“The Global Initiative for Chronic Obstructive Lung Disease (GOLD) is a program started by the National Heart, Lung, and Blood Institute (NHLBI) and the World Health Organization (WHO). Countries around the world use its guidelines and recommendations for diagnosing and treating COPD.
The GOLD system previously classified COPD from stages 1 to 4 solely based on your lung capacity measured by a spirometry test. But the newest GOLD guidelines combine your spirometry results with subjective measures of your symptom severity to determine your level of risk based on their ABCD rubric.
A spirometer test involves exhaling into a machine called a spirometer. The machine measures how hard and quickly you can exhale.
Your doctor examines two numbers in your results: your force vital capacity (FVC) and your force expiratory volume in one second (FEV1). Your FVC is the total amount of air you can breathe out and FEV1 is the amount you breathe out in one second. A ratio of your FEV1/FVC needs to be less than 0.7 for a diagnosis of COPD.
Doctors grade your spirometry results from grade 1 (least severe) to grade 4 (most severe). In the old system, these grades correlate with stage 1 to stage 4 COPD.”
Healthline (4 Stages of COPD: Symptoms, Treatment, Outlook)