From the Guidelines
A chest X-ray (CXR) is ordered after a positive Interferon-Gamma Release Assay (IGRA) to rule out active tuberculosis (TB) disease before initiating treatment for latent tuberculosis infection (LTBI). This approach is based on the guidelines for the management of latent Mycobacterium tuberculosis infection, which recommend that individuals with a positive IGRA result should be evaluated for the likelihood of M. tuberculosis infection and for symptoms and signs of active tuberculosis 1. The CXR is a crucial step in this evaluation, as it helps identify radiographic abnormalities consistent with active pulmonary TB, such as infiltrates, cavitations, nodules, or pleural effusions.
According to the updated guidelines for using interferon gamma release assays to detect Mycobacterium tuberculosis infection, a diagnosis of LTBI requires that active tuberculosis be excluded by medical evaluation, which should include taking a medical history and a physical examination to check for suggestive symptoms and signs, a chest radiograph, and, when indicated, testing of sputum or other clinical samples for the presence of M. tuberculosis 1.
The CXR is particularly important in individuals who have symptoms, signs, or radiographic evidence of active tuberculosis or who are at increased risk for progression to active tuberculosis if infected. In such cases, a positive result with either an IGRA or TST should be taken as evidence of M. tuberculosis infection, and further evaluation is necessary to determine if the person has LTBI or active tuberculosis 1.
If the CXR is normal, and the patient has no symptoms suggestive of active TB, LTBI treatment can be initiated. Standard LTBI treatment options include isoniazid for 6-9 months, rifampin for 4 months, or isoniazid plus rifapentine weekly for 3 months (3HP regimen) 1. However, if the CXR shows abnormalities consistent with TB or the patient has symptoms, further evaluation including sputum cultures and molecular testing is necessary to exclude active TB before starting LTBI treatment.
This approach is essential because treating active TB with an LTBI regimen would be inadequate and could lead to drug resistance. Therefore, a CXR is a critical component of the evaluation of individuals with a positive IGRA result, and it should be performed to rule out active TB disease before initiating treatment for LTBI.
From the Research
Investigation of Positive Interferon-Gamma Release Assay (IGRA)
- A Chest X-Ray (CXR) is ordered to investigate a positive Interferon-Gamma Release Assay (IGRA) as it can help identify individuals with active or latent tuberculosis (TB) 2.
- CXR is a valid tool for screening active pulmonary TB in immigrants originating from high-burden countries, and it is beneficial in detecting old healed tuberculosis (OHTB) in individuals who are at a higher risk for developing active PTB 3.
- The Timika CXR score is a simple and standard scoring system that can be used to correlate CXR findings with clinical and microbiological measures of disease severity in PTB patients 4.
Diagnostic Capabilities of CXR
- CXR has been used as an aid in the diagnosis of pulmonary tuberculosis (PTB) and to determine the extent of the disease 4.
- The sensitivity and specificity of CXR findings in diagnosing smear-negative PTB patients were 77.1% and 36.9%, respectively 5.
- CXR interpretation may not add diagnostic value in settings where Xpert MTB/RIF is available as a TB diagnostic tool, especially in high HIV/TB burden settings 6.
Comparison with Other Diagnostic Tools
- Chest computed tomography (CT) is superior to quantitative IGRA testing in identifying patients with active tuberculosis among those with positive IGRA testing, especially when the "tree-in-bud" pattern is present 2.
- The addition of CXR to Xpert MTB/RIF had overall sensitivity and specificity of 87.7% and 51.6%, respectively, in the diagnosis of culture-positive TB in HIV-positive individuals 6.