Introduction The background rate of latent tuberculosis infection (LTBI) in low prevalence regions of the UK is unknown. Interferon γ release assays (IGRAs) are sensitive and specific methods for detecting LTBI, and have accurately characterised the epidemiology of LTBI among high risk populations such as recent TB contacts or immigrants. However there are no current data on the incidence of IGRA positivity among the general adult population in the UK. Such data would be valuable for interpreting the significance of a positive IGRA result, and guiding cost-benefit analyses of new diagnostics.
Methods A TB outbreak occurred within a rural DGH. 481 individuals were identified as potential contacts and were tested by IGRA (TSpot.TB). Uniquely, for comparison, we recruited an additional large cohort of age matched controls from the same general wards but with no exposure to the outbreak.
Results 456 staff and patients were tested including 148 unexposed age-matched patient controls. Rates of positivity were 22% (95%CI, 14 to 29), 11% (6.1 to 16), 8.8% (4.2 to 13) and 9.5% (3.0 to 22) among exposed patients, exposed staff, unexposed patients and unexposed staff respectively. 8 cases of active TB (identical VNTR profile) and an estimated 35 cases of recently acquired LTBI can be attributed to exposure to the index case, out of 481 contacts. Characteristics of the unexposed controls are in Abstract S40 table 1. IGRA positivity was associated in multivariate analyses with history of previous TB treatment (OR 11, p=0.04) and use of corticosteroids (OR 5.9, p=0.02), but not with age. The age specific prevalences of IGRA positivity were 0 (N/A) for ages <40, 15.3% (12.2 to 29.4) for ages 40–59, 7.0% (0.92 to 13%) for age 60–79, and 10% (5.9 to 19) for ages =80.
Conclusions We observed a surprisingly high background rate of IGRA positivity among an unselected population typical of respiratory and general medical inpatients in a rural DGH. All controls were white-Caucasians, who comprise 92% of the UK population, and may represent a current minimum UK background rate. As rates were highest in the 5th and 6th decade, in the context of ageing populations and increasing iatrogenic immunosuppression, reactivation of LTBI may be a persistent hazard for several decades to come.