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Haemophilus influenzae is not the only species of Haemophilus that can be indole positive. The use of biochemical tests and Haemophilus species was very popular in the 1980's and 1990's when many labs were biotyping H. influenzae and H. parainfluenzae strains. These two species can be divided into various biotypes (I thru VIII) using the biochemical tests of indole production, urease, and ornithine decarboxylase.
With H. influenzae, Biotypes I, II, V, and VII are all indole positive -- so 4 other biotypes of H. influenzae are indole negative. With H. parainfluenzae, Biotypes IV, VI, VII, and VIII are also all indole positive -- so I'm afraid your idea for using this single biochemical test as an indicator for laboratory confirmation of H. influenzae would be a mistake.
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