A diagnostic system, method, and a computer-readable storage medium for determining a severity of a gastric condition, such as gastritis, in a subject are disclosed. The diagnostic system includes a processor that obtains an image of a stomach of the subject collected by an endoscope. The processor can compare the subject image with reference normal and abnormal stomach images. If the subject image is abnormal, the processor can generate a subject abnormality image indicative of the differences between the subject image and the normal image. By comparing the generated subject abnormality image with reference abnormality images representative of different severity levels of gastritis, the processor can diagnose the subject as having a particular severity level of gastritis.