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The outcome of a cohort of tuberculin-positive predominantly South Asian new entrants aged 16-34 to the UK: Blackburn 1989-2001.

SETTING
Blackburn, Hyndburn and Ribble Valley Local Government areas of England and Wales, the former a high tuberculosis (TB) prevalence district.


BACKGROUND
The incidence of tuberculosis in new entrants aged 16-34 with positive tuberculin skin tests but normal chest X-rays after initial entry is not definitely known, and was previously estimated from cross-sectional national surveys and derived data for the 2006 and 2011 NICE economic appraisals of new entrant TB screening.


METHODS
New entrants aged 16-34 years predominantly from South Asia (India, Pakistan and Bangladesh), with tuberculin tests inappropriately positive for their BCG history were identified for the years 1989-2001 inclusive from a new entrant database. These entrants were compared with the current GP registration database to see if local residence could be confirmed and the local TB notification database to October 2008. Survival analysis was carried out using Kaplan-Meier survival curves and a Cox Regression model.


RESULTS
Four hundred and seventy-nine such new entrants with normal initial chest X-rays were identified. Of these 402 (84%) registered with a General Practitioner in East Lancashire for a period of time and could be followed up by this study. The crude incidence density of active TB amongst these individuals with latent disease was 1297 per 100 000 person-years (95% CI; 991-1698 per 100 000 person-years). After 10 and 15 years of follow-up 13.5 and 16.3% of individuals, respectively, had progressed on to active disease.


CONCLUSION
This patient-derived, rather than estimated, data shows a minimum risk of TB disease of 16.3% at 15 years. The 2006 NICE economic appraisal, suggested that treatment for latent TB infection (LTBI) was cost-effective when the incidence of clinical TB over 15 years surpassed 18% in these populations. The 2011 NICE economic appraisal reduced this to 12% active TB over 15 years, and showed that at 16% active TB over 15 years a single interferon gamma release assay was the most cost-effective strategy. Further cohort studies are urgently needed to confirm or revise the assumptions behind the 2011 NICE economic appraisal.