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

Int J Tuberc Lung Dis. 2006 Feb;10(2):192-8.


Setting: An urban area, The Gambia.

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

Design: Tuberculosis cases >5 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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Antibodies, Bacterial / analysis*
  • Diagnosis, Differential
  • Enzyme-Linked Immunosorbent Assay / methods
  • Gambia / epidemiology
  • Humans
  • Mycobacterium tuberculosis / immunology*
  • Prevalence
  • Retrospective Studies
  • Sensitivity and Specificity
  • Tuberculin Test / methods
  • Tuberculosis / diagnosis*
  • Tuberculosis / epidemiology
  • Tuberculosis / microbiology
  • Urban Population


  • Antibodies, Bacterial