What Do FVC and DLCO Mean on Your ILD PFT Results?

When you receive your pulmonary function test (PFT) results for interstitial lung disease (ILD), the acronyms can look like an alphabet soup. Seeing numbers that are lower than you expect can be frightening, but it helps to know that these numbers are just one piece of the puzzle. The two commonly followed measures on these reports are FVC and DLCO.

By tracking these two numbers over time, your medical team can monitor how your lungs are functioning. However, doctors do not rely on FVC and DLCO alone; they combine them with other tests like Total Lung Capacity (TLC), oxygen levels, CT scans, and how you feel day-to-day to guide your care ^{[1] [2]}.

What is FVC (Forced Vital Capacity)?

FVC stands for Forced Vital Capacity. This is the total amount of air you can forcefully and completely blow out after taking the deepest breath possible ^[3].

In ILD, scarring (fibrosis) or inflammation can make the lung tissue stiffer and less stretchy. Because the lungs cannot expand fully, they hold less air. A lower FVC number can be a clue that your lungs are restricted by the disease ^[3].

However, FVC does not directly measure the exact amount of scarring in your lungs. A low FVC can also be caused by how hard you try during the test, your respiratory muscle strength, airway disease, or other factors like obesity ^{[4] [5]}. To confirm true lung restriction, your doctor will often look at another measurement called Total Lung Capacity (TLC) ^[3].

What is DLCO (Diffusing Capacity for Carbon Monoxide)?

DLCO stands for Diffusing Capacity of the Lungs for Carbon Monoxide. During this part of the test, you inhale a small, carefully measured, safe amount of carbon monoxide and an inert tracer gas, hold your breath for about 10 seconds, and exhale ^[6].

This test estimates gas transfer capacity—how easily gases cross over from the tiny air sacs in your lungs (alveoli) into your bloodstream ^[6]. Because ILD damages the tissue where this gas exchange happens, a lower DLCO often means your lung’s gas transfer is impaired ^[3].

It is important to know that DLCO does not directly measure the oxygen levels in your blood; your oxygen saturation can be completely normal at rest even if your DLCO is low. Furthermore, a low DLCO is not only caused by ILD scarring. It can be heavily influenced by anemia (low red blood cell count), smoking, emphysema, or pulmonary hypertension (high blood pressure in the blood vessels of the lungs) ^{[7] [8]}. Your doctor should ideally look at your “hemoglobin-corrected” DLCO to account for anemia.

Reading Your Report: Percentages and Predictions

Your PFT report will usually show your FVC and DLCO as a percentage of predicted (e.g., “FVC 75% predicted”). This number compares your results to reference equations based on a healthy population of the same age, height, and birth sex ^[5].

Different laboratories sometimes use different reference equations to calculate this percentage. Because of this, it is always best to have your PFTs done at the same laboratory so the trends in your numbers are a true “apples-to-apples” comparison.

How Doctors Use Trends to Guide Treatment

Because ILD is an umbrella term for many different diseases, treatment goals vary. Some inflammatory types of ILD may improve with medication, while fibrotic (scarring) types of ILD are usually treated with antifibrotic drugs that aim to slow down the decline of your lung function, rather than increase your numbers ^[9].

When tracking your disease, doctors look for meaningful changes. Depending on the guideline or specific ILD, doctors use different thresholds as a sign of progression.

• Absolute vs. Relative Change: It is important to understand the difference between absolute and relative drops. If your FVC falls from 80% to 72% predicted, that is an 8 percentage-point absolute decline, but a 10% relative decline (because 8 is 10% of 80).
• Examples of Progression Thresholds: In some fibrotic ILDs, a warning sign of progression is a 10% relative drop in FVC over a year ^[10]. Broad guidelines for progressive pulmonary fibrosis sometimes look for an absolute FVC decline of 5 percentage points, or an absolute DLCO decline of 10 percentage points, combined with worsening symptoms or CT scan changes ^[11].

The Big Picture: Because day-to-day PFT results can fluctuate based on fatigue, recent illness, or testing effort, a single lower number does not automatically mean your disease is worsening ^[12]. Stabilization or a slower decline is often a realistic sign of treatment success ^{[13] [14]}. Never start, stop, or change your medications based on a single PFT number alone.

Safety Note: While gradual changes in your PFTs are tracked over months, sudden changes in how you feel require immediate attention. Seek urgent medical help if you experience severe sudden breathlessness, chest pain, fainting, blue lips, or dangerously low oxygen levels.