SPECT-CT and PET-CT studies for extra-cardiac pathology, since some of these incidental findings warrant dedicated follow-up imaging.²⁴

If your report mentions an incidental finding and recommends further evaluation, it does not necessarily mean something dangerous was found. It means something was spotted that the images alone cannot fully characterize, and a closer look with a dedicated scan or follow-up in a few months is warranted to be safe.

Machine Learning and the Future of Interpretation

Automated software already plays a role in nuclear stress test interpretation, generating the polar maps and summed scores that appear in your report. The next frontier is machine learning models trained to read stress test data with greater consistency and fewer missed diagnoses. Some models applied to stress ECG interpretation have achieved sensitivity and specificity above 96% and reduced false positives by up to about a fifth.²⁵ These tools are still in development for widespread clinical use, but they hint at a future where the interpretation you receive is less dependent on individual reader experience and more standardized across labs. For now, the report you get is still reviewed and signed by a physician, and the machine-generated numbers serve as a starting point for that human interpretation.