LLN stands for Lower Limit of Normal, and it represents the lowest lung function value that’s still considered healthy for someone of your age, sex, height, and ethnicity. In spirometry, any measured value that falls below the LLN is classified as abnormal. Statistically, the LLN is set at the 5th percentile of a healthy, non-smoking population, meaning 95% of healthy people score above it.
How the LLN Is Calculated
The LLN isn’t a single number that applies to everyone. It’s personalized. Reference equations take your age, height, sex, and ethnic background as inputs and produce a predicted value for each spirometry measurement, along with a range of normal variation around that prediction. The bottom boundary of that range is the LLN.
In statistical terms, the LLN sits at negative 1.64 standard deviations below the predicted value (a z-score of −1.64). If your result falls below that line, it’s considered outside the normal range. The formula is straightforward: LLN = predicted value minus 1.645 multiplied by the residual standard deviation. But in practice, modern software handles this automatically using reference equations built into the spirometry system.
The current gold standard is the GLI 2012 reference equations, developed by the Global Lung Function Initiative and endorsed by both the American Thoracic Society (ATS) and the European Respiratory Society (ERS). These equations cover ages 3 to 95 and account for four major ethnic groups: Caucasian, African American, Northeast Asian, and Southeast Asian. For people of mixed or other ethnic backgrounds, the equations use an average of all four groups. Importantly, the scatter around predicted values changes with age, so the LLN isn’t a fixed distance from the predicted value. It shifts depending on how old you are and which measurement is being assessed.
What the LLN Applies To
Spirometry produces several key numbers. The two most important are FVC (the total amount of air you can forcefully exhale) and FEV1 (how much of that air comes out in the first second). The ratio of FEV1 to FVC tells clinicians whether your airways are obstructed. Each of these measurements has its own LLN, calculated separately based on your personal characteristics. Lower limits have also been established for less common ratios like FEV1/FEV6 and FEV3/FVC.
Z-Scores and Severity Grading
Your spirometry report may include a z-score alongside each measurement. The z-score tells you how far your result sits from the predicted value for a healthy person like you, measured in standard deviations. A z-score of zero means your result matches the predicted average exactly. A z-score below −1.64 means you’ve crossed below the LLN and the result is abnormal.
Z-scores also help grade severity:
- Mild abnormality: z-score between −1.65 and −2.5
- Moderate abnormality: z-score between −2.51 and −4.0
- Severe abnormality: z-score below −4.0
This grading system replaces older approaches that used “percent predicted” (your result as a percentage of the predicted value), which didn’t account for how much natural variation exists at different ages. A 70-year-old and a 30-year-old might both blow 80% of predicted, but the clinical meaning differs because the normal range of variation is wider in older adults.
LLN vs. the Fixed Ratio for COPD Diagnosis
One of the biggest debates in lung function testing is how to diagnose COPD. The traditional approach uses a fixed cutoff: if your FEV1/FVC ratio falls below 0.70 (meaning less than 70% of the air you exhale comes out in the first second), you’re diagnosed with airflow obstruction. The alternative approach uses the LLN for that ratio instead of 0.70.
The problem with the fixed 0.70 cutoff is that the FEV1/FVC ratio naturally declines with age. A healthy 75-year-old may have a ratio below 0.70 simply because of normal aging, not disease. This can lead to overdiagnosis in older adults. The LLN adjusts for age, so it avoids flagging normal age-related decline as disease. Conversely, the fixed ratio can miss obstruction in younger adults whose ratio is still above 0.70 but has dropped below their age-appropriate LLN.
However, the clinical picture is more nuanced than “LLN is always better.” A long-term follow-up study of 424 patients diagnosed with airflow obstruction by the fixed ratio found that 135 of them (about 32%) would not have been diagnosed using the LLN. These “discordant” patients, flagged only by the fixed ratio, actually lost lung function faster over time, and 81% of them eventually crossed below the LLN as well. Patients who met both criteria had nearly three times the risk of dying from COPD compared to those caught only by the fixed ratio, and they were hospitalized for flare-ups more often. But the study’s authors argued that using the LLN alone could delay diagnosis in milder cases, potentially missing a window for early intervention like aggressive smoking cessation.
In practice, many guidelines (including GOLD, the major international COPD initiative) still use the fixed 0.70 ratio for initial screening, while the ATS and ERS recommend reporting the LLN alongside it. Your clinician may consider both numbers when interpreting your results.
Why Your Personal Characteristics Matter
Lung size and function vary enormously across the population. A tall 25-year-old man has a much larger predicted FVC than a short 70-year-old woman, so the same raw number could be perfectly normal for one person and seriously abnormal for another. The LLN accounts for this by building in four core variables: age, height, sex, and ethnicity. Some newer reference equations also factor in weight, which can independently affect lung function, particularly in populations where existing equations perform poorly.
The GLI 2012 equations use a sophisticated statistical method called LMS modeling, which simultaneously adjusts for how the average, the spread of variation, and the shape of the distribution all change across the age range. This means the LLN curves smoothly from childhood through old age rather than jumping at arbitrary age cutoffs. It’s one reason the 2019 ATS/ERS spirometry standards recommend GLI 2012 as the default reference set.
Reading Your Spirometry Report
A typical spirometry report lists your measured values, the predicted values for someone matching your profile, the LLN for each measurement, and often a z-score. The key things to look at are whether your FEV1, FVC, and FEV1/FVC ratio fall above or below their respective LLNs. If your FEV1/FVC ratio is below the LLN, that suggests airflow obstruction, the hallmark of conditions like COPD and asthma. If your FVC is below the LLN but the ratio is normal, that pattern points toward restriction, where the lungs can’t fully expand.
Some reports still display results as “percent predicted,” showing your value as a percentage of the predicted average. While this is intuitive, it can be misleading because it doesn’t tell you where the boundary of normal actually falls. Two people might both blow 78% of predicted, but one could be within normal limits and the other below the LLN, depending on their age and the natural variability for their demographic group. The z-score and LLN give you a more reliable picture of whether your result is truly outside the healthy range.