Sometimes doctors want to find out what exactly is in your stomach, and to do that must take a sample of its contents. Pumping is sometimes used as a way to remove some of whatever's in a person's stomach for testing. For example, your doctor might want a sample of your stomach acid to test for ulcer-causing bacteria. Also, if your doctor suspects a blockage in your small intestine, he might want to test to see if any of the intestine's contents are backing up into the stomach [source: Dugdale]. In these cases, gastric lavage might be used to obtain the sample.
Because lavaging involves pumping liquid into the stomach and then removing it, doctors try to use the first sample removed from the stomach, otherwise the liquid pumped into the stomach could dilute the sample too much to be effective [source: Dart].