From the Guidelines
The most appropriate screening for this patient is B. interferon-gamma release assay (IGRA). This test is preferred for individuals who have previously received the BCG vaccine because it specifically detects Mycobacterium tuberculosis infection without cross-reactivity with BCG, as suggested by the 2023 guidelines 1. Unlike the tuberculin skin test (PPD), which can yield false-positive results in BCG-vaccinated individuals, IGRAs measure the release of interferon-gamma from T cells in response to M. tuberculosis-specific antigens. This makes IGRAs more specific for detecting latent tuberculosis infection in this medical student who received BCG vaccination in India.
The test requires a single blood draw and results are typically available within 24 hours. This approach avoids the need for the patient to return for skin test reading and eliminates the confusion of interpreting potentially false-positive skin test results due to prior BCG vaccination. According to the 2017 guidelines 1, performing an IGRA rather than a TST is recommended in individuals who are likely to be infected with Mtb, have a low or intermediate risk of disease progression, and have a history of BCG vaccination.
Some key points to consider when choosing a screening test for this patient include:
- The patient's history of BCG vaccination, which can affect the results of the TST
- The need for a test that can specifically detect M. tuberculosis infection without cross-reactivity with BCG
- The convenience and accuracy of the IGRA test, which requires a single blood draw and provides results within 24 hours. Given these considerations, IGRA is the most appropriate screening test for this patient, as it provides a specific and accurate diagnosis of latent tuberculosis infection, and is preferred over TST in individuals with a history of BCG vaccination 1.
From the Research
Screening for Tuberculosis
The patient in question is a 25-year-old medical student who had a bacillus Calmette-Guérin (BCG) vaccination in her late teens in India. Given this history, the most appropriate screening for this patient would be:
- B. interferon-gamma release assay This is because the BCG vaccination can cause a false-positive result on a tuberculin skin test (TST), but the interferon-gamma release assay (IGRA) is less affected by BCG vaccination 2.
Considerations for IGRA
It's worth noting that IGRA results can be affected by immunosuppressive therapy, with patients receiving immunosuppressive therapy being less likely to have a positive IGRA result 3. However, this patient is not mentioned to be on immunosuppressive therapy.
Comparison of Screening Options
The other options are:
- A. chest radiography: This is not a primary screening method for tuberculosis, but rather a diagnostic tool used in conjunction with other tests.
- C. 1-step PPD test: This is a type of TST, which may be less accurate in patients with a history of BCG vaccination.
- D. sputum testing for acid-fast bacilli: This is a diagnostic test used to confirm active tuberculosis, not a screening test for latent infection.
- E. 2-step PPD test: This is also a type of TST, which may be less accurate in patients with a history of BCG vaccination.
Additional Considerations
In patients with immunocompromised conditions, indeterminate IGRA results may represent an intermediate risk of progression to active tuberculosis 4. However, this patient is not mentioned to be immunocompromised. The diagnostic accuracy of IGRA in acquired immunodeficiency syndrome patients with suspected tuberculosis infection has been studied, with results suggesting that IGRA can be useful in diagnosing TB disease in these patients 5. Additionally, high rates of indeterminate IGRA results have been reported in liver transplantation candidates, associated with severity of liver disease and anergy 6.