The tests used to evaluate shortness of breath typically start simple and get more specific depending on what your doctor suspects. Most people with acute shortness of breath will get a chest X-ray, an electrocardiogram (ECG), basic blood work, and a pulse oximetry reading as a starting point. From there, additional tests narrow down whether the cause is related to your heart, lungs, blood clots, or something else entirely.
The First Round of Tests
When you show up with unexplained shortness of breath, the initial workup follows a predictable pattern. A chest X-ray is almost always first because it can reveal a wide range of problems quickly: fluid in or around the lungs, pneumonia, a collapsed lung, or an enlarged heart. An ECG checks for irregular heart rhythms or signs of a heart attack, both of which can cause sudden breathlessness.
Basic blood work usually includes a complete blood count to check for anemia (too few red blood cells to carry adequate oxygen) and electrolyte levels, including bicarbonate, which reflects how well your body is managing its acid-base balance. A finger clip pulse oximeter measures your blood oxygen level in seconds. These tests together help your doctor quickly sort through the most common and most dangerous causes before deciding what to investigate further.
Blood Tests That Point to Heart Failure or Blood Clots
Two specific blood markers play an outsized role in figuring out why someone is short of breath: one flags heart failure, and the other helps rule out blood clots in the lungs.
When the heart is struggling and its walls are under strain, it releases a protein that can be measured as BNP or NT-proBNP. These are considered the gold standard for identifying heart failure as the cause of sudden shortness of breath. The thresholds doctors use depend on your age. For people under 50, levels above roughly 125 pg/mL raise concern. For those 50 to 74, the threshold rises to around 250 pg/mL, and for people 75 and older, it’s about 500 pg/mL. A very high reading (above 2,000 pg/mL) signals serious cardiac strain regardless of age. Factors like obesity and other medical conditions can shift these numbers, so they’re interpreted alongside the rest of your clinical picture.
D-dimer is a blood marker that rises when your body is breaking down blood clots. It’s extremely useful for one specific purpose: ruling out a pulmonary embolism (a clot that has traveled to the lungs) when the suspicion is low. A normal D-dimer in a low-risk patient effectively takes PE off the table. But it’s not a great confirmation test. D-dimer levels rise with age, infection, surgery, pregnancy, and many other conditions, so an elevated result doesn’t mean you have a clot. It means more testing is needed.
Measuring How Well Your Lungs Work
Pulmonary function tests, often called PFTs, measure how much air your lungs can hold and how effectively you move air in and out. The core test is spirometry, where you take the deepest breath you can and blow out as hard and fast as possible into a tube connected to a sensor. This produces two key numbers: your forced vital capacity (FVC), which is the total volume of air you can exhale, and your forced expiratory volume in one second (FEV1), which is how much comes out in that first explosive second.
The ratio between these two numbers is what distinguishes the two main categories of lung disease. A ratio below 0.70 indicates an obstructive pattern, meaning air has trouble getting out. This is the hallmark of conditions like asthma and COPD. A normal or high ratio combined with reduced total lung capacity points to a restrictive pattern, where the lungs can’t fully expand. This shows up in conditions like pulmonary fibrosis or chest wall problems.
A third measurement, called diffusion capacity, tests how well oxygen crosses from your air sacs into your bloodstream. When this value is low but spirometry looks normal, it can point to pulmonary vascular disease, early interstitial lung disease, or emphysema. During the test, you’ll wear a nose clip, sit upright with your chin slightly elevated, and loosen any tight clothing. If your dentures are loose, you may be asked to remove them so they don’t interfere with the mouthpiece seal.
Echocardiogram for Suspected Heart Problems
When there’s a reasonable chance your shortness of breath is coming from the heart, an echocardiogram is the next step. This is an ultrasound of the heart that shows it beating in real time. It measures your ejection fraction, the percentage of blood your heart pumps out with each beat. A normal ejection fraction is typically 55% or higher. A low number confirms the heart isn’t pumping effectively and guides treatment decisions from that point forward.
Beyond pumping strength, the echo evaluates all four heart valves for narrowing or leaking, measures the size of heart chambers, checks for thickening of the heart walls, and can estimate pressure in the lung’s blood vessels. If your doctor heard a new heart murmur, suspects valve disease, or your BNP levels came back elevated, an echocardiogram is the test that translates those clues into a clear structural picture.
CT Scans for Blood Clots and Lung Disease
A CT pulmonary angiography (CTPA) is the go-to imaging test when a pulmonary embolism is suspected. It uses contrast dye injected into a vein to highlight the blood vessels in your lungs, making clots clearly visible. Doctors favor it because it gives a definitive yes-or-no answer and can also reveal other causes of your symptoms that a chest X-ray might miss, such as lung masses, fluid collections, or enlarged lymph nodes.
A high-resolution CT (HRCT) serves a different purpose. Rather than looking at blood vessels, it provides detailed images of the lung tissue itself. This is the preferred scan when your doctor suspects interstitial lung disease, unusual infections, or structural damage like bronchiectasis. The choice between CTPA and HRCT depends entirely on what your doctor thinks is most likely based on your symptoms, blood work, and initial imaging.
When Pulse Oximetry Isn’t Enough
The finger clip oximeter is a convenient screening tool, but it has real limitations. It can overestimate oxygen levels in people with darker skin tones and becomes unreliable when oxygen saturation drops below 90%. In carbon monoxide poisoning, it can read completely normal despite dangerously low oxygen delivery. Some COVID-19 patients showed 100% on pulse oximetry while their actual blood oxygen was critically low.
When accuracy matters most, or when your doctor suspects you’re not ventilating properly (meaning CO2 is building up even if oxygen looks adequate), an arterial blood gas test is necessary. This involves drawing blood from an artery, usually at the wrist. It measures oxygen and carbon dioxide levels directly, along with blood pH and bicarbonate, painting a far more complete picture of how well your lungs are doing their job. Pulse oximetry only tells you about oxygen. It says nothing about carbon dioxide, which is why suspected hypoventilation always warrants a blood gas.
Exercise Testing for Unexplained Cases
Sometimes all the resting tests come back normal or inconclusive, yet you’re clearly struggling with exertion. Cardiopulmonary exercise testing (CPET) fills this gap. You exercise on a treadmill or stationary bike while wearing a mask that measures your oxygen consumption and carbon dioxide output, along with continuous heart and breathing monitoring.
CPET is specifically recommended by the American College of Cardiology and American Heart Association for distinguishing cardiac from pulmonary causes of exercise-related breathlessness when the answer isn’t clear from other testing. One key measurement is the ventilatory anaerobic threshold, the point during exercise where your body shifts from aerobic to anaerobic energy production. If you fatigue before reaching this threshold, the limitation is more likely muscular or pulmonary rather than cardiac. Reaching or passing it before stopping suggests the heart may be the bottleneck. This test is particularly valuable for people whose shortness of breath only appears during activity and resolves at rest.