Oxygen saturation levels, usually measured through pulse oximetry, are normally 95-100%. In patients with COPD, however, oxygen saturation levels tend to drop below normal, especially when they're exerting themselves. This is due to chronically low levels of oxygen in the blood, otherwise known as hypoxemia. A drop in oxygen saturation levels is called desaturation.
When oxygen saturation levels drop below a certain level as determined by your doctor, further testing should be done to determine the need for supplemental oxygen.