Prevalence estimates of tuberculosis infection in adults in Denmark: a retrospective nationwide register-based cross-sectional study, 2010 to 2018

Background: Tuberculosis (TB) elimination requires identifying and treating persons with TB infection (TBI). Aim: We estimate the prevalence of positive interferon gamma release assay (IGRA) tests (including TB) and TBI (excluding TB) in Denmark based on TBI screening data from patients with inflammatory bowel disease (IBD) or inflammatory rheumatic disease (IRD). Methods: Using nationwide Danish registries, we included all patients with IBD or IRD with an IGRA test performed between 2010 and 2018. We estimated the prevalence of TBI and positive IGRA with 95% confidence intervals (CI) in adolescents and adults aged 15–64 years after sample weighting adjusting for distortions in the sample from the background population of Denmark for sex, age group and TB incidence rates (IR) in country of birth. Results: In 13,574 patients with IBD or IRD, 12,892 IGRA tests (95.0%) were negative, 461 (3.4%) were positive and 221 (1.6%) were indeterminate, resulting in a weighted TBI prevalence of 3.2% (95% CI: 2.9–3.5) and weighted positive IGRA prevalence of 3.8% (95% CI: 3.5–4.2) among adults aged 15–64 years in the background population of Denmark. Unweighted TBI prevalence increased with age and birthplace in countries with a TB IR higher than 10/100,000 population. Conclusion: Estimated TBI prevalence is low in Denmark. We estimate that 200,000 persons have TBI and thus are at risk of developing TB. Screening for TBI and preventive treatment, especially in persons born in high TB incidence countries or immunosuppressed, are crucial to reduce the risk of and eliminate TB.