You sit in a small booth, pinch your nose shut, and blow into a tube as hard and as long as you can. A technician coaches you: keep going, keep going, keep going. Then a printout appears, crowded with acronyms and percentages. FEV1. FVC. Predicted. 78%. What does any of this actually mean?

Spirometry is the foundational test of pulmonary function, used to diagnose asthma, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, and dozens of other conditions. Yet the numbers it produces are rarely self-explanatory. They describe volumes and flow rates that only make sense when compared against what your lungs should be able to do.

The test measures something deceptively simple—how much air you can move and how quickly—but its interpretation rests on a framework of ratios, predicted values, and pattern recognition. Understanding that framework transforms a confusing report into a readable map of how your lungs are functioning.

When you perform a spirometry maneuver, two core measurements emerge. FVC, or forced vital capacity, is the total volume of air you can exhale after taking the deepest breath possible. Think of it as the size of your usable lung reservoir, measured in liters.

FEV1, or forced expiratory volume in one second, is the amount of air you expel during the first second of that forceful exhalation. While FVC tells you how much air your lungs can hold and release, FEV1 tells you how quickly that air can move out.

The distinction matters because lungs can fail in two fundamentally different ways. They can fail to hold enough air, or they can hold air adequately but struggle to release it quickly. A healthy young adult might have an FVC of around 4 to 5 liters and exhale roughly 80% of that within the first second.

The spirometer also generates a flow-volume loop, a graphical curve that plots airflow against lung volume. The shape of this curve—whether it peaks sharply and descends smoothly, or sags in a characteristic scooped pattern—often reveals information that the numbers alone cannot.

Takeaway

Volume and flow are two independent dimensions of lung function. Your lungs can be large but slow, or small but efficient—and the clinical meaning of each is entirely different.

The most important number on your spirometry report isn't FEV1 or FVC individually—it's the ratio between them. The FEV1/FVC ratio is the diagnostic fulcrum that separates two entirely different categories of lung disease.

In an obstructive pattern, something is narrowing or blocking the airways. Asthma inflames them, COPD damages them, bronchiectasis scars them. Air can get in during a slow, relaxed breath, but when you try to force it out quickly, the narrowed passages create a bottleneck. FEV1 drops disproportionately compared to FVC, pushing the ratio below 0.70. You have plenty of air—you just can't release it fast enough.

In a restrictive pattern, the lungs themselves can't expand fully. Pulmonary fibrosis stiffens the tissue, obesity compresses the chest wall, neuromuscular disease weakens the breathing muscles. Both FEV1 and FVC drop together, but their ratio is preserved or even elevated. The lungs work efficiently within their reduced capacity—there's just less capacity to work with.

Mixed patterns exist, and distinguishing pure restriction requires additional testing like lung volume measurements. But the obstruction-versus-restriction framework remains the first and most decisive interpretive step, shaping every diagnosis and treatment decision that follows.

Takeaway

Lungs fail in two opposite ways: they lose their speed, or they lose their space. The FEV1/FVC ratio is the single number that tells you which.

Your FVC of 3.8 liters means nothing in isolation. A 3.8-liter FVC is excellent for a 65-year-old woman of average height, average for a 40-year-old man, and alarming for a 25-year-old athlete. This is why spirometry reports express results as a percentage of predicted.

Predicted values are generated from reference equations built on population studies. The inputs typically include age, height, sex, and—historically—ethnicity. Lung capacity naturally declines with age, scales with body size, and differs modestly between sexes. These equations estimate what a healthy person matching your profile would produce.

The use of ethnicity-specific correction factors has become contentious. Recent research suggests that race-based adjustments may reflect historical inequities in the reference populations rather than true biological differences, potentially underdiagnosing disease in certain groups. Many professional societies now recommend race-neutral equations, though the transition is ongoing.

A result above 80% of predicted is generally considered normal, though the threshold varies by measurement and context. More importantly, a single measurement captures one moment. The real diagnostic power of spirometry emerges from trends—how your numbers change over months and years, and how they respond to treatments like bronchodilators.

Takeaway

A number becomes meaningful only when compared to the right reference. Your results aren't judged against perfection, but against what your body should reasonably be capable of producing.

Spirometry translates breath into data, but the data only speaks when you know the grammar. Volume and flow describe capacity and speed. The FEV1/FVC ratio distinguishes obstruction from restriction. Percentage of predicted places your numbers in the context of who you are.

None of these measurements diagnose disease on their own. They suggest patterns, raise questions, and invite follow-up. A borderline result in a symptomatic patient means something very different from the same result in someone breathing comfortably.

The next time you see a spirometry report, read it as a dialogue between your lungs and a statistical model of what lungs like yours should do. The gap between the two is where the clinical story begins.