Diagnosing Latent Tuberculosis Infection with IGRA

Key Clinical Advantages

The Centers for Disease Control and Prevention recommends IGRAs for individuals who have received BCG vaccination, due to their superior specificity, particularly in BCG-vaccinated populations, and equal or better positive predictive value for progression to active TB disease 1, 2
IGRAs use M. tuberculosis-specific antigens that are absent from BCG vaccine strains and most nontuberculous mycobacteria, eliminating false-positive results from BCG vaccination, which is particularly important in BCG-vaccinated populations where TST results correlate poorly with true infection status 2, 3
IGRAs require only a single patient visit, eliminating the need for return visits, and results are objective and laboratory-based rather than subjective 1, 2, 4

The positive predictive value of IGRAs for development of active tuberculosis is equal to or better than TST due to higher specificity and similar or higher sensitivity 3, 5
IGRAs demonstrate strong negative predictive value in immunocompetent individuals, especially when combined with negative TST 3, 6

The CDC recommends IGRAs may be used in all situations where TST is recommended, including contact investigations of active TB cases, evaluation of recent immigrants from high-incidence countries, screening of healthcare workers with TB exposure, and assessment of immunocompromised patients at risk for reactivation 1, 2

Indeterminate results occur in 10-25% of severely immunocompromised patients due to impaired mitogen reactivity, particularly with advanced disease 7, 8
The negative predictive value in immunocompromised patients remains incompletely established 3, 8

Consider dual testing to maximize sensitivity in severely immunocompromised patients, though this reduces specificity 2, 9
Annual risk of progression to active TB increases dramatically to 5-15% in immunocompromised individuals versus 0.1% in immunocompetent hosts 5, 6
Children aged <5 years and immunocompromised patients exposed to smear-positive cases require priority screening due to high risk of progression to severe disease 5, 6
Household contacts and those with >40 hours cumulative contact time warrant aggressive screening 7

The Centers for Disease Control and Prevention recommends that prior BCG vaccination is the leading cause of TST-positive/IGRA-negative discordance because tuberculin PPD contains antigens present in BCG vaccine strains that trigger false-positive skin test reactions 10
The European Respiratory Society notes that BCG vaccination received even decades earlier can cause persistent TST positivity without true M. tuberculosis infection 11
The CDC guidelines state that IGRAs use M. tuberculosis-specific antigens (ESAT-6 and CFP-10) that are absent from BCG vaccine strains, eliminating this cross-reactivity and providing superior specificity 10
Environmental exposure to certain nontuberculous mycobacteria can cause positive TST results through cross-reactive antigens in tuberculin PPD, according to the CDC 10

The CDC recommends that in healthy persons with low probability of M. tuberculosis infection, a single positive TST result should not be considered reliable evidence of infection because false-positive results are more likely than true infection 12
The European Respiratory Journal suggests that in the absence of TB risk factors, the positive predictive value of TST drops dramatically, making false-positive results more likely than true infection 11
National guidelines, including the CDC and European Respiratory Society, recommend using IGRA to confirm positive TST results, particularly in BCG-vaccinated individuals 11

The CDC recommends that an immunocompetent patient with positive TST but negative IGRA and no TB risk factors does not require treatment for latent TB infection 12
The European Respiratory Journal notes that the negative IGRA has superior specificity and should be considered the definitive test result in this discordant scenario 11
The CDC guidelines state that treatment based solely on TST results in this context would represent overtreatment and unnecessary exposure to isoniazid hepatotoxicity risk 11