The stomach acid test assesses the ability of the stomach's parietal cells to release acid. Aspirating liquids through a tube put down the esophagus into the stomach is used to perform the test. The results can help doctors understand how well your body is producing acid and whether any damage is occurring from lack of acid or excess acid. Many things can cause differences in gastric acid levels including food preferences, diet, medications, etc. People with high acid levels may have problems digesting protein because their digestive enzymes are destroyed by the acid. People with low acid levels may develop precancerous lesions or cancer of the stomach or intestines. The test requires that you first swallow a capsule that contains an alkaline substance called sodium biphosphate. This swells up in the stomach like a balloon and creates a large volume that can be measured using ultrasound. The amount of time it takes for the capsule to pass through the stomach provides information about its rate of movement and the strength of the acid therein. A weak acid environment will allow the capsule to pass through more quickly while a stronger acid environment will keep the capsule in the stomach longer. People who are unable to swallow capsules may receive a direct injection of fluorescent markers into their stomachs instead.