The citrate test screens a bacterial isolate for the ability to utilize citrate as its carbon and energy source. A positive diagnostic test rests on the generation of alkaline by-products of citrate metabolism. The subsequent increase in the pH of the medium is demonstrated by the color change of a pH indicator. The citrate test is often part of a battery of tests used to identify gram-negative pathogens and environmental isolates. For instance, test kits, such as the API-20E (bioMerieux) and Enterotube II (BD Diagnostics), include citrate utilization medium as one of the diagnostic tests.