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Open Access Research article

Analysis of latent tuberculosis and mycobacterium avium infection data using mixture models

José I Villate12*, Berta Ibáñez3, Valentín Cabriada4, José I Pijoán5, Jorge Taboada1 and Arantza Urkaregi6

Author Affiliations

1 Preventive Medicine Department, Cruces Hospital, Barakaldo, Spain

2 Preventive Medicine and Public Health Department, Basque Country University, Leioa, Spain

3 Basque Foundation for Health Innovation and Research, Sondika, Spain

4 Neumology Department, Cruces Hospital, Barakaldo, Spain

5 Clinical Epidemiology Unit, Cruces Hospital, Barakaldo, Spain

6 Mathematics Department, Science Faculty, Basque Country University, Leioa, Spain

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BMC Public Health 2006, 6:240  doi:10.1186/1471-2458-6-240

Published: 28 September 2006

Abstract

Background

Estimation of the frequency of latent tuberculosis infection (LTBI) is difficult in areas with low tuberculosis infection rates and high exposure to non-tuberculous mycobacteria (NTM), including BCG vaccination. The objective was to assess LTBI and M avium infection and to estimate their probability based on skin tests responses in an infant population from a region with the aforementioned characteristics.

Methods

A population-based tuberculin skin test (TST) and sensitin (M avium) survey was conducted on seven years old infants in Biscay, a province from The Basque Country (Spain). 2268 schoolchildren received sensitin and 5277 TST. Participation rate was 89%. Commonly used estimation methods were compared with a method based on the fit of mixture models using the Expectation Maximization algorithm. Functions estimating the probabilities of LTBI and M avium infection given the observed skin tests responses were developed for vaccinated and unvaccinated children.

Results

LTBI prevalences varied widely according to the estimation method. The mixture model provided prevalences higher than expected although intermediates between those obtained by currently recommended approaches. Exposure to previous BCG vaccine produces an upward shift of an average of about 3 mm on the induration size to attain the same probability of infection.

Conclusion

Our results confirm the commonplace exposure to NTM which effect should be taken into account when performing and assessing tuberculin surveys. The use of mixture analysis under the empirical Bayes framework allows to better estimate the probability of LTBI in settings with presence of other NTM and high BCG-vaccination coverage. An estimation of the average effect of BCG vaccination on TST induration is also provided. These models maximise information coming from classical tuberculin surveys and could be used together with the newly developed blood tests to improve survey's specificity and cost-effectiveness.