Interferon-Gamma R elease Assays Robert Belknap,
MD
a,
*, Charles L. Daley,
MD
b
KEYWORDS Latent tuberculosis infection Interferon-gamma release assays Tuberculosis Tuberculin skin test QuantiFERON T-SPOT KEY POINTS Diagnosis of latent tuberculosis infection (LTBI) should be targeted toward individuals and groups with high risk of progression to active tuberculosis (TB). Low-risk populations should not be screened. Interferon-gamma release assays (IGRAs) perform as well or better than the tuberculin skin test (TST) in most targeted populations. IGRAs are preferred for bacille CalmetteGue´rin (BCG)-vaccinated populations. A positive IGRA in a person at low risk for TB exposure should be confirmed with a repeat test or another method before recommending LTBI treatment. The choice of which IGRA to use is generally based on the costs and feasibility of performing the test.
INTRODUCTION
TB remains a major global health problem. Worldwide, there are an estimated 2 billion people infected with Mycobacterium tuberculosis and from this large reservoir approximately 8.6 million people develop TB each year.1 A staggering 1.3 million people die from TB annually, including more than 300,000 with HIV infection. Although the rates of TB are declining at approximately 2% per year, HIV coinfection and the emergence of drug-resistant strains of M tuberculosis threaten to undermine global TB control. Identification and treatment of LTBI can substantially reduce the risk of developing TB and is a major focus of TB control in the United States.2 Identification of all persons with LTBI would require screening large numbers of low-risk individuals that would not be cost-effective and would result in many false-positive test results.3 Instead, the Centers for Disease Control and Prevention (CDC) recommends targeted testing in order to identify persons with LTBI who are at greater risk of progressing to TB and who
a Denver Public Health Department, University of Colorado School of Medicine, 605 Bannock Street, Denver, CO 80204, USA; b National Jewish Health, University of Colorado School of Medicine, 1400 Jackson Street, Denver, CO 80206, USA * Corresponding author. E-mail address: [email protected]
Clin Lab Med - (2014) -–http://dx.doi.org/10.1016/j.cll.2014.02.007 labmed.theclinics.com 0272-2712/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
2
Belknap & Daley
would benefit from treatment of LTBI.2 Persons with increased risk for developing TB include those who have been recently infected with M tuberculosis and those who have medical conditions that are associated with an increased risk of developing TB. For nearly a century, the TST has been used to identify persons with TB infection. The TST is performed by the intradermal injection of purified protein derivative (PPD) that contains more than 200 proteins.4 With such a diverse collection of antigens, it is not surprising that the TST lacks sensitivity and specificity, resulting in false-positive and false-negative test results. The 2 most important causes of falsepositive results are infection with nontuberculous mycobacteria (NTM) and prior BCG vaccination. IGRAs are in vitro blood tests that measure the production of interferon gamma after stimulation with more specific mycobacterial antigens.5 Because these antigens are not contained within BCG strains or most NTM, they provide a more specific test than the TST. This review focuses on IGRAs and their strengths and limitations for screening for LTBI. MICROBIOLOGY
The genus Mycobacterium consists of slow-growing organisms that are widely distributed throughout the world and range from organisms that cause no human disease to those like M tuberculosis and M leprae that cause enormous morbidity and mortality.6 TB is caused by members of the M tuberculosis complex that includes the clinically relevant species M tuberculosis, M bovis, and M africanum. All members of the M tuberculosis complex, except BCG substrains, contain a region of the genome referred to as the deleted region 1 (RD1); this region distinguishes virulent strains of M bovis from all BCG strains and is thought to represent part of the original attenuation during 1908–1921.7 Within RD1 are genes that encode for the antigens, early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10); these antigens are more specific than PPD for M tuberculosis and are absent from all available strains of M bovis BCG.8 NTM refer to nonlepromatous organisms that are not members of the M tuberculosis complex.6 NTM have been referred to as mycobacteria other than TB, atypical mycobacteria, and environmental mycobacteria. The latter designation refers to their widespread presence in the environment. NTM have several features that distinguish them from M tuberculosis. They have a wide range of pathogenicity, are not always associated with disease, and, unlike M tuberculosis, are not transmissible from human to human. Importantly, the incidence of NTM disease is increasing in many areas of the world, and the cause for this increase is unknown.9 NTM share many antigens with M tuberculosis and thus can result in a false-positive TST result. Most strains of NTM, however, except M marinum, M kansasii, M szulgai, and M flavescens, do not encode for the antigens ESAT-6 and CFP-10, so they do not affect the results of IGRAs.10 EPIDEMIOLOGY
Understanding the epidemiology of TB is necessary to develop successful TB control interventions. As discussed previously, there were an estimated 8.6 million people who developed TB in 2012.1 Twenty-two high-burden countries accounted for 81% of all estimated incident cases worldwide, with rates of approximately 150 to 300 cases per 100,000 population. In these high-burden countries, stopping transmission through TB case detection and treatment is the most important TB control intervention.
Interferon-Gamma Release Assays
The prevalence of LTBI in the United States was estimated to be approximately 4.2% in 1999–2000; an estimated 11,213,000 individuals had LTBI.11 Higher prevalence rates were noted in those who were foreign born (18.7%), non-Hispanic blacks (7.0%), Mexican Americans (9.4%), and individuals living in poverty (6.1%). This reservoir of infected persons was the source for most of the 9951 new TB cases in 2012, resulting in an incidence rate of 3.2 per 100,000 population12; this represents a 6.1% decline compared with 2011. Four states reported half of the total cases (California, Texas, New York, and Florida) but rates of TB were highest in Alaska (9.0/100,000) and Hawaii (8.4/100,000). In countries like the United States, where the number of active TB cases are relatively few, prevention of cases through identification and treatment of LTBI is paramount; developing effective targeted testing programs requires being able to identify individuals and populations with higher rates of disease. One such population is those who are foreign born, who make up a disproportionate burden of TB cases in the United States. Foreign-born cases in the United States made up 63% of all cases with an incidence of 15.8 per 100,000 population.12 In contrast, the rate of TB among US-born cases was only 1.4 per 100,000 population. More than half of the foreign-born cases originated from 5 countries: Mexico (20.9%), Philippines (8.5%), India (8.5%), Vietnam (7.2%), and China (5.6%). Studies have shown that the risk of TB in those who are foreign born is substantially higher than that in US-born persons. This rate is highest in the first year likely due to prevalent TB at the time of immigration but remains elevated above the US-born rate for several years.13,14 The highest rates are in immigrants and refugees from high-incidence countries (defined as 100 cases/100,000 population), students/exchange visitors and temporary workers from high-incidence countries, and immigrants and refugees from medium-incidence countries.15 According to a recent study, the risk of reactivation among Filipino immigrants was 25 times higher than that of the 2012 TB rate among US-born persons and did not decline significantly over a 9-year period.14 Molecular genotyping studies have documented that more than 80% of TB in those who are foreign born in the United States is due to reactivation of LTBI.16 Therefore, targeted testing for LTBI among those who are foreign born is a major strategic component of TB control. The CDC currently recommends that foreign-born individuals from high-prevalence countries be tested for LTBI if they have been in the United States for less than 5 years. The studies, cited previously, however, as well as a cost-effective analysis argue for testing this population regardless of the amount of time in the United States.17 Persons who have been recently infected with M tuberculosis as well as certain medical conditions (Table 1) are at high risk of developing TB and, therefore, should be targeted for screening.2 Recently infected persons, such as contacts to infectious TB cases and persons who have converted their TST (or IGRA) from negative to positive over the previous 2 years, have a high rate of progression to TB (discussed later). In a controlled clinical trial evaluating the efficacy of isoniazid treatment of LTBI among contacts to infectious cases, the rate of developing TB in the first year among converters in the placebo arm was 12.9 cases per 100,000 compared with 1.6 per 100,000 in the subsequent 7 years of the study.18 More than half of the risk of developing TB is in the first 1 year after infection.2 CLINICAL PRESENTATION
TB represents a spectrum of disease ranging from LTBI, in which patients are asymptomatic, to active TB, where patients are symptomatic and infectious. LTBI is defined solely by evidence of sensitization to mycobacterial proteins in the absence of
3
4
Belknap & Daley
Table 1 Medical conditions associated with high risk of progression to active TB HIV infection
Injection drug use
Other immunosuppressive diseases
End-stage renal disease
Diabetes mellitus
Silicosis
Immunosuppressive drugs
Intestinal bypass
Hematologic/reticuloendothelial malignancies
Malabsorption
Postgastrectomy
MD
a,
*, Charles L. Daley,
MD
b
KEYWORDS Latent tuberculosis infection Interferon-gamma release assays Tuberculosis Tuberculin skin test QuantiFERON T-SPOT KEY POINTS Diagnosis of latent tuberculosis infection (LTBI) should be targeted toward individuals and groups with high risk of progression to active tuberculosis (TB). Low-risk populations should not be screened. Interferon-gamma release assays (IGRAs) perform as well or better than the tuberculin skin test (TST) in most targeted populations. IGRAs are preferred for bacille CalmetteGue´rin (BCG)-vaccinated populations. A positive IGRA in a person at low risk for TB exposure should be confirmed with a repeat test or another method before recommending LTBI treatment. The choice of which IGRA to use is generally based on the costs and feasibility of performing the test.
INTRODUCTION
TB remains a major global health problem. Worldwide, there are an estimated 2 billion people infected with Mycobacterium tuberculosis and from this large reservoir approximately 8.6 million people develop TB each year.1 A staggering 1.3 million people die from TB annually, including more than 300,000 with HIV infection. Although the rates of TB are declining at approximately 2% per year, HIV coinfection and the emergence of drug-resistant strains of M tuberculosis threaten to undermine global TB control. Identification and treatment of LTBI can substantially reduce the risk of developing TB and is a major focus of TB control in the United States.2 Identification of all persons with LTBI would require screening large numbers of low-risk individuals that would not be cost-effective and would result in many false-positive test results.3 Instead, the Centers for Disease Control and Prevention (CDC) recommends targeted testing in order to identify persons with LTBI who are at greater risk of progressing to TB and who
a Denver Public Health Department, University of Colorado School of Medicine, 605 Bannock Street, Denver, CO 80204, USA; b National Jewish Health, University of Colorado School of Medicine, 1400 Jackson Street, Denver, CO 80206, USA * Corresponding author. E-mail address: [email protected]
Clin Lab Med - (2014) -–http://dx.doi.org/10.1016/j.cll.2014.02.007 labmed.theclinics.com 0272-2712/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
2
Belknap & Daley
would benefit from treatment of LTBI.2 Persons with increased risk for developing TB include those who have been recently infected with M tuberculosis and those who have medical conditions that are associated with an increased risk of developing TB. For nearly a century, the TST has been used to identify persons with TB infection. The TST is performed by the intradermal injection of purified protein derivative (PPD) that contains more than 200 proteins.4 With such a diverse collection of antigens, it is not surprising that the TST lacks sensitivity and specificity, resulting in false-positive and false-negative test results. The 2 most important causes of falsepositive results are infection with nontuberculous mycobacteria (NTM) and prior BCG vaccination. IGRAs are in vitro blood tests that measure the production of interferon gamma after stimulation with more specific mycobacterial antigens.5 Because these antigens are not contained within BCG strains or most NTM, they provide a more specific test than the TST. This review focuses on IGRAs and their strengths and limitations for screening for LTBI. MICROBIOLOGY
The genus Mycobacterium consists of slow-growing organisms that are widely distributed throughout the world and range from organisms that cause no human disease to those like M tuberculosis and M leprae that cause enormous morbidity and mortality.6 TB is caused by members of the M tuberculosis complex that includes the clinically relevant species M tuberculosis, M bovis, and M africanum. All members of the M tuberculosis complex, except BCG substrains, contain a region of the genome referred to as the deleted region 1 (RD1); this region distinguishes virulent strains of M bovis from all BCG strains and is thought to represent part of the original attenuation during 1908–1921.7 Within RD1 are genes that encode for the antigens, early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10); these antigens are more specific than PPD for M tuberculosis and are absent from all available strains of M bovis BCG.8 NTM refer to nonlepromatous organisms that are not members of the M tuberculosis complex.6 NTM have been referred to as mycobacteria other than TB, atypical mycobacteria, and environmental mycobacteria. The latter designation refers to their widespread presence in the environment. NTM have several features that distinguish them from M tuberculosis. They have a wide range of pathogenicity, are not always associated with disease, and, unlike M tuberculosis, are not transmissible from human to human. Importantly, the incidence of NTM disease is increasing in many areas of the world, and the cause for this increase is unknown.9 NTM share many antigens with M tuberculosis and thus can result in a false-positive TST result. Most strains of NTM, however, except M marinum, M kansasii, M szulgai, and M flavescens, do not encode for the antigens ESAT-6 and CFP-10, so they do not affect the results of IGRAs.10 EPIDEMIOLOGY
Understanding the epidemiology of TB is necessary to develop successful TB control interventions. As discussed previously, there were an estimated 8.6 million people who developed TB in 2012.1 Twenty-two high-burden countries accounted for 81% of all estimated incident cases worldwide, with rates of approximately 150 to 300 cases per 100,000 population. In these high-burden countries, stopping transmission through TB case detection and treatment is the most important TB control intervention.
Interferon-Gamma Release Assays
The prevalence of LTBI in the United States was estimated to be approximately 4.2% in 1999–2000; an estimated 11,213,000 individuals had LTBI.11 Higher prevalence rates were noted in those who were foreign born (18.7%), non-Hispanic blacks (7.0%), Mexican Americans (9.4%), and individuals living in poverty (6.1%). This reservoir of infected persons was the source for most of the 9951 new TB cases in 2012, resulting in an incidence rate of 3.2 per 100,000 population12; this represents a 6.1% decline compared with 2011. Four states reported half of the total cases (California, Texas, New York, and Florida) but rates of TB were highest in Alaska (9.0/100,000) and Hawaii (8.4/100,000). In countries like the United States, where the number of active TB cases are relatively few, prevention of cases through identification and treatment of LTBI is paramount; developing effective targeted testing programs requires being able to identify individuals and populations with higher rates of disease. One such population is those who are foreign born, who make up a disproportionate burden of TB cases in the United States. Foreign-born cases in the United States made up 63% of all cases with an incidence of 15.8 per 100,000 population.12 In contrast, the rate of TB among US-born cases was only 1.4 per 100,000 population. More than half of the foreign-born cases originated from 5 countries: Mexico (20.9%), Philippines (8.5%), India (8.5%), Vietnam (7.2%), and China (5.6%). Studies have shown that the risk of TB in those who are foreign born is substantially higher than that in US-born persons. This rate is highest in the first year likely due to prevalent TB at the time of immigration but remains elevated above the US-born rate for several years.13,14 The highest rates are in immigrants and refugees from high-incidence countries (defined as 100 cases/100,000 population), students/exchange visitors and temporary workers from high-incidence countries, and immigrants and refugees from medium-incidence countries.15 According to a recent study, the risk of reactivation among Filipino immigrants was 25 times higher than that of the 2012 TB rate among US-born persons and did not decline significantly over a 9-year period.14 Molecular genotyping studies have documented that more than 80% of TB in those who are foreign born in the United States is due to reactivation of LTBI.16 Therefore, targeted testing for LTBI among those who are foreign born is a major strategic component of TB control. The CDC currently recommends that foreign-born individuals from high-prevalence countries be tested for LTBI if they have been in the United States for less than 5 years. The studies, cited previously, however, as well as a cost-effective analysis argue for testing this population regardless of the amount of time in the United States.17 Persons who have been recently infected with M tuberculosis as well as certain medical conditions (Table 1) are at high risk of developing TB and, therefore, should be targeted for screening.2 Recently infected persons, such as contacts to infectious TB cases and persons who have converted their TST (or IGRA) from negative to positive over the previous 2 years, have a high rate of progression to TB (discussed later). In a controlled clinical trial evaluating the efficacy of isoniazid treatment of LTBI among contacts to infectious cases, the rate of developing TB in the first year among converters in the placebo arm was 12.9 cases per 100,000 compared with 1.6 per 100,000 in the subsequent 7 years of the study.18 More than half of the risk of developing TB is in the first 1 year after infection.2 CLINICAL PRESENTATION
TB represents a spectrum of disease ranging from LTBI, in which patients are asymptomatic, to active TB, where patients are symptomatic and infectious. LTBI is defined solely by evidence of sensitization to mycobacterial proteins in the absence of
3
4
Belknap & Daley
Table 1 Medical conditions associated with high risk of progression to active TB HIV infection
Injection drug use
Other immunosuppressive diseases
End-stage renal disease
Diabetes mellitus
Silicosis
Immunosuppressive drugs
Intestinal bypass
Hematologic/reticuloendothelial malignancies
Malabsorption
Postgastrectomy
