The CLO test is one way of testing for H. pylori infection. A scope is inserted through the mouth into the stomach. This scope is used to look around the stomach to see any obvious signs of damage in the form of ulcers. A biopsy sample is then taken from the inside of the stomach (the mucosa) and then removed. The sample is placed in a liquid to determine if there will be a chemical reaction. If the liquid turns red it is positive. If it stays yellow, it is negative for an infection.
The reason for using the CLO tests is to quickly tell if there is H. pylori infection. If the test shows an infection of H. pylori, treatment can be started. In many cases, doctors may prescribe treatment for the infection even if the results are negative.