From the Guidelines
A 15-month-old child who has received the BCG vaccine is unlikely to have a positive QuantiFERON-TB Gold test result, as the test measures the immune response to specific proteins found in Mycobacterium tuberculosis that are not present in the BCG vaccine strain. The QuantiFERON test is generally not affected by prior BCG vaccination, unlike the traditional tuberculin skin test (TST) 1. According to the guidelines for using the QuantiFERON-TB Gold test, the test detects the release of interferon-gamma (IFN-g) in response to synthetic peptides simulating two proteins present in M. tuberculosis: early secretory antigenic target--6 (ESAT-6) and culture filtrate protein--10 (CFP-10), which are absent from all BCG vaccine strains 1.
However, it is essential to consider the clinical context and the potential for false-positive results, particularly in recently vaccinated infants or in areas with high TB prevalence. The American Thoracic Society, Infectious Diseases Society of America, and Centers for Disease Control and Prevention clinical practice guidelines suggest that the TST might be more sensitive than IGRAs in young children, but IGRAs may be more specific, particularly in those given BCG 1.
Key points to consider:
- The QuantiFERON test is specific for M. tuberculosis and not affected by prior BCG vaccination 1
- Most children who have only received the BCG vaccine without actual tuberculosis infection will test negative on the QuantiFERON test
- There is still a small possibility of false-positive results, particularly in recently vaccinated infants or in areas with high TB prevalence
- Further clinical evaluation, including chest X-ray and assessment of TB exposure risk, would be necessary to determine if a positive QuantiFERON test result represents true TB infection requiring treatment or is a false positive result 1.
From the Research
Vaccine Response
- The BCG vaccine is known to have a valuable protective effect against tuberculosis (TB) and is the only vaccine available for this purpose 2.
- However, the vaccine can occasionally cause systemic infection in immunocompromised hosts, and its susceptibility to anti-tuberculosis drugs has been studied 2.
Quantiferon Test
- The QuantiFERON-TB Gold assay (QFT-G) is more specific for infection with Mycobacterium tuberculosis than the tuberculin skin test (TST), especially among persons vaccinated with BCG 3.
- A study found that 30% of BCG-vaccinated adults with a positive TST result also had a positive QFT-G result 3.
- Predictors for a positive QFT-G test in BCG-vaccinated adults with a positive TST include having an abnormal chest radiograph consistent with healed TB, a TST induration of ≥16mm, and birth in a high-incidence country 3.
BCG Vaccination and Quantiferon Test in Children
- A study found that the risk of a false-positive TST result due to BCG vaccination disappears 3 years after vaccine administration 4.
- The use of interferon-gamma release assays (IGRAs) such as QFT-G is recommended for children under 3 years of age who have been vaccinated with BCG, as BCG can interfere with TST results and cause false positives 4.
- There is limited information available on the response to the Quantiferon test in 15-month-old children who have received the BCG vaccine, and more research is needed to determine the accuracy of the test in this age group.
Vaccine Effects
- A study found that BCG vaccination at birth did not have a significant effect on the incidence of infections, growth, or development in children 5.
- However, a subgroup analysis found that children of mothers who were BCG-vaccinated had a reduced incidence of infections from 0 to 3 months among BCG-vaccinated children 5.