Incidence and Predictors of Gram-Negative Bacilli in Hospitalized People Who Inject Drugs with Injection Drug Use-Attributable Infections

The objective of this study was to quantify incidence and determine predictors of Gram-negative bacilli (GNB) in people who inject drugs (PWID) with injection-drug use (IDU)-related infections. The investigation was a retrospective cohort of hospitalized PWID from January 2017 to December 2019. Inclusion criteria were age of ≥18 years, active IDU, treated IDU-attributable infection, and organism growth from microbiology cultures. Infection types included infective endocarditis (IE), acute bacterial skin/skin structure infection (ABSSSI), osteoarticular infection (OAI), and other bloodstream infections (BSI). Primary outcome was GNB identification from microbiologic culture; descriptive statistics were used to describe the cohort. Multivariable regression was used to identify variables associated with GNB infection. A total of 230 PWID were included, 65 (28%) with GNB infections and 165 (72%) with Gram-positive infections. The median (interquartile range [IQR]) population age was 38 (31 to 45) years. Most patients were women (56%); 37% had no insurance. Infection types were as follows: IE, 41%; ABSSSI, 37%; OAI, 20%; and other BSI, 2%. A total of 278 organisms were isolated from 230 patients. The most common organisms were methicillin-resistant Staphylococcus aureus (43%), Streptococcus spp. (19%), methicillin-susceptible S. aureus (17%), and Serratia marcescens (8%); 10% of infections were mixed GNB and Gram-positive infections. A total of 80% of patients received empirical Pseudomonas aeruginosa coverage; only 7% had P. aeruginosa infections. In multivariable regression, age of >50 years (adjusted odds ratio [adjOR], 2.9; 95% confidence interval [CI], 1.2 to 7.2), prior hospitalization within 90 days (adjOR, 2.2; 95% CI, 1.2 to 4.3), and OAI (adjOR, 3.2; 95% CI, 1.5 to 6.6) were associated with GNB infection. GNB in PWID with IDU-attributable infections were more frequently observed in recently hospitalized, older patients with OAI. The majority of patients received empirical antipseudomonal antibiotic coverage, but P. aeruginosa was infrequent. PWID are a potential population to target improved empirical antibiotic use.