Chronic obstructive pulmonary disease is measured in four stages using the Global Initiative for Obstructive Lung Disease classifications, according to WebMD. Pulmonary function tests are used to measure the patient's airflow limitation, and the results indicate in which of the four stages of COPD the patient is classified.
Pulmonary function tests measure the patient's forced expiratory volume in a one-second time frame. The forced vital capacity, which registers the total breath exhaled by the patient, is also measured. Both results are measured in liters.
The forced expiratory volume is at least 70 percent of the forced vital capacity in individuals with normal lung function, notes WebMD. Forced expiratory volume typically declines as COPD progresses. The four stages of COPD, as measured using the Global Initiative for Obstructive Lung Disease classifications, include mild, moderate, severe and very severe.
The forced expiratory volume in patients with mild COPD measures at least 80 percent of normal, states WebMD. Moderate COPD patients have forced expiratory volume between 50 percent and 79 percent of normal. The range for severe COPD patients is between 30 percent and 49 percent of normal. Individuals with severe COPD have forced expiratory volume of less than 30 percent of normal, or less than 50 percent of normal with chronic respiratory failure.
Symptoms of COPD include wheezing, fatigue, chronic cough, shortness of breath and frequent respiratory infections, reports Healthline. Advanced symptoms of COPD include swollen feet and ankles and morning headaches.