Lower Limit of Normal (LLN)

Last updated: July 28, 2018

What Does Lower Limit of Normal (LLN) Mean?

The lower limit of normal (LLN) within the context of health and wellness, refers to a group who has a drastically reduced lung capacity. This clinical guideline number represents a fifth percentile of a healthy, non-smoking population whose test results fall below the standardized mean value of ninety percent of people comprising a particular demographic.

In spirometry, clinicians measure airflow from the lungs by application of forced expiratory volume per second in which a patient blows out air as forcefully and quickly as possible as an assessable measure of COPD.

The LLN value gives a threshold number when considering test results for COPD. While false positives are possible, the LLN value (when compared to other threshold standards), usually results in more false negatives than positives in COPD testing.


WorkplaceTesting Explains Lower Limit of Normal (LLN)

The lower limit of normal, or LLN, relates to a fifth percentile of a healthy, non-smoking demographic coinciding proportionally to a mass ninety percent of the larger population representing a standardized model of normal airflow capacity of healthy lungs.

When testing for COPD or other respiratory conditions, the forced expiratory volume per second of airflow that a person can expel at a forceful and fast rate reflects several anthropocentric factors including, age, gender, height, and race. These variables contribute widely to clinical analysis in terms of identifying COPD and/or other harmful bronchopulmonary diseases that can develop, targeting distinctive populations whose lung function can be compromised.

Though spirometry is the primary method for diagnosing pulmonary diseases like asthma or COPD, interpreting LLN results comparatively with a normative value system of the greater population can be subject to error. Consequently, people who fall below the LLN cutoff threshold may be misdiagnosed with false positives and those above the threshold may receive false negatives, warranting consistent spirometry testing by a clinician to monitor latent COPD development and its potential progressive nature.


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