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Free Content Identifying ELISPOT and skin test cut-offs for diagnosis of Mycobacterium tuberculosis infection in The Gambia

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SETTING: An urban area, The Gambia.

OBJECTIVE: To identify ELISPOT and PPD skin test cut-offs, targeting sensitivity and specificity equivalence.

DESIGN: Tuberculosis cases >15 years of age and their household contacts underwent ELISPOT, HIV and PPD skin tests. Cases and contacts sleeping in a different house were used to estimate sensitivity and specificity, providing two planes for estimating cut-offs. Specificity was adjusted for infection from previous exposure using a multivariate discrimination algorithm.

RESULTS: The point on the line of intersection of the planes that maximised sensitivity and specificity equivalence occurred at 4 spots (95% confidence interval [CI] 3.5–5, multiplier = 0) for CFP-10 and 5.5 spots (4.5–8, multiplier = 0 for ESAT-6), yielding a sensitivity and specificity of 76% for both antigens. Combining ESAT-6 and CFP-10 using an ‘or’ statement yielded a maximum equivalence sensitivity and specificity of 76.5% at 6 spots for ESAT-6 and 11.5 spots for CFP-10. For the PPD skin test sensitivity and specificity, an equivalence of 78% occurred at 11 mm induration (9–13 mm).

CONCLUSION: An ELISPOT cut-off for ESAT-6 or CFP-10 could be set at 4–8 spot forming units (20–40 spots per million), with little benefit from combining the results. A cut-off of 9–13 mm for the PPD skin test is reasonable when comparing with the ELISPOT.

Keywords: ELISPOT; PPD skin test; cut-off; sensitivity; specificity

Document Type: Regular Paper

Affiliations: Medical Research Council Labs, Bacterial Diseases Programme, Banjul, The Gambia

Publication date: February 1, 2006

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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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