Elsevier

The Lancet

Volume 361, Issue 9364, 5 April 2003, Pages 1168-1173
The Lancet

Fast track — Articles
Comparison of T-cell-based assay with tuberculin skin test for diagnosis of Mycobacterium tuberculosis infection in a school tuberculosis outbreak

https://doi.org/10.1016/S0140-6736(03)12950-9Get rights and content

Summary

Background

The diagnosis of latent tuberculosis infection relies on the tuberculin skin test (TST), which has many drawbacks. However, to find out whether new tests are better than TST is difficult because of the lack of a gold standard test for latent infection. We developed and assessed a sensitive enzyme-linked immunospot (ELISPOT) assay to detect T cells specific for Mycobacterium tuberculosis antigens that are absent from Mycobacterium bovis BCG and most environmental mycobacteria. We postulated that if the ELISPOT is a more accurate test of latent infection than TST, it should correlate better with degree of exposure to M tuberculosis.

Methods

A large tuberculosis outbreak in a UK school resulted from one infectious index case. We tested 535 students for M tuberculosis infection with TST and ELISPOT. We compared the correlation of these tests with degree of exposure to the index case and BCG vaccination.

Findings

Although agreement between the tests was high (89% concordance, κ=0·72, p<0·0001), ELISPOT correlated significantly more closely with M tuberculosis exposure than did TST on the basis of measures of proximity (p=0·03) and duration of exposure (p=0·007) to the index case. TST was significantly more likely to be positive in BCG-vaccinated than in non-vaccinated students (p=0·002), whereas ELISPOT results were not associated with BCG vaccination (p=0·44).

Interpretation

ELISPOT offers a more accurate approach than TST for identification of individuals who have latent tuberculosis infection and could improve tuberculosis control by more precise targeting of preventive treatment.

Introduction

Identification and treatment of people who have latent tuberculosis infection by targeted tuberculin skin testing and preventive therapy is a cornerstone of tuberculosis control in developed countries.1 The main drawback of the tuberculin skin test (TST) is poor specificity, since previous Mycobacterium bovis BCG vaccination and environmental mycobacterial exposure can lead to false-positive results.2, 3, 4 More than half the burden of tuberculosis in developed countries is carried by foreign-born immigrants from high-prevalence countries, among whom BCG vaccination and environmental mycobacterial exposure are common.5, 6 The TST also has several operational drawbacks, including the need for a return visit and operator-dependent variability in placement and reading of the test. A more accurate rapid test for latent infection is a major priority for improved tuberculosis control.7

The identification of genes in the M tuberculosis genome that are absent from M bovis BCG8 and most environmental mycobacteria9 offers an opportunity to develop more specific tests for M tuberculosis infection.10 Early secretory antigen target-6 (ESAT-6) and culture filtrate protein 10 (CFP10) are two such gene products that are strong targets of the cellular immune response in tuberculosis patients and contacts.11, 12 The presence of ESAT-6-specific T cells, detected by the rapid ex-vivo enzyme-linked immunospot (ELISPOT) assay for interferon-gamma,13 is a highly sensitive and specific marker of M tuberculosis infection in patients who have culture-confirmed tuberculosis; its sensitivity is substantially higher than that for the TST.14, 15 In a UK pilot study of 50 contacts at risk of latent tuberculosis infection, we noted a correlation between ESAT-6 ELISPOT results and the extent of exposure to tuberculosis cases,16 whereas unexposed people were uniformly ELISPOT-negative.17, 18

In February, 2001, a secondary school student who had had a chronic cough for 9 months was diagnosed with sputum-smear-positive cavitatory pulmonary tuberculosis. The health authority screened 1128 of 1208 students at the school with TST and diagnosed 69 secondary cases of active tuberculosis and 254 cases of latent infection. This outbreak presented a unique opportunity to compare the effectiveness of the ELISPOT assay with the TST.

In the absence of a gold standard reference test, direct assessment of the sensitivity and specificity of a new test for latent tuberculosis infection is impossible.4 However, since airborne transmission of M tuberculosis is promoted by increasing duration and proximity of contact with an infectious case,19, 20, 21 a key determinant of infection is the amount of time spent sharing room air with the source 22, 23 We formed the hypothesis that if the ELISPOT assay is a more sensitive and specific test than the TST, it should correlate more closely than the TST with degree of exposure to M tuberculosis and should be independent of BCG vaccination status. Two measures of exposure were prespecified at the time of study design: proximity to the index case, based on school class and year, and hours of direct classroom contact. Three features of this outbreak made it particularly suitable for this investigation: there was one infectious index case with several hundred contacts; the outbreak occurred in an enclosed environment; and school timetables permitted precise quantification of the amount of time each child spent sharing room air with the source case.

Section snippets

Participants

The study was approved by the Leicestershire research ethics committee. We invited 963 students, aged 11–15 years, from the same school as the index case to participate. We obtained written informed consent from 594 (62%) children and their parents. In May and June, 2001, the school nurses interviewed 550 (57% of the total invited) of these children about place of birth and history of tuberculosis exposure outside school. At the same time they drew 10 mL blood samples that were stored in

Results

ELISPOT and TST results were available for 535 students—44·3% of the school. Our sample was representative in terms of the proportion of non-white children (97% in our sample vs 93% in the whole school); UK-born children (86 vs 86%); children diagnosed with active tuberculosis (5 vs 6%); and participants deemed to have latent tuberculosis infection on the basis of TST result (24 vs 23%, table 1).

The odds of a test result being positive for each increase across the four stratified exposure

Discussion

In the absence of a gold standard test for latent tuberculosis infection, the sensitivity and specificity of the ELISPOT assay or the TST cannot be directly quantified.4 However, given that the likelihood of latent tuberculosis infection is determined by exposure to M tuberculosis,19, 20, 21, 22, 23 we were able to rank the tests according to their diagnostic accuracy. Agreement between TST and ELISPOT results was high, but discordance in 11% of students shows that the tests are not equivalent.

References (32)

  • MahairasGG et al.

    Molecular analysis of genetic differences between Mycobacterium bovis BCG and virulent M bovis

    J Bacteriol

    (1996)
  • HarboeM et al.

    Evidence for occurrence of the ESAT-6 protein in Mycobacterium tuberculosis and virulent Mycobacterium bovis and for its absence in Mycobacterium bovis BCG

    Infect Immun

    (1996)
  • LeinAD et al.

    Cellular immune responses to ESAT-6 discriminate between patients with pulmonary disease due to Mycobacterium avium complex and those with pulmonary disease due to Mycobacterium tuberculosis

    Clin Diagn Lab Immunol

    (1999)
  • ArendSM et al.

    Detection of active tuberculosis infection by T cell responses to early-secreted antigenic target 6-kDa protein and culture filtrate protein 10

    J Infect Dis

    (2000)
  • LalvaniA et al.

    Rapid effector function in CD8+ memory T cells

    J Exp Med

    (1997)
  • LalvaniA et al.

    Rapid detection of Mycobacterium tuberculosis infection by enumeration of antigen-specific T cells

    Am J Respir Crit Care Med

    (2001)
  • Cited by (0)

    View full text