Pulmonary arterial hypertension (PAH) is diagnosed through a multi-step process that starts with noninvasive screening and ends with a procedure called right heart catheterization, which directly measures blood pressure inside the lung arteries. A diagnosis requires the mean pulmonary artery pressure to be above 20 mmHg, with additional measurements confirming the problem originates in the lung vessels rather than the left side of the heart. Getting to that final confirmation typically involves several weeks of testing to rule out other conditions that can mimic PAH.
Why Diagnosis Takes Multiple Steps
High blood pressure in the lungs can stem from many causes: left-sided heart failure, chronic lung disease, blood clots, and more. PAH (classified as Group 1 pulmonary hypertension) is a specific condition where the small arteries in the lungs narrow and stiffen on their own, not because of another disease driving the problem. The diagnostic workup exists to separate PAH from these other causes, because treatment differs dramatically depending on the underlying reason.
The process generally moves from broad screening toward increasingly specific tests. At each stage, certain conditions are either confirmed or crossed off the list, narrowing the possibilities until the type of pulmonary hypertension can be classified with confidence.
Initial Evaluation and Symptoms
Most people first raise suspicion of PAH through symptoms that are frustratingly nonspecific: shortness of breath during routine activities, fatigue, chest pressure, or feeling lightheaded with exertion. These overlap with dozens of other heart and lung conditions, which is one reason PAH often goes undiagnosed for months or years.
Doctors classify symptom severity using the WHO functional classification system, which has four levels. Class I means no limitation of physical activity. Class II involves slight limitation, with symptoms during ordinary exertion but not at rest. Class III means marked limitation, where even mild activity causes symptoms. Class IV describes an inability to perform any physical activity without discomfort, sometimes with symptoms at rest and signs of right-sided heart failure. This classification doesn’t diagnose PAH by itself, but it plays a major role in guiding treatment urgency and tracking disease progression over time.
Echocardiogram: The Key Screening Test
A transthoracic echocardiogram (a heart ultrasound) is the most important noninvasive screening test for pulmonary hypertension. It can’t confirm the diagnosis on its own, but it estimates the pressure in the lung arteries and flags whether further investigation is warranted.
The echocardiogram measures the speed of a small jet of blood that leaks backward through the tricuspid valve, called the tricuspid regurgitant velocity (TRV). This velocity correlates with pulmonary artery pressure. Guidelines classify the probability of pulmonary hypertension based on TRV readings: a peak velocity of 2.8 m/s or lower suggests low probability, 2.9 to 3.4 m/s indicates intermediate probability, and above 3.4 m/s points to high probability. Additional findings like an enlarged right side of the heart can bump an intermediate reading into the high-probability category.
If the echocardiogram shows intermediate or high probability, the diagnostic workup continues. A low-probability result in someone without risk factors generally means PAH is unlikely.
Blood Tests and Biomarkers
Blood work during the initial evaluation serves two purposes: screening for conditions that cause or accompany PAH, and gauging how much strain the heart is under. Thyroid disease is common in PAH patients and is routinely checked. Screening for connective tissue diseases (like scleroderma), hepatitis, and HIV is also standard, since all of these can trigger PAH.
A biomarker called NT-proBNP, released when the heart is under stress, helps assess severity. Levels below 300 ng/L fall in the low-risk category, while levels above 1,400 ng/L indicate high risk. In patients with scleroderma, an NT-proBNP above 395 ng/L has a positive predictive value above 95% for associated PAH. These numbers don’t diagnose PAH directly, but they add important context to the clinical picture and help predict outcomes.
Lung Function and Imaging Tests
Pulmonary function tests help distinguish PAH from chronic lung diseases. The hallmark finding in PAH is a reduced DLCO, a measure of how efficiently the lungs transfer gas from air into the bloodstream. In one study of PAH patients with connective tissue disease, 96% had a decline in DLCO, with 60% showing moderate to severe reductions. Airflow measurements like the ratio of air exhaled in one second to total lung capacity tend to remain relatively normal, which helps separate PAH from conditions like COPD or pulmonary fibrosis where airflow is more significantly impaired.
A ventilation/perfusion (V/Q) scan is required in all patients being evaluated for pulmonary hypertension. This nuclear medicine test checks whether blood is flowing evenly through the lungs. Its primary role is to screen for chronic thromboembolic pulmonary hypertension (CTEPH), a form caused by old blood clots lodged in the lung arteries. CTEPH is treated very differently from PAH, sometimes with surgery, so ruling it out is essential. V/Q scans have excellent sensitivity and specificity for this purpose.
A chest CT scan provides additional clues. It can reveal enlarged pulmonary arteries, signs of lung disease, or evidence of blood clots. Together with the V/Q scan, it helps build a complete picture of the lungs’ structure and blood supply.
Right Heart Catheterization: The Definitive Test
Every patient suspected of having PAH must undergo right heart catheterization (RHC) before a diagnosis is finalized. This is the only test that directly measures the pressures inside the heart and lung arteries, and it remains the gold standard. The procedure involves threading a thin catheter through a vein (typically in the neck or groin) into the right side of the heart and then into the pulmonary artery. At experienced centers, the complication rate is about 1.1%, with a mortality rate of 0.055%.
During the procedure, several measurements are taken. The mean pulmonary artery pressure (mPAP) establishes whether pulmonary hypertension exists. Cardiac output, the volume of blood the heart pumps per minute (normally 4 to 6 liters), reveals how well the heart is compensating. Mixed venous oxygen saturation shows whether the body is getting enough oxygen from the blood being circulated. Right atrial pressure reflects how much backup pressure exists in the veins returning to the heart.
The Numbers That Define PAH
Until 2022, pulmonary hypertension was defined as a mean pulmonary artery pressure of 25 mmHg or higher. The European Society of Cardiology and European Respiratory Society lowered this threshold to above 20 mmHg, based on evidence that the upper limit of normal pressure is 20 mmHg and that significant vascular damage may already exist once pressure exceeds that level. Research suggests that when mPAP rises above 20 mmHg, more than 50% of the pulmonary vascular bed may already be obstructed.
To qualify specifically as PAH (pre-capillary pulmonary hypertension), three criteria must be met simultaneously: mPAP above 20 mmHg, pulmonary vascular resistance (PVR) above 2 Wood units, and a pulmonary arterial wedge pressure (PAWP) of 15 mmHg or below. The wedge pressure is the critical distinction. It estimates pressure on the left side of the heart. A wedge pressure above 15 mmHg suggests the elevated lung pressure is being driven by left heart disease, not by problems in the lung arteries themselves. That distinction changes the diagnosis entirely and redirects treatment toward the heart rather than the lungs.
It’s worth noting that most existing treatment evidence for PAH medications comes from patients who met the older, higher thresholds (mPAP of 25 mmHg or above, PVR of 3 Wood units or above). Data on treating patients who fall between the old and new cutoffs is still limited.
Vasoreactivity Testing
For a specific subset of PAH patients, those with idiopathic, heritable, or drug-induced forms, an additional test is performed during catheterization called vasoreactivity testing. This involves administering a short-acting vasodilator (a substance that relaxes blood vessels) and watching whether the pulmonary artery pressure drops significantly. Patients who respond positively may be candidates for a particular class of medications, calcium channel blockers, which are far simpler and less expensive than the advanced therapies most PAH patients require. Only a small minority of patients test positive, but identifying them matters because their long-term outlook is considerably better.
Putting the Diagnosis Together
No single test diagnoses PAH. The final classification comes from layering results: the echocardiogram raises suspicion, blood tests and imaging narrow the differential, and catheterization confirms the hemodynamic profile. The complete workup typically involves an electrocardiogram, echocardiogram, blood panels, pulmonary function tests, a V/Q scan, chest CT, and right heart catheterization, often with additional specialized tests depending on what surfaces along the way.
For people in high-risk groups, particularly those with scleroderma or other connective tissue diseases, family history of PAH, or portal hypertension, screening echocardiograms may be performed even before symptoms appear. Early detection at lower pressures is the goal of the updated diagnostic thresholds, catching the disease before the right side of the heart has been significantly damaged.