Risk of Staphylococcus aureus bacteraemia in patients with rheumatoid arthritis and the effect of orthopaedic implants on the risk: a nationwide observational cohort study

Objective It remains disputed how much the risk of Staphylococcus aureus bacteraemia (SAB) is increased in patients with rheumatoid arthritis (RA), and the extent to which orthopaedic implants explain the risk. We assessed SAB incidence rates (IRs) and incidence rate ratios (IRRs), comparing RA patients with a general population cohort (GPC) and individuals with versus without orthopaedic implants. Method Danish residents aged >= 18 years without prior RA or SAB (=GPC) were followed up for RA and microbiologically verified SAB events (1996-2017). IRRs were calculated by age- and sex-stratified Poisson regression adjusted for age, comorbidities, calendar year, and socioeconomic status. Results The GPC comprised 5 398 690 individuals. We identified 33 567 incident RA patients (=RA cohort) (median follow-up 7.3 years, IQR 3.6-12.3). We observed 25 023 SAB events (n = 224 in the RA cohort). IRs per 100 000 person-years were 81.0 (RA cohort) and 29.9 (GPC). IRs increased with age. Adjusted IRRs in 18-59-year-old RA patients were 2.6 (95% confidence interval 1.8-3.7) for women and 1.8 (1.1-3.1) for men, compared with same sex and age group GPC. IRRs declined with age. Compared with the GPC without implants, IRRs for RA patients with implants ranged from 1.9 (1.3-2.8) (women >= 70 years) to 5.3 (2.2-12.8) (18-59-year-old men). Conclusion In this nationwide registry-based cohort study RA was a risk factor for SAB, and orthopaedic implants further increased the risk. Clinicians should be aware of potential SAB in patients with RA and orthopaedic implants.