About

Pulmonary Function Tests (PFTs) quantify lung mechanics. This calculator uses the Global Lung Function Initiative (GLI) 2012 equations, the global standard for spirometry interpretation. It accounts for age, height, biological sex, and ethnicity to determine the Lower Limit of Normal (LLN). Accurate interpretation distinguishes between obstructive disorders, such as COPD or Asthma, and restrictive disorders, like Pulmonary Fibrosis. The tool calculates the Z-score to normalize results across demographics.

Formulas

The Lower Limit of Normal (LLN) is the 5th percentile of the healthy population distribution. Using the Lambda-Mu-Sigma (LMS) method:

LLN = M × (1 + L × S × (−1.645))^1/L

The Z-Score defines how many standard deviations a value is from the mean:

Z = (Measured / M)^L − 1L × S

Reference Data

Condition	FEV1	FVC	FEV1/FVC Ratio
Normal	≥ LLN	≥ LLN	≥ LLN
Obstructive	Decreased	Normal	Decreased (< 0.70)
Restrictive	Decreased	Decreased	Normal / Increased
Mixed	Decreased	Decreased	Decreased

Frequently Asked Questions

FEV1 (Forced Expiratory Volume in 1 second) measures how much air you can forcefully exhale in the first second of a breath. It is the primary marker for airway obstruction.

The Global Initiative for Chronic Obstructive Lung Disease (GOLD) classifies COPD severity based on FEV1 % Predicted. Stage 1 is Mild (FEV1 ≥ 80%), while Stage 4 is Very Severe (FEV1 < 30%).

Thoracic geometry differs among ethnic groups. GLI 2012 provides specific coefficients for Caucasian, African American, North East Asian, and South East Asian populations to prevent misdiagnosis.

A ratio below the Lower Limit of Normal (often approx 0.70) indicates airflow limitation, the hallmark of obstructive diseases like asthma or COPD.