Differentiation of Haemophilus spp. in Respiratory isolate cultures by an indole spot test

Abstract

Indole spot tests using isolated, nonhemolytic colonies of Haemophilus species were positive for 90 of 151 (60%) respiratory isolates of Haemophilus influenzae, whereas 67 to 72 (93%) isolates of H. influenzae from cerebrospinal fluid and blood specimens were indole positive. Only 4 of 117 (3%) Haemophilus parainfluenzae isolates were positive for indole spot tests. Thus, indole-positive, nonhemolytic Haemophilus isolates in respiratory cultures can be presumptively identified as H. influenzae.