What Is Spirometry Used for in Cases of Lung Cancer?

It's a reasonable thing to find confusing. Lung cancer is detected on scans and confirmed through biopsies. So what exactly is a lung function test going to add?

Quite a lot, as it turns out.

Spirometry doesn't diagnose lung cancer. It can't see a tumour or tell you whether cancer has spread. But once lung cancer is in the picture, lung function testing becomes one of the more useful tools your clinical team has. It helps plan treatment. It informs decisions about surgery. It identifies other conditions that frequently travel alongside lung cancer. And it turns out to have a real bearing on prognosis.

What Spirometry Actually Measures

Spirometry is a lung function test. It's the most commonly used one, and in many ways the most practical.

The basic idea is straightforward. You breathe in as deeply as you can, then blow out as hard and fast as you can for several seconds. The machine records what happens.

There are three measurements that matter most.

• FVC, or forced vital capacity. This is the total volume of air you can forcibly breathe out from full lungs. Think of it as a measure of lung capacity. How much is in there?
• FEV1, or forced expiratory volume in one second. This is the volume of air you can push out in the first second of that manoeuvre. It's a measure of how freely air moves through your airways. Narrow or obstructed airways slow that flow down.
• The FEV1/FVC ratio. This is the relationship between the two. A low ratio, meaning your FEV1 is disproportionately reduced compared to your total lung capacity, is the hallmark of obstructive lung disease. Conditions like COPD and emphysema show this pattern. The airways are narrowed, and air takes longer to move through them.

Spirometry sits within a broader family of pulmonary function tests. Other tests measure different aspects of lung performance, including how efficiently oxygen crosses from the lungs into the bloodstream. But spirometry is typically the starting point, and for most purposes in lung cancer care, it does much of the heavy lifting.

It's a simple, non-invasive test. No injections. No radiation. You breathe into a mouthpiece, and the machine does the rest. The main requirement is effort.

Why Someone with Lung Cancer Might Need Spirometry

There are several distinct reasons, and they're worth separating because they tend to come up at different points in the journey.

Establishing a baseline

Before any treatment begins, it helps to know where lung function stands. Many patients with lung cancer, particularly those with a smoking history, already have some degree of compromise in their breathing before the cancer is even found. Having a clear baseline means that any changes during or after treatment can be properly interpreted.

Assessing whether surgery is safe

If surgery is being considered to remove a tumour, the clinical team needs to know how much lung function the patient has, and critically, how much they can afford to lose. This is one of the most important uses of spirometry in lung cancer, and it gets its own section below.

Planning radiotherapy

Radiation to the chest can affect surrounding lung tissue. Understanding pre-treatment lung function helps calibrate the approach and makes it possible to monitor for any changes that develop as a result of treatment.

Identifying COPD

A significant proportion of lung cancer patients also have chronic obstructive pulmonary disease, often without having been formally diagnosed. Spirometry is the only way to confirm it. And as the research makes clear, the presence and severity of COPD at the time of lung cancer diagnosis have an independent bearing on outcomes.

Monitoring during and after treatment

Certain treatments can affect lung function over time. Spirometry gives the team an objective way to track those changes before they become symptomatic.

Prognostic assessment

This one surprises many patients. Lung function values at the time of diagnosis are meaningful predictors of long-term survival in non-small cell lung cancer. It's not the only factor, but it's a real one, and it's worth understanding why.

Spirometry and Surgery: The Detail That Matters Most

Surgery offers the best chance of cure for early-stage lung cancer, but it means removing part of a lung. The question is whether a patient has enough reserve to tolerate that.

Before any resection, spirometry is used to predict what lung function will look like after the operation. Good pre-operative function substantially reduces risk. When the predicted post-operative value falls below certain thresholds, the risk of serious complications rises, and other treatment approaches may need to be considered instead.

Spirometry is rarely used alone. Cardiopulmonary exercise testing is often done alongside it. But spirometry provides the essential foundation. Without it, the surgical conversation cannot happen properly.

If your consulting oncologist and surgical team tell you they need lung function tests before deciding on surgery, this is precisely why. It isn't a bureaucratic hurdle. The results directly shape what's safe to offer you.

COPD, Lung Cancer and Why the Overlap Matters

Chronic Obstructive Pulmonary Disease (COPD) and lung cancer share the same primary risk factor. They occur together far more often than chance would predict. And yet a substantial number of people with lung cancer have never had their lung function formally tested, meaning their COPD, if present, has never been confirmed.

Spirometry changes that. The specific pattern it identifies, a reduced FEV1/FVC ratio that doesn't fully reverse after a bronchodilator, is the defining feature of obstructive airway disease. Without the test, you're guessing. With it, you know.

Why does this matter in lung cancer? Because the severity of COPD at diagnosis appears to be independently associated with survival. Patients with preserved lung function can typically tolerate a wider range of treatments. Those with significant obstruction face an additional physiological burden, and some options may be restricted when lung reserve is low.

A COPD diagnosis doesn't disqualify anyone from treatment. But identifying it matters. Knowing it's there allows the clinical team to plan accordingly.

Spirometry as a Window Into Prognosis

This is the part of spirometry's role in lung cancer that tends to surprise people most.

Both FEV1 and FVC at diagnosis are significantly associated with overall survival. Lower values tend to mean worse outcomes. Higher values, better ones. And the relationship follows a gradient rather than a simple divide.

The association holds across different cancer stages, which suggests it's capturing something real about underlying physiological reserve, not just reflecting tumour burden.

Why might this be? A few mechanisms seem plausible. Patients with better lung function at diagnosis tend to tolerate treatment better and remain eligible for more options throughout their care. They're more resilient when treatment takes a toll. Reduced lung reserve may also reflect a broader picture of physiological compromise that extends beyond the lungs themselves.

I want to be careful about how this is framed, because it's easy to read this and feel like a number on a report is determining your fate. It isn't. Spirometry values are one input into a complex clinical picture. A lower FEV1 doesn't mean treatment isn't worthwhile. What it means is that the team has more information to work with, and that's generally a good thing. Honest prognostic information is what allows realistic, well-informed decisions to be made.

Spirometry is increasingly discussed as a standard part of risk stratification at diagnosis. Not just for patients being considered for surgery, but across the board.

What to Expect During the Test

Most patients approach spirometry with vague dread, usually because nobody has explained what's involved. It's fairly undramatic.

Before the test, preparation is minimal. You may be asked to avoid certain inhalers beforehand. Avoid a large meal, strenuous exercise, and smoking on the day.

During the test, you'll be seated with a nose clip in place. The technician will ask you to take the deepest breath you can, then blow out as hard and fast as possible for at least six seconds. It requires real effort. The test is repeated two or three times to ensure consistent results.

In some cases, you'll be given a short-acting inhaler and retested after a short wait. This checks whether any airflow limitation reverses with a bronchodilator, which helps distinguish between different lung conditions.

The appointment usually takes thirty to forty-five minutes in total. Blowing that hard, that many times, can feel briefly uncomfortable, and some patients feel a little lightheaded. It passes quickly and isn't harmful.

Understanding Your Results

Results are expressed in two ways: as actual values in litres, and as a percentage of what would be predicted for someone of the same age, sex, and height. That second figure is often more useful in practice. An FEV1 of 80% predicted means your airflow is 80% of what's typical for someone like you.

The FEV1/FVC ratio is the key marker for obstructive lung disease. A ratio below the lower limit of normal indicates obstruction, which can be graded from mild to very severe depending on how far the FEV1 falls below predicted.

The main thing to take away: don't try to interpret your results in isolation. A number without context doesn't tell the full story. Your clinical team will read it alongside your symptoms, history, scans, and everything else.

If you receive your results and don't understand what they mean, ask your lung cancer specialist. It's a completely reasonable question. A clear explanation of what the numbers mean for your particular situation is something you're entitled to.

About Dr James Wilson

Dr James Wilson is a consultant clinical oncologist specialising in lung and skin cancer, based in central London. He offers rapid access consultations for patients seeking diagnosis, treatment, or a second opinion.