Since 1992, chronic gastritis lesions are classified according to the Sydney system. Metaplasia
Mucous gland metaplasia, the reversible replacement of differentiated cells, occurs in the setting of severe damage of the gastric glands, which then waste away (atrophic gastritis) and are progressively replaced by mucous glands. Gastric ulcers may develop; it is unclear if they are the causes or the consequences. Intestinal metaplasia typically begins in response to chronic mucosal injury in the antrum and may extend to the body. Gastric mucosa cells change to resemble intestinal mucosa and may even assume absorptive characteristics. Intestinal metaplasia is classified histologically as complete or incomplete. With complete metaplasia, gastric mucosa is completely transformed into small-bowel mucosa, both histologically and functionally, with the ability to absorb nutrients and secrete peptides. In incomplete metaplasia, the epithelium assumes a histologic appearance closer to that of the large intestine and frequently exhibits dysplasia. Diagnosis
Often, a diagnosis can be made based on  patients description of their symptoms. Other methods which may be used to verify gastritis include:

Blood tests:
Blood cell count
Presence of H. pylori
Liver, kidney, gallbladder, or pancreas functions
Urinalysis
Stool sample, to look for blood in the stool
X-rays
Endoscopy, to check for stomach lining inflammation and mucous erosion
Stomach biopsy, to test for gastritis and other conditionsThe OLGA staging frame of chronic gastritis on histopathology. Atrophy is scored as the percentage of atrophic glands and scored on a four-tiered scale. No atrophy (0%) = score 0; mild atrophy (1–30%) = score 1; moderate atrophy (31–60%) = score 2; 9 severe atrophy (>60%) = score 3.
NSAIDs inhibit cyclooxygenase-1, or COX-1, an enzyme responsible for the biosynthesis of eicosanoids in the stomach, which increases the possibility of peptic ulcers forming. Also, NSAIDs, such as aspirin, reduce a substance that protects the stomach called prostaglandin. These drugs used in a short period are not typically dangerous. However, regular use can lead to gastritis. Additionally, severe physiologic stress ("stress ulcers") from sepsis, hypoxia, trauma, or surgery is also a common etiology for acute erosive gastritis. This form of gastritis can occur in more than 5% of hospitalized patients.Also, note that alcohol consumption does not cause chronic gastritis. It does, however, erode the mucosal lining of the stomach; low doses of alcohol stimulate hydrochloric acid secretion. High doses of alcohol do not stimulate secretion of acid. Chronic
Chronic gastritis refers to a wide range of problems of the gastric tissues. The immune system makes proteins and antibodies that fight infections in the body to maintain a homeostatic condition. In some disorders the body targets the stomach as if it were a foreign protein or pathogen; it makes antibodies against, severely damages, and may even destroy the stomach or its lining. In some cases bile, normally used to aid digestion in the small intestine, will enter through the pyloric valve of the stomach if it has been removed during surgery or does not work properly, also leading to gastritis. Gastritis may also be caused by other medical conditions, including HIV/AIDS, Crohns disease, certain connective tissue disorders, and liver or kidney failure.