The GOLD (Global Initiative for Chronic Obstructive Lung Disease) system is a widely used classification system for the severity of COPD. The GOLD system categorizes COPD into one of four stages — mild, moderate, severe, and very severe — based on spirometry test results.
The two most important spirometry measurements are FEV1 (forced expiratory volume in one second) and FVC (forced vital capacity). FEV1 measures the air that a person can exhale in one second, while FVC measures the total amount of air that a person can exhale forcefully, according to Healthline.
The GOLD system uses the FEV1/FVC ratio to determine the presence of airflow limitation. An FEV1/FVC ratio of less than 0.70 is considered COPD, as it indicates that the airflow going in and out of your lungs is restricted. Based on the severity of airflow limitation, the GOLD system will classify COPD from mild to very severe.