Septic Arthritis and Its Mimics: Assessing the Incidence, Mortality Rate, Cause-of Death and Preventability in a Population-Based Cohort Study.

Background: Septic arthritis (SA) is considered the most harmful etiology in patients presenting with an acutely painful swollen knee. This study aimed to estimate the incidence, mortality rate, and cause of death among patients diagnosed with SA and compare it to non-infectious causes. Methods: We conducted a population-based observational cohort study in Skåne, southern Sweden, with a population of 1.4 million people. We included adults in the years 2020 and 2021 with cultures obtained at the emergency department from a native knee, where SA was considered a differential diagnosis based on medical chart review. Data on demographics, comorbidities, microbiological etiology, mortality, and treatment were collected. Results: Out of 709 knee joint cultures, 60 (8%) cultures were considered SA. The median age of included patients was 71 years (range 19-97) and 67% were men. The incidence of SA was 2.7 cases per 100,000 person-years. Gout and pyrophosphate arthritis were more prevalent than SA (5.7/100,000 person-years and 6.1/100,000 person-years, respectively. The 90-day all-cause mortality rate for SA was 8% (4/51), compared to mortality rates of 4% (5/126) for gout arthritis and 4% (5/139) for pyrophosphate arthritis, respectively. Possibly one death within 90-days was considered directly associated to SA (2%). No deaths were considered preventable due to inadequate antimicrobial therapy or management. Conclusion: We observed a low incidence of native knee SA, with gout and pyrophosphate arthritis occurring twice as frequently. In our cohort, the infection-related mortality from SA was low and considered non-preventable.