Pepsinogen is the precursor of pepsin, which is an organic substance secreted by the gastric mucosa before the formation of pepsin. In the stomach, pepsinogen becomes pepsin when it interacts with stomach acid or pepsin already formed. Pepsin is an important substance involved in digestion in the stomach and is mainly responsible for breaking down proteins into smaller components such as fat and peptides. Pepsinogen is an important indicator for the four tests of gastric function. Pepsinogen I reflects the health of the gastric fundus and gastric mucosal function; Pepsinogen II reflects the state and function of the gastric sinus mucosa; and the Pepsinogen Ratio is used to enhance the sensitivity of the Pepsinogen I and Pepsinogen II tests. These results can be used to detect the function and damage of the gastric mucosa, screen for chronic gastritis, gastric ulcers, and detect tumors and gastric cancer.