Serotype and ampicillin susceptibility of Haemophilus influenzae causing systemic infections in children: 3 years of experience.

Over a 3-year period, 96% of systemic infections in children caused by Haemophilus influenzae were of serotype b. Of 346 invasive infections, 15 (4%) were caused by non-type b H. influenzae. The monthly prevalence of ampicillin resistance in all isolates was highly variable (0 to 63%). Ampicillin resistance in H. influenzae causing invasive disease occurred in 13% of non-type b and 21.8% of type b isolates. There was no significant difference (x2 - 0.21; p greater than 0.10) in the rate of ampicillin resistance between type b and non-type b H. influenzae causing systemic illness in children over a 3-year period.