LTB Infection Estimated with Interferon-γ Release Assay

Latent tuberculosis (LTB), also called latent tuberculosis infection (LTBI) is when a person is infected with Mycobacterium tuberculosis, but do not have active tuberculosis. Active tuberculosis can be contagious while latent tuberculosis is not, and it is therefore not possible to get infected from someone with latent tuberculosis.

In Japan, university students, including foreign-born students, undergo TB screening with chest radiograph; however, a chest radiograph cannot detect LTBI; it detects only pulmonary TB. Because immigrants may develop TB after entry, screening with chest radiograph might be ineffective; therefore, screening for LTBI may be necessary to prevent TB outbreaks.

Scientists at Keio University School of Medicine (Tokyo, Japan) and their colleagues enrolled 177 participants 20–42 years of age (median 23 years), of whom 98 (55.1%) were female. Participants were from China (55 students), Indonesia (24 students), France (19 students), Germany (nine students), and Thailand (eight students); the remaining participants were from 28 different countries; including 50 from countries with estimated TB incidence rates greater than 100 cases/100,000 persons.

The team collected whole blood specimens for the T-SPOT.TB test, an Interferon-γ Release Assay (IGRA) available in Japan. All participants were screened for pulmonary TB with chest radiograph. They interviewed participants using a structured questionnaire on identification and demographic information, the date of first arrival in Japan, and history of TB.

The scientists found that overall, eight (4.5%) students tested positive on IGRA (two each from China and Thailand and one each from Ghana, Indonesia, South Korea, and the Philippines). The rate of the positive IGRA result for students from countries with an estimated TB incidence rate of >100 cases/100,000 persons was 10.0% and relative risk was 4.2, whereas the rate for students from countries with an estimated TB incidence rate of <100 cases/100,000 persons was 2.4%. The IGRA positivity of students 20–29 years of age from countries with estimated TB incidence rates of >100 cases/100,000 persons was 9.4%.

The authors concluded that estimated LTBI rates for foreign-born students in Japan from countries with high TB incidence rates were higher than those for students from countries with low TB incidence rates and for students from Japan. Based on their findings, they recommend that universities screen for LTBI using IGRAs in students from countries with high TB incidence rates (i.e., >100 cases/100,000 persons). The study was published in the November 2018 issue of the journal Emerging Infectious Diseases.

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Keio University School of Medicine