Mustafa Bakir, MD; Kerry A. Millington, DPhil; Ahmet Soysal, MD; Jonathan J. Deeks, PhD; Serpil Efee; Yasemin Aslan, SRN; Davinder P.S. Dosanjh, DPhil; Ajit Lalvani, DM
Note: Drs. Bakir and Millington contributed equally to this work.
Acknowledgment: The authors thank the children who took part in the study and their parents. They also acknowledge the crucial support of the Istanbul Association for the Fight Against Tuberculosis and the physicians and nurses of the 7 government-run tuberculosis clinics in the Anatolian side of Istanbul.
Grant Support: By the Wellcome Trust and the United Nations (International) Children's Fund/United Nations Development Programme/World Bank/World Health Organization Special Programme for Research and Training in Tropical Diseases, a UK Department of Health Senior Fellowship in Evidence Synthesis (Dr. Deeks), and the Sir Halley Stewart Trust (Dr. Dosanjh's PhD studentship). Dr. Lalvani is a Wellcome Senior Research Fellow in Clinical Science, and Dr. Millington was a Wellcome Trust Prize PhD student. Both are members of the Wellcome Trust-funded Centre for Respiratory Infection, Imperial College London.
Potential Financial Conflicts of Interest:Consultancies: A. Lalvani (Oxford Immunotec Ltd. [nonexecutive director from 2003 to 2007]). Stock ownership or options (other than mutual funds): A. Lalvani (Oxford Immunotec Ltd.), University of Oxford (Oxford Immunotec Ltd.). Patents received: A. Lalvani (T-cell–based diagnosis of tuberculosis infection.) Patents pending: K.A. Millington (T-cell–based diagnosis of tuberculosis infection.), D.P.S. Dosanjh (T-cell–based diagnosis of tuberculosis infection.), A. Lalvani (T-cell–based diagnosis of tuberculosis infection.)
Reproducible Research Statement:Study protocol: Not available. Statistical code and data set: Available to academic investigators from Dr. Lalvani (e-mail, email@example.com) after agreement is made by written request.
Requests for Single Reprints: Ajit Lalvani, DM, Tuberculosis Immunology Group, Department of Respiratory Medicine, National Heart and Lung Institute, Imperial College London, Norfolk Place, London W2 1PG, United Kingdom; e-mail, firstname.lastname@example.org.
Current Author Addresses: Drs. Bakir and Soysal, Ms. Efee, and Ms. Aslan: Department of Paediatrics, Marmara University School of Medicine, Tophanelioglu Caad. No. 13-15, Altunizade, Istanbul, Turkey.
Drs. Millington, Dosanjh, and Lalvani: Tuberculosis Immunology Group, Department of Respiratory Medicine, National Heart and Lung Institute, Imperial College London, Norfolk Place, London W2 1PG, United Kingdom.
Dr. Deeks: Unit of Public Health, Epidemiology and Biostatistics, School of Population Health and Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
Author Contributions: Conception and design: M. Bakir, A. Lalvani.
Analysis and interpretation of the data: M. Bakir, K.A. Millington, A. Soysal, J.J. Deeks, D.P.S. Dosanjh, A. Lalvani.
Drafting of the article: K.A. Millington, A. Lalvani.
Critical revision of the article for important intellectual content: K.A. Millington, J.J. Deeks, A. Lalvani.
Final approval of the article: M. Bakir, K.A. Millington, A. Soysal, J.J. Deeks, S. Efee, Y. Aslan, D.P.S. Dosanjh, A. Lalvani.
Provision of study materials or patients: M. Bakir, A. Soysal, S. Efee, Y. Aslan.
Statistical expertise: K.A. Millington, J.J. Deeks, A. Lalvani.
Obtaining of funding: M. Bakir, A. Lalvani.
Administrative, technical, or logistic support: M. Bakir, A. Soysal, S. Efee, Y. Aslan, A. Lalvani.
Collection and assembly of data: M. Bakir, K.A. Millington, A. Soysal, S. Efee, Y. Aslan, D.P.S. Dosanjh, A. Lalvani.
Bakir M, Millington KA, Soysal A, Deeks JJ, Efee S, Aslan Y, et al. Prognostic Value of a T-Cell–Based, Interferon-γ Biomarker in Children with Tuberculosis Contact. Ann Intern Med. 2008;149:777-786. doi: 10.7326/0003-4819-149-11-200812020-00248
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Published: Ann Intern Med. 2008;149(11):777-786.
Enzyme-linked immunospot (ELISpot) assay is an increasingly widely used, T-cellâ€“based, interferon-Î³â€“release assay for diagnosing tuberculosis infection, but whether positive results are prognostic of active tuberculosis is not known.
To determine whether ELISpot results predict the development of active tuberculosis among persons with recent tuberculosis exposure.
Longitudinal cohort study of children and adolescents with tuberculosis contact recruited from October 2002 to April 2004.
Community-based contact investigations in Turkey.
908 children and adolescents with recent household tuberculosis exposure.
Enzyme-linked immunospot assay, incorporating early secretory antigenic target-6 and culture filtrate protein-10, and tuberculin skin test were done at baseline.
Incidence rates ratios of progression to active tuberculosis for contacts with positive tuberculin skin test and ELISpot results, and relative incidence rates comparing contacts with positive and negative test results.
Isoniazid preventive therapy was given to 688 (76%) contacts according to local guidelines. Fifteen contacts developed active tuberculosis over 1201 person-years of follow-up. Of 381 contacts with positive ELISpot results, 11 developed active tuberculosis over 536 person-years of follow-up (incidence rate, 21 per 1000 person-years [95% CI, 10.2 to 36.7 per 1000 person-years]), a statistically significant 3- to 4-fold increased risk for progression relative to ELISpot-negative contacts. Of 550 contacts with positive tuberculin skin test results, 12 developed active tuberculosis over 722 person-years of follow-up (incidence rate, 17 per 1000 person-years [CI, 8.6 to 29.0 per 1000 person-years]).
Only 3 of the 15 incident cases were confirmed by culture.
Positive ELISpot results predict subsequent development of active tuberculosis in recent tuberculosis contacts. Although tuberculosis contacts with positive ELISpot results have an incidence rate of tuberculosis similar to that of contacts with positive tuberculin skin test results, ELISpot testing could allow more focused targeting of preventive therapy to fewer contacts.
Interferon-γ–release assays, such as enzyme-linked immunospot (ELISpot), are increasingly used to help diagnose latent tuberculosis infection, but information about the ability of these tests to predict development of tuberculosis among exposed persons is scant.
This longitudinal study followed 908 children and adolescents with recent household exposure to tuberculosis, most of whom received preventive therapy. Of 381 children with positive ELISpot results, 11 subsequently developed active tuberculosis compared with 12 of 550 children with positive tuberculin skin test results.
Household contacts with positive ELISpot results have about a 3- to 4-fold increased risk for progression to active tuberculosis compared with contacts with negative ELISpot results. Compared with the tuberculin skin test, ELISpot could allow more focused targeting of preventive therapy to fewer contacts.
The use of the tuberculin skin test (TST) to diagnose latent tuberculosis infection was established and validated in large studies, which observed that persons with a positive TST result had a higher risk for active tuberculosis than those with a negative TST result (1–7). However, diagnostic specificity of the TST is confounded in bacille Calmette–Guérin (BCG)–vaccinated persons, and sensitivity is reduced in vulnerable populations, including immunocompromised persons and children (8).
The advent of T-cell interferon-γ–release assays (IGRAs) may offer a realistic alternative to the TST (9–15). Currently, there are only 2 forms of IGRAs using antigens specific to Mycobacterium tuberculosis: Either interferon-γ secretion is measured in whole blood by the enzyme-linked immunosorbent (ELISA) assay, or interferon-γ–secreting T cells are enumerated by the enzyme-linked immunospot (ELISpot) assay. Much published data indicate that IGRAs are more specific than the TST (12, 15, 16), and cross-sectional studies have shown good correlation of IGRA results with tuberculosis exposure in contacts (17–22).
Treatment of recent tuberculosis contacts with positive IGRA results, as recommended by some national guidelines (23, 24), will only provide clinical benefit if these persons are at increased risk for progression to tuberculosis compared with those with negative results. Thus, there is a widely recognized, urgent need for large longitudinal studies with clinical outcomes to determine the prognostic value of positive IGRA results for development of tuberculosis (12, 15, 23–28). A recent report of predominantly immunocompetent adults with tuberculosis contact who declined preventive therapy showed that a higher proportion of those who initially had a positive result by whole-blood ELISA (QuantiFERON-TB Gold In-Tube, Cellestis, Carnegie, Victoria, Australia) developed active tuberculosis during 2 years of follow-up compared with contacts with positive TST results, using a 5-mm cutoff point (29). However, this important observation, which was based on 6 incident tuberculosis cases, cannot be extrapolated to ELISpot because of substantial discordance between the 2 assays (12) or to contact investigations involving high-risk populations (30).
We conducted a longitudinal cohort study to determine the prognostic value of positive interferon-γ ELISpot results for development of active tuberculosis in a key high-risk population: children and adolescents with recent household exposure to tuberculosis. Because tuberculosis infection in children is usually recently acquired, it serves as a key marker for M. tuberculosis transmission in the general population and triggers extensive contact investigations to identify infectious adult source cases. Moreover, prompt isoniazid preventive therapy in infected children reduces the risk for active tuberculosis, which is associated with high morbidity and mortality (31, 32). Validation of IGRAs for diagnosis of asymptomatic tuberculosis infection by determining the predictive value of positive test results in this population is therefore a global public health priority.
All adults with newly diagnosed sputum smear–positive pulmonary tuberculosis at the 7 government-funded tuberculosis clinics on the Anatolian side of Istanbul over 18 months (from October 2002) were asked whether they had children living in the household, and we invited them to participate in the study. A total of 443 patients had 1 or more children in the household (household contacts). All agreed to participate and gave written informed consent on behalf of their children (17). When the index case was not the parent, consent was given by the child's grandparents or legal guardian. Contacts were included if they were 16 years or younger. There were no exclusion criteria for enrollment. Follow-up comprised 6 monthly symptom reviews and continued until incident tuberculosis was diagnosed or 2 years of follow-up was completed. We contacted parents or guardians who did not attend clinical follow-up appointments by telephone, and we assessed the health status of the child by using a standardized questionnaire. In addition, all children had a repeated ELISpot assay at 6-month follow-up. The institutional review board of Marmara University School of Medicine, Istanbul, Turkey; the Turkish Ministry of Health, Ankara, Turkey; and the World Health Organization Steering Committee on Research Involving Human Subjects, Geneva, Switzerland, granted ethical approval for the study.
Children were vaccinated intradermally with BCG Pasteur 1173-P2 from age 2 to 3 months and were given a booster vaccination in the first year of primary school and at age 6 to 7 years, as recommended by Turkish Ministry of Health guidelines. We documented BCG vaccination by the number of BCG vaccination scars. We diagnosed and excluded prevalent cases of active tuberculosis from our cohort by history, physical examination, chest radiography, and microbiological investigations at enrollment.
We administered a 6-month course of isoniazid preventive therapy to 3 groups of contacts on the basis of age and TST results in accordance with the Turkish Ministry of Health guidelines (33). The guidelines include all children younger than 6 years, regardless of baseline or subsequent TST results; children 6 years or older with a positive result in the first TST (that is, ≥10 mm in unvaccinated children and ≥15 mm in vaccinated children); and children 6 years or older with a negative result from the first TST but a converted TST result (that is, induration that increased by ≥6 mm from the first TST result and second TST induration ≥10 mm in unvaccinated children and ≥15 mm in vaccinated children). Parents or guardians, who were provided with tablets at 2 monthly intervals, administered isoniazid to their children. We questioned all parents or guardians who returned for follow-up about adherence to preventive treatment, and all reported full adherence.
We performed ELISpot assays, using peptides spanning early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) (10 μg/mL final concentration of each peptide; Louisiana State University Health Sciences Center Core Laboratories, New Orleans, Louisiana) and recombinant ESAT-6 (10 μg/mL; Veterinary Laboratories Agency, Weybridge, United Kingdom) and recombinant CFP-10 (10 μg/mL; Lionex Diagnostics and Therapeutics, Braunschweig, Germany), as described elsewhere (17, 21). The peptide assay was subsequently commercialized into the regulatory-approved T-SPOT.TB (Oxford Immunotec, Oxford, United Kingdom). Because the diagnostic potential of ESAT-6– and CFP-10–derived peptides and recombinant antigens has been assessed in published work, we determined the prognostic value of both forms of these M. tuberculosis–specific antigens.
We counted and scored ELISpot plates by using an automated ELISpot counter (AID, Strassberg, Germany), using predefined size and intensity spot settings. We scored responses to peptides as positive if duplicate test wells contained a mean of 5 or more spot-forming cells (SFCs) more than the mean of the negative control wells and if this number was at least twice the mean of the negative control wells. We used this predefined threshold in all of our previous studies (17, 20–22, 34–38). We scored responses to antigen as positive if duplicate test wells contained a mean of at least 10 SFCs more than the mean of the negative control wells and if this number was at least twice the mean of the negative control wells. For simplicity, cells stimulated with ESAT-6 or CFP-10 peptides are referred to as “ELISpot.”
We administered TST by the Mantoux method using 0.1 mL (2 tuberculin units) of purified protein derivative RT23 (Statens Serum Institut, Copenhagen, Denmark). The study pediatricians, who were blinded to ELISpot results, did the test and read and interpreted the results. They noted the cutaneous appearance of peau d'orange in all participants, confirming intradermal inoculation of purified protein derivative. Induration was measured after 72 to 96 hours with a ruler. For our analysis, we scored a positive TST response if the induration diameter was 5 mm or greater, as recommended by the American Thoracic Society and Centers for Disease Control and Prevention guidelines (39).
Each child contact was to be clinically followed up every 6 months for 2 years, but we asked parents to return with the child immediately for further clinical assessment if the child developed intercurrent symptoms. The study pediatricians, who were blinded to the ELISpot results, diagnosed incident tuberculosis on the basis of clinical, radiologic, and microbiological criteria. Two independent clinicians separately assessed the clinical and radiologic evidence for culture-negative incident cases, and in each case, the diagnosis was further confirmed by a documented successful clinical and radiologic response to antituberculosis treatment. We treated children with a diagnosis of tuberculosis with 6 months of standard chemotherapy, and we extended the continuation phase of isoniazid and rifampicin to 10 months for children with miliary tuberculosis. We treated children with a diagnosis of multidrug-resistant tuberculosis with second-line agents on the basis of antibiotic susceptibility results.
We defined each child's entry into the study as the date first examined and tested with the ELISpot and TST (baseline). The end point of the study for each child contact was development of active tuberculosis or the last follow-up assessment (whether by telephone or a clinic visit). We compared differences in baseline characteristics of participants who developed active tuberculosis and those who did not by using the chi-square test for binary variables and the Mann–Whitney test for continuous variables. We used Poisson regression to estimate incidence rates of progression to active tuberculosis per 1000 person-years of follow-up, together with 95% CIs. We estimated incidence rate ratios to compare the prognostic value of test-positive with test-negative results, both unadjusted and adjusted for preventive therapy. We compared differences in SFCs at baseline by using the Mann–Whitney test. We did all analyses in Stata, version 9.1 (StataCorp, College Station, Texas).
The Wellcome Trust and the United Nations (International) Children's Fund/United Nations Development Programme/World Bank/World Health Organization Special Programme for Research and Training in Tropical Diseases provided funding for the study. The funding sources had no role in the study design, data collection, data analysis, data interpretation, or writing of the report.
The final cohort comprised 908 children whom we followed for 1201 person-years; thus, the mean duration of follow-up was 1.3 years (Figure). There were 456 (50%) male children. The mean age of the cohort was 7.5 years (range, 1 month to 16 years). Of the final cohort, 728 (80%) children had been vaccinated with BCG.
Study flow diagram detailing the follow-up of 908 children with complete baseline results for enzyme-linked immunospot (ELISpot) and tuberculin skin test (TST). IPT = isoniazid preventive therapy; IR = incidence rate.
* Index patients and child contacts were recruited at the 7 government-run tuberculosis clinics in the Anatolian side of Istanbul.
† Pediatric Infectious Disease Clinic at Marmara University Hospital, Istanbul.
‡ When sputum microscopy and culture reports for all 443 index cases were obtained and checked, 4 contacts had index cases who did not have sputum smear–positive results and 2 contacts had index cases whose sputum grew nontuberculosis atypical mycobacteria.
§ Two contacts were removed because of loss of ELISpot plates, and 33 contacts were removed because of an episode of bacterial contamination of peptide pool reagents.
∥ Twenty (5%) contacts had positive ELISpot results and 37 (7%) contacts had negative ELISpot results at recruitment.
¶ Isoniazid preventive therapy was administered on the basis of age and TST result and was interpreted in accordance with Turkish Ministry of Health guidelines. Eighteen contacts who had positive ELISpot and negative TST results at recruitment and 49 contacts who had negative ELISpot and TST results at recruitment were given IPT because their TST converted. In total, 688 contacts received IPT, of whom 41 were exposed to index cases with multidrug-resistant tuberculosis. Thirteen incident cases received IPT: 6 had positive ELISpot and TST results, 4 had positive ELISpot and negative TST results (2 of whom had converted TST results), and 3 had negative ELISpot and negative TST results (2 of whom had converted TST results). None of the incident cases had negative ELISpot and positive TST results.
** Per 1000 person-years of follow-up.
All 908 children in the analytic data set had complete baseline ELISpot and TST results available. There were 381 (42%) children with positive ELISpot results and 550 (61%) with positive TST results. The prevalence of positive TST results was statistically significantly higher among BCG-vaccinated contacts than among unvaccinated contacts (462 of 728 vs. 88 of 180; P < 0.001).
We lost 63 children to follow-up after clinical assessment at 6 months; withdrawal rates did not differ according to ELISpot status (6% of contacts with positive ELISpot results vs. 7% with negative results). Of the cohort, 270 children completed 24 months of follow-up with 6 monthly clinical reviews and 560 children did not attend clinical follow-up after 12 months. These 560 children were followed every 6 months by a study nurse who administered a standardized telephone questionnaire.
Fifteen contacts developed active tuberculosis during follow-up. Development of active tuberculosis was not related to sex (incidence in girls vs. boys, 9 of 452 (2.0%) vs. 6 of 456 (1.3%); P = 0.43), although children with incident cases tended to be younger (median age, 4 years [interquartile range, 1 to 8 years] vs. 8 years [interquartile range, 4 to 11 years]; P = 0.074).
Table 1 shows demographic characteristics and test results of the 15 patients with incident tuberculosis. Table 2 shows clinical manifestations at diagnosis: At enrollment, all patients with incident tuberculosis were asymptomatic, with a normal chest radiograph, and had no history of tuberculosis. We detected 8 cases of incident tuberculosis at scheduled follow-up visits, and the remaining 7 patients presented to clinics outside of scheduled follow-up appointments. The proportion of incident cases detected by positive ELISpot results and diagnosed at scheduled clinical follow-up visits (5 of 11 cases) was no greater than the corresponding proportion of ELISpot-negative incident cases diagnosed at scheduled clinical follow-up (3 of 4 cases). All 15 patients with incident tuberculosis had clinical and radiologic findings strongly suggestive of tuberculosis; 3 of the patients also had positive cultures for M. tuberculosis from clinical specimens (Table 2). All patients with incident tuberculosis responded fully to antituberculosis treatment, including those treated with second-line drugs for multidrug-resistant tuberculosis, with complete clinical and radiologic resolution.
At recruitment, 11 patients with incident tuberculosis had positive ELISpot results and 12 had positive TST results. Ten patients with incident tuberculosis had both positive ELISpot and TST results, 1 had a positive ELISpot result and a negative TST result, 2 had negative ELISpot results and positive TST results, and 2 had negative ELISpot and TST results (Table 1). Two of 4 patients with incident tuberculosis who initially had negative ELISpot results had positive results when retested 6 months later. Although the quantitative definition of ELISpot conversion is evolving because of the natural biological variability around the threshold for a positive result, both converted results were strongly positive (376 and 140 SFCs per million peripheral blood mononuclear cells, respectively). Of the 3 patients who had negative TST results, all were retested 2 to 4 months later, and 1 had a converted result to 15-mm induration (Table 1).
Of the 381 contacts who had positive ELISpot results, 11 progressed to active tuberculosis within 536 person-years of follow-up (incidence rate, 20.5 per 1000 person-years [95% CI, 10.2 to 36.7 per 1000 person-years]). Of the 550 contacts with positive TST results, 12 progressed to tuberculosis in 722 person-years (incidence rate, 16.6 per 1000 person-years [CI, 8.6 to 29.0 per 1000 person-years]) (Table 3). Thus, ELISpot detected a similar number of incident cases of tuberculosis from a smaller group of positive test results. Moreover, a significantly higher proportion of contacts had positive TST results than positive ELISpot results (550 of 908 vs. 381 of 908, respectively; P < 0.001), which is consistent with the higher specificity of ELISpot.
Children with positive ELISpot results were 3.4 times more likely than those with negative results to develop active disease (P = 0.036) (Table 3). Children with positive TST results were 2.7 times more likely than those with negative results to develop disease, but this difference was not significant (P = 0.131) (Table 3).
We prescribed 688 children isoniazid preventive therapy. Because we administered isoniazid in part on the basis of TST results, interpreted in accordance with the Turkish Ministry of Health guidelines, and because agreement between TST and ELISpot results was high (κ = 0.65), a higher proportion of contacts with positive ELISpot results received isoniazid therapy than did those with negative results (353 of 381 (93%) vs. 335 of 527 (64%); P < 0.001). Thus, administration of isoniazid preventive therapy is partially related to the test result and could confound the incidence rate. Adjustment for treatment mildly increased the relative incidence rates between positive and negative test results (Table 3).
Because a high proportion (71%) of contacts exposed to index cases infected with M. tuberculosis susceptible to isoniazid received preventive therapy, including 10 of the 15 cases of incident tuberculosis, we had limited data on contacts who had not received preventive therapy to estimate the incidence rate. In these untreated contacts, 54 had positive ELISpot results, of whom 4 progressed to active tuberculosis within 93 person-years of follow-up (incidence rate, 43.0 per 1000 person-years [CI, 11.7 to 110.2 per 1000 person-years]). Contacts who had positive ELISpot results were statistically significantly more likely than those with negative results to develop active tuberculosis (incidence rate ratio, 10.6 [CI, 1.19 to 95.2]; P = 0.034). Of the 83 contacts who had positive TST results, 3 progressed to tuberculosis within 114 person-years of follow-up (incidence rate, 26.2 per 1000 person-years [CI, 5.4 to 76.6 per 1000 person-years]).
Of the 333 contacts who had positive ELISpot results to recombinant ESAT-6/CFP-10 antigen, 11 progressed to active tuberculosis within 462 person-years of follow-up (incidence rate, 23.8 per 1000 person-years [CI, 11.9 to 42.6 per 1000 person-years]). The incidence rate in contacts who had positive results to purified protein derivative was similar to that in contacts who had positive results to the nontuberculosis control antigen streptokinase streptodornase, and children who had positive ELISpot results to these antigens were no more likely than those who had negative results to develop active disease (Table 3).
Of the 337 contacts who had positive results for both tests, 10 progressed to tuberculosis within 451 person-years of follow-up (incidence rate, 22.2 per 1000 person-years [CI, 10.6 to 40.8 per 1000 person-years]) (Figure). The low number of cases of incident tuberculosis with discordant test results (positive ELISpot and negative TST results [1 case from 44 contacts] and negative ELISpot and positive TST results [2 cases from 213 contacts]) resulted in poor estimates of incidence rates with very wide CIs for these test combinations.
Within ELISpot-positive contacts, the size of the baseline interferon-γ response to peptides did not significantly differ between contacts who developed tuberculosis and those who remained well (median SFCs per million peripheral blood mononuclear cells, 176 [interquartile range, 88 to 1332] vs. 182 [interquartile range, 70 to 534]; P = 0.27). This was also true for responses to recombinant antigen (median SFCs per million peripheral blood mononuclear cells, 246 [interquartile range, 200 to 918] vs. 264 [interquartile range, 124 to 526]; P = 0.183).
We found that child tuberculosis contacts with positive ELISpot results had a statistically significant 3- to 4-fold increased risk for progression to active tuberculosis compared with contacts with negative ELISpot results. The incidence rate of tuberculosis in contacts who had positive ELISpot results was similar to that of contacts who had positive TST results, but ELISpot detected a similar number of incident cases from fewer contacts with positive test results.
Enzyme-linked immunospot is a quantitative assay; thus, interpretation of ELISpot results is not restricted to a binary read-out. The size of the baseline interferon-γ response did not statistically significantly differ between incident cases and contacts who did not develop tuberculosis, suggesting that although a positive ELISpot result is prognostic of progression to tuberculosis, the magnitude of this response does not further refine the risk for progression. This is in contrast to the TST, in which the size of the tuberculin reaction correlates with the risk for subsequent progression to tuberculosis (1–7).
None of the incident cases had a history of tuberculosis, and tuberculosis was excluded at recruitment by the absence of symptoms and normal radiologic and clinical examinations. Thus, a positive ELISpot result at recruitment reflected asymptomatic M. tuberculosis infection and not active tuberculosis or relapse of tuberculosis. We evaluated all household members clinically and with chest radiography, and we treated any secondary cases of tuberculosis promptly at the time of recruitment of child contacts. Hence, reexposure of child contacts in the household would have been minimal, although we cannot exclude reinfection from exposure in the community after recruitment.
Very few prospective studies have assessed the relationship between IGRA results and clinical outcome, and to our knowledge, our study is the first to focus on a key high-risk group. In a small study of 24 household contacts of sputum smear–positive pulmonary tuberculosis in Ethiopia, 6 of the 7 incident cases of tuberculosis were positive to recombinant ESAT-6 in a 5-day interferon-γ research ELISA compared with 3 of 17 contacts who did not develop active tuberculosis (40). In a more recent household contact study in Germany, a significantly higher proportion of untreated household contacts with positive ELISA results progressed to active tuberculosis than contacts who had positive TST results when using the 5-mm threshold (P < 0.003) (29). However, this proportion was not significantly greater than the proportion of contacts who had positive TST results and progressed to tuberculosis when using a 10-mm threshold (P = 0.1). Hill and coworkers (41) identified 26 incident cases during 2 years of follow-up of household contacts and, although contacts did not receive isoniazid preventive therapy, neither TST (using a 10-mm threshold) nor ELISpot (using a substantially higher threshold for positive results than that used in our current and previous studies and in the commercial kit) was prognostic of subsequent progression to tuberculosis. The reason for the lack of prognostic power of both TST and ELISpot in the African study is unclear but may relate to the highly endemic setting, in which community transmission outside households may cause a substantial proportion of tuberculosis cases, even in household contacts (42).
Our study has several limitations. Only 3 of the incident cases in this study were culture-positive; however, clinical specimens from children are difficult to obtain and are usually culture-negative (43, 44). For the culture-negative cases, the clinical and radiologic findings strongly suggested active tuberculosis, and all responded well to antituberculosis treatment. The number of incident cases in our study was small and the CIs around the point estimates were wide, but the risk for subsequent tuberculosis in the recent child contact who had positive ELISpot results was statistically significant.
Because a high proportion of contacts and incident cases received isoniazid preventive therapy, we had very limited data on contacts who had not received preventive therapy. As a result, our estimates of incidence rates in untreated contacts who had positive ELISpot and TST results were less robust but did suggest a stronger prognostic value of each test result in this group. Given the ethical imperative for prompt isoniazid preventive therapy in young children exposed to tuberculosis, it may not be possible to robustly assess whether IGRAs are more strongly prognostic of subsequent tuberculosis than TST in the absence of preventive treatment.
Despite administration of isoniazid, 13 children with incident cases developed tuberculosis, 3 of whom were exposed to isoniazid-resistant M. tuberculosis. Reasons for progression to disease in the remaining 10 children include the fact that isoniazid preventive therapy for 6 months is only 60% effective (31, 32), and adherence to the therapy may not have been complete in all children, even though parents or guardians reported full adherence of their children when they returned to collect more tablets every 2 months. Although there may theoretically have been potential bias in terms of greater failure to complete the isoniazid regimen among those who had negative TST results (interpreted according to guidelines from the Turkish Ministry of Health) and among those who received BCG vaccination, all parents were informed that a negative TST result did not rule out infection and that BCG vaccination did not adequately protect against disease. Our finding that ELISpot is prognostic of active tuberculosis holds regardless of the extent of adherence to isoniazid preventive therapy in the study population.
A positive interferon-γ ELISpot result is a useful and valid marker of latent tuberculosis infection because it predicts the subsequent development of active tuberculosis, which suggests that contacts with a diagnosis of latent tuberculosis infection on the basis of ELISpot results could benefit from preventive therapy. Although the risk for progression to active tuberculosis in contacts with a positive ELISpot result was similar to that in contacts with a positive TST result, if contacts who had positive test results were targeted with preventive treatment, statistically significantly more contacts who had positive TST results than those who had positive ELISpot results would need to be treated to prevent a similar number of incident cases. Thus, ELISpot testing could allow more focused targeting of preventive therapy to fewer contacts. Moreover, contacts' awareness of the risk for progression may improve adherence to the preventive treatment regimen (45). Confirming whether the prognostic power of ELISpot is significantly higher than that of TST will require larger studies identifying more cases of clinical disease. There is potential to further improve the prognostic value of T-cell biomarkers by measuring interferon-γ responses to additional novel M. tuberculosis–specific antigens (46) or by measuring additional cytokines or chemokines (47–49). Although our study is generalizable because it was conducted in unselected child household tuberculosis contacts in community-based contact investigations, similar studies are now required to validate and quantify the prognostic value of ELISpot in other high-risk groups, including persons coinfected with HIV (50).
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