Testing for pulmonary fibrosis involves several steps, starting with a physical exam and breathing tests, then moving to a high-resolution CT scan of the lungs. No single test confirms the diagnosis on its own. Instead, doctors piece together findings from imaging, lung function measurements, blood work, and sometimes a lung biopsy to reach a definitive answer.
What Happens During the Physical Exam
The first clue often comes from listening to your lungs with a stethoscope. Pulmonary fibrosis produces a distinctive crackling sound during the early part of each breath, concentrated in the lower back portions of the lungs. These crackles have a dry, fine quality that doctors describe as sounding like Velcro being pulled apart. If you’ve been told your lungs sound like Velcro, that’s a recognized hallmark of the condition.
Your doctor will also look at your fingertips. About 50% of people with idiopathic pulmonary fibrosis develop a condition called clubbing, where the fingertips widen and the nails curve downward. Combined with the Velcro crackles, these findings are enough to prompt further testing, but they can’t confirm the diagnosis alone.
High-Resolution CT Scan
A high-resolution CT scan (HRCT) of the chest is the single most important test for diagnosing pulmonary fibrosis. It provides detailed cross-sectional images of the lungs and can often confirm the diagnosis without any need for a biopsy.
Radiologists look for a specific pattern called usual interstitial pneumonia, or UIP. A definitive UIP pattern requires all four of these features: the damage is concentrated in the lower, outer edges of the lungs; there’s a net-like (reticular) pattern of scarring; there’s honeycombing (clusters of small air-filled cysts, typically under 10 mm each, with well-defined walls); and there are no features that point toward a different diagnosis. When all four criteria are present, the scan alone is considered diagnostic.
When the scan shows the right location and scarring pattern but no honeycombing, it’s classified as “possible UIP.” In those cases, a biopsy may be recommended to get a more definitive answer. The distinction between honeycombing and dilated airways can be tricky even for experienced radiologists, so doctors sometimes scroll through thin cross-sectional slices or review images from different angles to tell them apart.
Pulmonary Function Tests
Breathing tests, formally called pulmonary function tests, measure how well your lungs move air and transfer oxygen into your blood. You’ll breathe into a mouthpiece while a machine records several measurements. The whole process takes about 30 to 45 minutes and is painless.
Pulmonary fibrosis causes a “restrictive” pattern, meaning the lungs can’t expand fully because scar tissue has stiffened them. The key indicators are a forced vital capacity (the total amount of air you can forcefully exhale) below 80% of the predicted value for your age, sex, and height, while the ratio between how fast you exhale in the first second versus total exhalation stays normal or even increases above 0.70. In simpler terms, the lungs are smaller but the airways themselves aren’t blocked.
Another critical measurement is your lungs’ ability to transfer oxygen into the bloodstream, called diffusing capacity. A value below 75% of predicted suggests the gas-exchanging tissue is damaged. This measurement can drop even before other breathing tests show abnormalities, making it useful for catching early-stage disease.
The Six-Minute Walk Test
This straightforward test measures how far you can walk in six minutes on a flat surface while a small clip on your finger tracks your oxygen levels. Doctors are looking for exertional desaturation, a drop in blood oxygen below 90% during activity. This tells them how much the scarring is affecting your body’s ability to get oxygen where it needs to go during everyday movement, not just at rest. The results also serve as a baseline to track whether your condition is stable or progressing over time.
Blood Tests to Rule Out Other Causes
Before confirming idiopathic (meaning “cause unknown”) pulmonary fibrosis, doctors need to rule out conditions that cause similar lung scarring but require different treatment. Autoimmune diseases like rheumatoid arthritis, lupus, and inflammatory muscle diseases can all damage the lungs in ways that mimic pulmonary fibrosis on imaging.
A panel of blood tests checks for autoantibodies, proteins the immune system produces when it mistakenly attacks the body’s own tissues. The panel typically includes antinuclear antibodies (ANA), rheumatoid factor, and antibodies against a protein called cyclic citrullinated peptide. A positive result on any of these points toward an autoimmune condition as the underlying driver, which changes both the diagnosis and the treatment approach.
It’s worth knowing that a negative result doesn’t completely rule out autoimmune involvement, and a positive ANA alone isn’t definitive since it can show up in people without autoimmune disease. That’s why doctors interpret these results alongside everything else rather than in isolation.
Bronchoscopy and Lung Fluid Analysis
In some cases, doctors use a thin, flexible camera called a bronchoscope to look inside the airways and collect a sample of fluid from the lungs. This procedure, called bronchoalveolar lavage, involves flushing a small amount of saline into a section of the lung and then suctioning it back out. The recovered fluid is analyzed under a microscope to count the types of immune cells present.
Different patterns of immune cells point toward different diagnoses. In pulmonary fibrosis with a UIP pattern, neutrophil counts tend to be elevated, with a median around 5 to 7%. Lymphocyte counts vary but tend to be higher in non-UIP patterns of fibrosis. These cell profiles help distinguish between types of interstitial lung disease when imaging alone doesn’t provide a clear answer.
Lung Biopsy
When imaging and other tests can’t confirm the diagnosis, a lung biopsy may be necessary. There are two main approaches, and each has trade-offs.
Surgical lung biopsy is the traditional method and remains the more reliable option. It requires general anesthesia and involves removing small wedge-shaped pieces of lung tissue, usually through a few small incisions. The tissue samples are large enough for pathologists to see the full pattern of scarring, which is why surgical biopsies leave the diagnosis unclassifiable in only about 7% of cases.
Transbronchial cryobiopsy is a newer, less invasive alternative performed through a bronchoscope. A probe is cooled to very low temperatures, freezing a small piece of lung tissue that is then extracted. Recovery is generally easier, but the smaller samples come with a diagnostic trade-off. Studies comparing the two approaches found that cryobiopsy agreed with the surgical biopsy diagnosis about 62% of the time, and the rate of nondiagnostic or low-confidence results was significantly higher with cryobiopsy (68%) compared to surgical biopsy (37%). Consistency between different medical centers was also much lower for cryobiopsy readings.
Pathologists examining biopsy tissue look for a characteristic combination: dense scarring that distorts the lung’s normal architecture, patchy involvement (areas of scarring next to areas of relatively normal tissue), and clusters of actively dividing cells called fibroblast foci at the edges of the scar tissue.
How the Diagnosis Comes Together
Pulmonary fibrosis is one of those conditions where no single specialist makes the final call alone. International guidelines recommend a multidisciplinary discussion involving a pulmonologist, a radiologist, and a pathologist (if biopsy tissue is available). Each expert reviews the case from their angle, and the group reaches a consensus diagnosis together.
This team-based approach has been shown to improve agreement between clinicians, change the initial working diagnosis in a significant number of cases, and reduce the proportion of cases that remain unclassifiable. The diagnostic process follows a structured path: first, rule out known causes of lung scarring like occupational exposures, medications, and autoimmune disease. Then, if HRCT shows a definitive UIP pattern, the diagnosis can be made without biopsy. If the CT findings are less clear, specific combinations of imaging and biopsy results guide the final diagnosis.
The entire workup, from initial suspicion to confirmed diagnosis, can take weeks to months depending on how clear the imaging is and whether biopsy is needed. If your doctor suspects pulmonary fibrosis, expect to visit multiple specialists and undergo several of these tests before receiving a definitive answer.