What Do FVC and DLCO Mean in Sarcoidosis PFTs?

When you have pulmonary sarcoidosis, breathing tests—specifically pulmonary function tests (PFTs)—are a critical part of your routine care. Your doctor looks closely at two specific numbers: your FVC and your DLCO. Simply put, FVC measures how much air your lungs can hold, and DLCO measures how well your lungs transfer oxygen into your blood ^[1]. Because sarcoidosis can cause inflammation or scarring in the lungs, doctors track these percentages over time ^[2]. A significant drop in these numbers between visits is a key indicator of disease progression, signaling that it may be time to investigate further or adjust your treatment ^[3][4].

Depending on how active your sarcoidosis is, you can expect to repeat these PFTs every 3 to 6 months, or annually if your condition is stable. Tip: Try to have your PFTs done at the same pulmonary lab each time, as different machines and calibrations can cause slight variations in your results.

What is FVC (Forced Vital Capacity)?

FVC stands for Forced Vital Capacity. It measures the total amount of air you can forcefully blow out of your lungs after taking the deepest breath possible.

Think of FVC as a measurement of your lung size or volume. When sarcoidosis causes inflammation (granulomas) or permanent scarring (fibrosis) in the lung tissue, the lungs can become stiff ^[5]. Stiff lungs cannot expand fully to take in as much air, which causes your FVC percentage to go down. FVC is widely considered the most accurate single measurement for tracking how sarcoidosis is affecting your lungs over time ^[1][6].

What is DLCO (Diffusing Capacity)?

DLCO stands for Diffusing Capacity of the Lungs for Carbon Monoxide. While it sounds complicated, it simply measures how efficiently your lungs transfer oxygen from the air you breathe into your bloodstream.

Even if your lungs can hold a normal amount of air (a normal FVC), inflammation or scarring from sarcoidosis can thicken the delicate membranes where oxygen enters the blood. If these membranes are thick or damaged, your DLCO percentage will drop ^[7]. It is also important to know that temporary issues, like anemia (low hemoglobin), can artificially lower your DLCO score, which is why your doctor will often look at routine bloodwork alongside your breathing tests.

If your FVC is normal but your DLCO drops significantly, this can be an early warning sign of pulmonary hypertension (high blood pressure in the blood vessels of the lungs), a known complication of sarcoidosis ^[8][9].

Why Doctors Watch for a 10% to 15% Drop

Doctors do not just look at a single PFT result in isolation; they look at the trend over months or years. Your results are given as a “percent predicted,” comparing your numbers to what is expected for someone of your age, height, sex, and ethnicity.

When looking for disease progression, pulmonologists look for an absolute drop in your percent predicted points ^[3]. For example, if your FVC was 80% predicted last time and is 70% predicted this time, that is a 10-point (or 10%) absolute drop.

• For FVC: A drop of 10% or more (10 points) is considered a clinically significant sign of decline.
• For DLCO: Because this test naturally fluctuates more from day to day, doctors typically look for a drop of 15% or more (15 points) to definitively confirm progression, though they will closely watch any downward trend.

A significant decline may indicate:

• Active Inflammation: Your sarcoidosis may be flaring up.
• Disease Progression: Granulomas may be causing permanent scarring, known as pulmonary fibrosis ^[5][10].
• Need for Treatment Adjustment: Your current medications may not be controlling the disease effectively ^[4].
• Other Complications: A drop in DLCO might prompt your doctor to screen for pulmonary hypertension with an echocardiogram ^[11][12].

Conversely, an increase in your FVC of more than 5% (5 points) is often viewed as a reliable sign that your therapy is working ^[2].

Will I Feel This Drop?

Not always. The reason PFTs are so important is that they can detect lung decline before you feel it in your daily life. Sometimes a 10% drop will cause you to feel noticeably more short of breath or fatigued when climbing stairs, but for others, it happens silently. If you do suddenly feel more breathless or experience a new cough, contact your doctor immediately, even if your next breathing test isn’t scheduled for months.

Next Steps When Your Numbers Drop

If your numbers have dropped significantly, your doctor will likely want to look deeper. They may order a High-Resolution CT (HRCT) scan or a PET scan to see if the decline is caused by active inflammation (which can often be treated) or irreversible scarring ^[13][5]. This helps you and your care team make informed decisions about whether to start, increase, or change your immunosuppressive therapy.