Manish Pareek

Interferon gamma release assays: principles and practice

The last decade has witnessed significant advances in mycobacterial genomics and cellular research which have resulted in the development of two new blood tests, the enzyme-linked immunospot assay (ELISpot) (TSPOT.TB, Oxford Immunotec, Oxford, UK) and the enzyme-linked immunosorbent assay (ELISA) (QuantiFERON-TB Gold In-Tube, Cellestis, Carnegie, Australia). These tests, which are collectively known as interferon gamma release assays (IGRAs), detect latent tuberculosis infection (LTBI) by measuring interferon (IFN)-gamma release in response to antigens present in Mycobacterium tuberculosis, but not bacille Calmette-Guerin (BCG) vaccine and most nontuberculous mycobacteria. This is done through enumeration of IFN-gamma-secreting T cells (ELISpot) or by measurement of IFN-gamma concentration (ELISA). The evidence base for these tests has expanded rapidly and now demonstrates that IGRAs are more specific than the tuberculin skin test (TST) as they are not confounded by previous BCG vaccination. In addition, with active tuberculosis (TB) as a surrogate for LTBI, it appears that the ELISA has a similar sensitivity to the TST, whereas the ELISpot is more sensitive. Using degree of exposure to TB as a surrogate for LTBI, both assays correlate at least as well with TB exposure as the TST. Recent longitudinal data have now demonstrated the prognostic power of positive IGRA results in recent contacts for the subsequent progression to active TB. Deployment of IGRAs, driven by new guidelines internationally, will impact on clinical practice in several ways. Their high specificity means that BCG-vaccinated individuals with a false-positive TST will not receive unnecessary preventive treatment, whereas improved sensitivity in individuals with weakened cellular immunity at highest risk of progressing to active TB (for example HIV-positive individuals) enables more reliable targeted testing and treatment of these vulnerable groups. The role of IGRAs in active TB is less clear but they may be useful as adjunctive tests in the diagnostic work-up of an individual with suspected TB. Finally, recent developments and future directions in IGRA development are reviewed.

A 100 year update on diagnosis of tuberculosis infection

BACKGROUND Diagnosis and treatment of latent tuberculosis infection (LTBI) is a cornerstone of tuberculosis (TB) control in the developed world. In the last century, the tuberculin skin test (TST) was the only means of diagnosing LTBI. ELISpot and whole-blood ELISA, collectively known as interferon-gamma release assays (IGRAs), are promising new tools. AREAS OF AGREEMENT IGRAs are more specific than TST for diagnosis of LTBI as they are not confounded by previous bacille Calmette-Guerin (BCG) vaccination. Assessing IGRA sensitivity in the absence of a gold standard for LTBI is challenging. Studies have therefore used surrogate markers such as active TB and correlation with degree of TB exposure in contact investigations. These studies suggest that sensitivity of ELISpot is higher than TST while whole-blood ELISA has similar sensitivity to TST. Recent longitudinal studies demonstrating the prognostic power of these tests for development of active TB provide definitive evidence that positive IGRA results reflect infection with dormant yet viable bacilli. AREAS OF CONTROVERSY Is the prognostic power of IGRAs greater than the TST? What are the false-negative rates in immunocompromised individuals with LTBI at high risk of progressing to active TB? GROWING POINTS IGRAs have been incorporated into national guidelines, although their optimal deployment in diagnostic algorithms is evolving. The health economic benefits of utilizing IGRAs are increasingly recognized, partly because their high specificity avoids unnecessary chemoprophylaxis in BCG-vaccinated persons with false-positive TST results. AREAS TIMELY FOR DEVELOPING RESEARCH Current IGRAs are being improved and next-generation tests, with improved sensitivity, could enable the reliable exclusion of LTBI in immunocompromised individuals.

Evaluation of Immigrant Tuberculosis Screening in Industrialized Countries

Improvements are needed in current screening, which is insufficient and ineffective.

Tuberculosis screening of migrants to low-burden nations: insights from evaluation of UK practice

Tuberculosis (TB) primarily occurs in the foreign-born in European countries, such as the UK, where increasing notifications and the high proportion of foreign-born cases has refocused attention on immigrant (new entrant) screening. We investigated how UK primary care organisations (PCOs) screen new entrants and whether this differs according to TB burden in the PCOs (incidence <20 or ≥20 cases per 100,000 per annum). An anonymous, 20-point questionnaire was sent to all 192 UK PCOs asking which new entrants are screened, who is screened for active TB/latent TB infection (LTBI) and the methods used. Descriptive analyses were undertaken. Categorical responses were compared using the Chi-squared test. 177 (92.2%) out of 192 PCOs responded; all undertook screening action in response to abnormal chest radiographs, but only 107 (60.4%) screened new entrants for LTBI. Few new entrants had active TB diagnosed (median 0.0%, interquartile range (IQR) 0.0–0.5%) but more were identified with LTBI (median 7.85%, IQR 4.30–13.50%). High-burden PCOs were significantly less likely to screen new entrants for LTBI (OR 0.26, 95% CI 0.12–0.54; p<0.0001). Among PCOs screening for LTBI, there was substantial deviation from national guidance in selection of new entrant subgroups and screening method. Considerable heterogeneity and deviation from national guidance exist throughout the UK new entrant screening process, with high-burden regions undertaking the least screening. Forming an accurate picture of current front-line practice will help to inform future development of European new entrant screening policy.

Ethnicity and mycobacterial lineage as determinants of tuberculosis disease phenotype

Background Emerging evidence suggests that Mycobacterium tuberculosis (Mtb) lineage and host ethnicity can determine tuberculosis (TB) clinical disease patterns but their relative importance and interaction are unknown. Methods We evaluated prospectively collected TB surveillance and Mtb strain typing data in an ethnically heterogeneous UK population. Lineage assignment was denoted using 15-loci mycobacterial interspersed repetitive units containing variable numbers of tandem repeats (MIRU-VNTR) and MIRU-VNTRplus. Geographical and ethnic associations of the six global Mtb lineages were identified and the influence of lineage and demographic factors on clinical phenotype were assessed using multivariate logistic regression. Results Data were available for 1070 individuals with active TB which was pulmonary only, extrapulmonary only and concurrent pulmonary–extrapulmonary in 52.1%, 36.9% and 11.0% respectively. The most prevalent lineages were Euro-American (43.7%), East African Indian (30.2%), Indo-Oceanic (13.6%) and East Asian (12.2%) and were geo-ethnically restricted with, for example, Indian subcontinent ethnicity inversely associated with Euro-American lineage (OR 0.23; 95% CI 0.14 to 0.39) and positively associated with the East African-Indian lineage (OR 4.04; 95% CI 2.19 to 7.45). Disease phenotype was most strongly associated with ethnicity (OR for extrathoracic disease 21.14 (95% CI 6.08 to 73.48) for Indian subcontinent and 14.05 (3.97 to 49.65) for Afro-Caribbean), after adjusting for lineage. With East Asian lineage as the reference category, the Euro-American (OR 0.54; 95% CI 0.32 to 0.91) and East-African Indian (OR 0.50; 95% CI 0.29 to 0.86) lineages were negatively associated with extrathoracic disease, compared with pulmonary disease, after adjusting for ethnicity. Conclusions Ethnicity is a powerful determinant of clinical TB phenotype independently of mycobacterial lineage and the role of ethnicity-associated factors in pathogenesis warrants investigation.

Vitamin D deficiency and TB disease phenotype

Background Extrapulmonary TB is increasingly common, yet the determinants of the wide clinical spectrum of TB are poorly understood. Methods We examined surveillance data (Birmingham, UK: 1980–2009 and USA Centers for Disease Control: 1993–2008) to identify demographic factors associated with extrapulmonary TB. We then directly tested association of these factors and serum 25-hydroxycholecalciferol (25(OH)D) concentration with extrapulmonary TB by multivariable analysis in a separate UK cohort. Results Data were available for 10 152 and 277 013 TB cases for Birmingham and US, respectively. Local-born individuals of white ethnicity had a lower proportion of extrapulmonary disease when compared with local-born non-whites (p<0.0001); both groups had a lower proportion of extrapulmonary disease when compared with foreign-born non-whites (p<0.0001). In a separate UK cohort (n=462), individuals with extrapulmonary TB had lower mean serum 25(OH)D concentration than those with pulmonary TB (11.4 vs 15.2 nmol/L, respectively, p=0.0001). On multivariable analysis, vitamin D deficiency was strongly associated with extrapulmonary TB independently of ethnicity, gender and other factors. Doubling in serum 25(OH)D concentration conferred substantially reduced risk of extrapulmonary disease (OR 0.55, 95% CI 0.41 to 0.73). Conclusions We identify vitamin D deficiency as a probable risk factor for extrapulmonary dissemination in TB, which may account for the associations of dark-skinned ethnicity and female gender with extrapulmonary disease. Our findings implicate vitamin D status in Mycobacterium tuberculosis containment in vivo and, given the high prevalence of deficiency, may inform development of novel TB prevention strategies.

High rates of Mycobacterium tuberculosis among socially marginalized immigrants in low-incidence area, 1991-2010, Italy.

Migration from low- and middle-income countries to high-income countries increasingly determines the severity of tuberculosis (TB) cases in the adopted country. Socially marginalized groups, about whom little is known, may account for a reservoir of TB among the immigrant populations. We investigated the rates of and risk factors for Mycobacterium tuberculosis transmission, infection, and disease in a cohort of 27,358 socially marginalized immigrants who were systematically screened (1991–2010) in an area of Italy with low TB incidence. Overall TB and latent TB infection prevalence and annual tuberculin skin testing conversion rates (i.e., incidence of new infection) were 2.7%, 34.6%, and 1.7%, respectively. Prevalence of both TB and latent TB infection and incidence of infection increased as a function of the estimated TB incidence in the immigrants’ countries of origin. Annual infection incidence decreased with time elapsed since immigration. These findings have implications for control policy and immigrant screening in countries with a low prevalence of TB.

Comparison of screening strategies to improve the diagnosis of latent tuberculosis infection in the HIV-positive population: a cohort study

Background HIV is the most important risk factor for progression of latent tuberculosis infection (LTBI) to active tuberculosis (TB). Detection and treatment of LTBI is necessary to reduce the increasing burden of TB in the UK, but a unified LTBI screening approach has not been adopted. Objective To compare the effectiveness of a TB risk-focused approach to LTBI screening in the HIV-positive population against current UK National Institute for Health and Clinical Excellence (NICE) guidance. Design Prospective cohort study. Setting Two urban HIV treatment centres in London, UK. Participants 114 HIV-infected individuals with defined TB risk factors were enrolled prospectively as part of ongoing studies into HIV and TB co-infection. Outcome measures The yield and case detection rate of LTBI cases within the research study were compared with those generated by the NICE criteria. Results 17/114 (14.9%, 95% CI 8.3 to 21.5) had evidence of LTBI. Limiting screening to those meeting NICE criteria for the general population (n=43) would have detected just over half of these, 9/43 (20.9%, 95% CI 8.3 to 33.5) and those meeting criteria for HIV co-infection (n=74) would only have captured 8/74(10.8%, 95% CI 3.6 to 18.1) cases. The case detection rates from the study and NICE approaches were not significantly different. LTBI was associated with the presence of multiple TB risk factors (p=0.002). Conclusion Adoption of a TB risk-focused screening algorithm that does not use CD4 count stratification could prevent more cases of TB reactivation, without changing the case detection rate. These findings should be used to inform a large-scale study to create unified guidelines.

Immigrant screening for TB: a missed opportunity to improve TB control in the United Kingdom

Abstract Tuberculosis in the United Kingdom and other high-income countries is primarily a disease of the foreign-born arising from the synergy of migration from high TB burden regions and the reactivation of remotely acquired latent TB infection. UK immigrant screening policy primarily aims to identify active, rather than latent, TB although mounting evidence indicates that implementing latent TB screening for new entrants from intermediate and high incidence countries could cost-effectively reduce TB incidence in the UK.

UK immigrant screening is inversely related to regional tuberculosis burden

We read with interest the editorial by Moore-Gillon et al ,1 which advocated a more comprehensive system of immigrant screening/treatment for latent tuberculosis infection (LTBI) as a means of augmenting tuberculosis (TB) control in the UK. A recent comprehensive, national evaluation of local TB services/primary care organisations(PCOs) in the UK, which provides key insights into UK screening practices,2 found that the existing …