What is the FEV1 percentage predicted considered below the lower limit of normal (LLN)?

The percentage of FEV1 (Forced Expiratory Volume in one second) considered below the lower limit of normal (LLN) is typically set at 80% of the predicted value. This threshold is important in the context of evaluating lung function and diagnosing conditions such as obstructive and restrictive lung diseases. A value below 80% indicates a significant deviation from what is statistically expected for a healthy individual of the same age, sex, height, and ethnicity.

When interpreting pulmonary function tests, values below this LLN can warrant further investigation into a patient's respiratory health. The use of 80% as a threshold aligns with established guidelines and clinical practice in assessing airflow limitation and guiding treatment decisions. Recognizing this value helps clinicians differentiate between normal lung function and potential pathological conditions.