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Free Content Mixture analysis of tuberculin survey data from northern Malawi and critique of the method

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SETTING: Various methods have been used to estimate the prevalence of Mycobacterium tuberculosis infection from tuberculin survey data. All are complicated by prior sensitisation to environmental mycobacteria and bacille Calmette-Guérin (BCG) vaccination. Mixture analysis has recently been proposed as a means of overcoming misclassification and improving infection prevalence estimates.

OBJECTIVE: To compare conventional and mixture model estimates of M. tuberculosis infection prevalence.

DESIGN: Mixture models with two or three univariate normal components were fitted to the results of 53909 tuberculin tests conducted in northern Malawi during 1980–1984. Data were stratified by BCG status, sex and age and corrected for digit preference. Prevalence estimates derived from mixture models were compared with those of conventional methods.

RESULTS: The optimal model was age-dependent, with three- and one-component solutions preferred in younger and older age groups, respectively. In contrast with findings from elsewhere, a component corresponding to BCG vaccination was indistinguishable from that attributable to environmental mycobacterial exposure, and infection prevalence estimates in younger individuals with a BCG scar were inflated, irrespective of the method used.

CONCLUSION: The validity of infection prevalence and incidence estimates based on mixture modelling is probably locale-dependent, and the assumptions underlying mixture models may not realistically reflect underlying immunological processes.

Keywords: BCG vaccine; epidemiology; mathematical model; tuberculin tests; tuberculosis

Document Type: Regular Paper

Affiliations: 1: Department of Pharmacology, University of Liverpool, Liverpool, Merseyside, United Kingdom 2: Department of Infectious Diseases and Tropical Diseases, London School of Hygiene & Tropical Medicine, London, United Kingdom 3: Department of Infectious Diseases and Tropical Diseases, London School of Hygiene & Tropical Medicine, London, United Kingdom; and Modelling and Economics Unit, Health Protection Agency, London, United Kingdom

Publication date: 2006-09-01

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  • The International Journal of Tuberculosis and Lung Disease publishes articles on all aspects of lung health, including public health-related issues such as training programmes, cost-benefit analysis, legislation, epidemiology, intervention studies and health systems research. The IJTLD is dedicated to the continuing education of physicians and health personnel and the dissemination of information on tuberculosis and lung health world-wide.

    Certain IJTLD articles are selected for translation into French, Spanish, Chinese or Russian. They are available on the Union website

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