Neutropenia is a risk factor for gram-negative bacillus bacteremia in human immunodeficiency virus-infected patients: results of a nested case-control study.

A previous cohort study demonstrated a relation between neutropenia and bacteremia due to gram-negative bacilli among patients infected with human immunodeficiency virus (HIV). To explore further the relation between neutropenia and bacteremia due to Escherichia coli, Klebsiella pneumoniae, or Pseudomonas aeruginosa among HIV-infected patients, controlling for confounding factors, the authors conducted a nested case-control study with matching and risk-set sampling of controls. The cohort included 1,645 HIV-infected patients followed at the University of California, San Diego, Medical Center in San Diego, California, between 1991 and 1995. Absolute neutrophil count (ANC) was summarized as mean ANC during the 7-day interval preceding the index date of bacteremia. Covariates were ascertained by medical record review. The matching ratio was 6:1 (controls:cases). Odds ratios were estimated using conditional logistic regression. Forty-four incident cases of bacteremia were identified. After adjustment for covariates, the estimated odds ratio for the effect of neutropenia (ANC=500 vs. >500/microl) during the 7 days preceding the index date was 8.1 (95% confidence interval confidence interval 1.5-43.1). The rate of bacteremia due to E. coli, K. pneumoniae, or P. aeruginosa is increased eightfold if the average current-week ANC is less than or equal to 500/microl compared with more than 500/microl.