Mycobacterium tuberculosis is a strongly nitrate positive organism. This test is valuable for the identification of M. tuberculosis, M. kansasii, M. szulgai and M. fortuitum. Rapid growers such as M. fortuitum can be tested within 2 weeks, but slow growers should be tested after 3-4 weeks of luxuriant growth. Both chemical procedure and commercially available nitrate strips are available. Control strains must be used while performing and interpreting the result.