The impact of opioid agonist treatment on hospitalisations for injecting-related diseases: a retrospective data linkage study

Background: Opioid agonist treatment (OAT) has demonstrated to be protective against drug-related mortality. Among people who inject drugs, this is partially due to less frequent injecting while in treatment. Injecting-related diseases (IRD) are expensive contributors to morbidity in this population and highly correlated with frequent injecting. The aim of this study was to determine the impact of OAT engagement on hospitalisations for IRD.

Methods: We conducted a retrospective state-wide cohort study using linked administrative data. The cohort included 47 163 individuals entering OAT between 1 August 2001 and 31 December 2017 in New South Wales, Australia, with 454 951 person-years (PY) of follow-up information. Outcomes were incident hospitalisations for IRD, including (ICD-10) skin and soft tissue infections, endocarditis, sepsis, osteomyelitis, septic arthritis, venous infections/diseases, and other bacterial diseases, as principal or secondary diagnosis. The primary exposure was OAT status, categorised as either (a) out of OAT; (b) OAT induction (i.e., the first four weeks); or (c) OAT retention (i.e., the remainder of time on treatment). Covariates included year, demographic characteristics, and recent hospitalisations.

Results: Of the 47,163 patients in the cohort, 8349 (17.7%) presented with an IRD hospitalisation in the study period. Compared to time out of treatment, retention on OAT was associated with a reduced risk of IRD (adj rate ratio=0.92;95CI confidence intervals [CI]=0.87-0.97;p=0.03). The age-adjusted incidence rates of hospitalisations increased from 34.8 (95%CI=30.2-40.0) per 1000 person years in 2001 to 54.9 (95%CI=51.3-58.8) per 1000 person years in 2017.

Conclusion: OAT retention is associated with reduced hospitalisations for IRD, particularly skin and soft tissue infections, sepsis, septic arthritis, and other bacterial diseases. Our findings also suggest an increase in these hospitalisations over time that warrants further investigation.