Heart failure is diagnosed through a combination of symptom assessment, blood tests, and imaging, with an echocardiogram being the single most important test. No one test confirms the diagnosis on its own. Instead, doctors layer several pieces of evidence together to determine whether the heart is failing, what type of heart failure is present, and how severe it is.
What Happens During the Physical Exam
The diagnostic process starts with your symptoms and a hands-on exam. Fatigue that doesn’t match your activity level, shortness of breath (especially when lying flat), and swelling in your legs or feet are the classic complaints that prompt a closer look. Some people first notice they can’t fit into their usual shoes because of fluid buildup in the ankles and feet.
During the exam, your doctor will check for a few specific signs. One is jugular venous distention, where the veins in your neck appear visibly swollen. You’ll be positioned at a 45-degree angle while the doctor measures how high the blood is backing up in those veins. A measurement more than 4 to 5 centimeters above a bony landmark on your sternum suggests elevated pressure inside the heart. This sign is highly specific (about 90% of the time, if it’s present, something is genuinely wrong) but not very sensitive, meaning many people with heart failure won’t show it. Your doctor will also listen to your lungs for crackling sounds that indicate fluid buildup, and press on your lower legs to check for pitting edema, where a thumbprint leaves a temporary dent in swollen skin.
Blood Tests: BNP and NT-proBNP
When the heart is under strain, its muscle cells release a hormone called B-type natriuretic peptide (BNP) and a related fragment called NT-proBNP. A simple blood draw measures these levels, and the results are most useful for ruling heart failure out. If your NT-proBNP is below 300 pg/mL in an acute setting, heart failure is very unlikely regardless of your age. For people with milder, slower-onset symptoms, a level below 125 pg/mL serves the same purpose.
The “rule-in” thresholds are more nuanced because they shift with age. For NT-proBNP, levels above 450 pg/mL in people under 50, above 900 pg/mL in those between 50 and 75, and above 1,800 pg/mL in people over 75 strongly suggest acute heart failure. For the older BNP test, a level above 100 pg/mL raises suspicion, though values between 100 and 500 pg/mL fall into a gray zone where other conditions like kidney disease, lung disease, or obesity can muddy the picture. Think of these blood tests as a sorting tool: low levels are reassuring, high levels push your doctor toward imaging, and middle-range levels mean more investigation is needed.
The Electrocardiogram
An ECG (or EKG) records the electrical activity of your heart through sticky patches placed on your chest and limbs. It takes about 10 minutes and is painless. While an ECG alone can’t confirm heart failure, certain patterns raise a red flag. Signs of thickening in the left ventricle, a widened electrical signal (called a prolonged QRS duration), left bundle-branch block (where the electrical signal takes an abnormal path through the heart), and ST/T-wave abnormalities all increase suspicion. A completely normal ECG makes heart failure less likely, so it’s a useful early screening step.
Different ECG patterns can also hint at which type of heart failure you might have. Electrical conduction delays and signs of left ventricular thickening tend to point toward heart failure with a weak pump, while a faster resting heart rate and certain changes in the P-wave (reflecting how the upper chambers fire) are more associated with heart failure where the pump is stiff rather than weak.
Echocardiogram: The Central Test
The echocardiogram is an ultrasound of your heart and the single most informative test in the diagnostic process. It’s noninvasive, uses no radiation, and typically takes 30 to 60 minutes. A technician presses a probe against your chest while you lie on your side, and the resulting images show your heart beating in real time.
The key measurement is your ejection fraction (EF), the percentage of blood your left ventricle pumps out with each beat. A normal heart ejects about 55% to 70% of its blood per squeeze. The ejection fraction determines which category of heart failure you have:
- Reduced ejection fraction (HFrEF): 40% or lower. The heart muscle is too weak to pump effectively.
- Mildly reduced ejection fraction (HFmrEF): 41% to 49%. A relatively new category recognizing a middle ground.
- Preserved ejection fraction (HFpEF): 50% or higher. The heart pumps adequately but has become too stiff to fill properly between beats.
Beyond ejection fraction, the echocardiogram reveals the thickness and mass of your heart walls (thickening suggests the heart has been working too hard for too long), the size of the left atrium (enlargement suggests chronic pressure buildup), how well your valves open and close, and estimated pressure in the pulmonary artery. It can also assess how well the right side of the heart is functioning, which matters because right-sided failure often develops as a consequence of left-sided problems. In patients who are difficult to image due to body size, lung disease, or chest wall shape, contrast dye injected through an IV can sharpen the pictures.
When More Advanced Imaging Is Needed
For most people, an echocardiogram plus blood work provides enough information. But in certain situations, additional tests fill in gaps. If your doctor suspects that coronary artery disease is the underlying cause of your heart failure, a stress test may be ordered. This involves walking on a treadmill or receiving a medication that mimics exercise while your heart is monitored with imaging. Stress testing is most useful when the likelihood of blocked arteries is moderate, roughly between 10% and 75% based on your risk factors and symptoms.
Cardiac MRI is sometimes used when the echocardiogram images are inconclusive or when your doctor needs to understand the heart muscle tissue itself. Using a contrast agent called gadolinium, cardiac MRI can detect scarring from a previous heart attack, identify inflammation, and distinguish between different diseases that cause the heart to thicken or weaken. It provides the most precise measurement of ejection fraction available and is particularly helpful when the cause of heart failure isn’t obvious from standard tests.
How Severity Is Classified
Once heart failure is confirmed, your doctor will classify how much it limits your daily life using the New York Heart Association (NYHA) system. This isn’t based on a test result. It’s based on how you feel.
- Class I: No limitations. Everyday activities like walking, climbing stairs, or carrying groceries don’t cause unusual fatigue or breathlessness.
- Class II: Slight limitation. You’re comfortable at rest, but ordinary activities like brisk walking or yard work bring on fatigue, shortness of breath, or chest discomfort.
- Class III: Marked limitation. You’re fine sitting still, but even light activity like getting dressed or walking across a room can trigger symptoms.
- Class IV: Symptoms at rest. Any physical activity makes them worse.
This classification matters because it directly shapes treatment decisions and gives your care team a common language for tracking whether you’re getting better or worse over time. Your class can change. Someone diagnosed at Class III who responds well to treatment might improve to Class II, while someone whose condition progresses could shift from Class II to III. It’s reassessed at every visit, making it one of the simplest but most practical tools in heart failure management.