Tuberculosis (TB) is an infectious disease primarily affecting the lungs, caused by the bacterium Mycobacterium tuberculosis. Diagnosing TB promptly and accurately is crucial for effective treatment and prevention of transmission. Here, we’ll explore some of the most common tests for diagnosing TB, how they work, and their relative advantages and limitations.
Chest X-Ray
Chest X-rays can reveal abnormalities in the lungs commonly associated with tuberculosis (TB). Chest X-rays produce images of the lungs, showing areas that may be affected by Tuberculosis (TB). Typical findings include cavities, enlarged lymph nodes and miliary nodules. Other findings which might be seen with Tuberculosis (TB) but might be common with other conditions are pneumonias, pleural effusion etc. Chest Xray is a quick and non-invasive test which aids in diagnosing active Tuberculosis (TB) when combined with other tests. It is a very important initial screening test. Although, chest x ray may not be very accurate as other lung conditions may show similar patterns.
Sputum Testing
For diagnosing active tuberculosis (TB), sputum examination is commonly used. A sample of sputum (mucus coughed up from the lungs) is stained and examined under a microscope for the presence of acid-fast bacilli. A positive result indicates TB bacteria in the sputum, confirming active TB.
Molecular tests, such as the GeneXpert MTB/RIF, are newer technologies used for rapid detection of TB and rifampicin resistance. These tests detect the DNA of TB bacteria and determine if the bacteria are resistant to rifampicin, a key first-line TB drug, within a few hours. GeneXpert MTB/RIF is highly accurate and fast, with results in under 2 hours. It is very useful in diagnosing drug-resistant TB. GeneXpert MTB/RIF is now widely available and considered to be the first line test by the National TB program of India.
Culturing Mycobacterium tuberculosis is the gold standard for TB diagnosis because of its high sensitivity and ability to test for drug resistance. AFB culture or MGIT TB culture is a very accurate test put on sputum samples, even in cases with low bacterial load. Culture test also allows for drug susceptibility testing to guide effective treatment. An important drawback of culture is its slow process, taking up to 6-8 weeks for results.
Children who are unable to bring out sputum alternate methods are used to collect the samples for testing. Common methods are obtaining a gastric lavage specimen or an induced sputum test.
Bronchoscopy and Bronchoalveolar Lavage
Bronchoscopy is a valuable diagnostic tool in certain cases of tuberculosis (TB), especially when standard methods like sputum tests are inconclusive. This procedure allows doctors to directly visualize the airways and obtain samples from the lungs (bronchoalveolar lavage), making it especially useful in cases where TB is suspected but difficult to confirm. Bronchoscopy can also help assess TB-related damage within the airways, such as strictures, obstructions, or inflammatory changes, which can impact treatment planning.
CT Scans Chest
CT (Computed Tomography) scans have become an essential tool, especially in complex or uncertain cases. While not used as a first-line diagnostic test, CT scans can provide detailed, cross-sectional images of the chest, making them invaluable in detecting TB’s impact on the lungs and other areas. While highly informative, CT scans alone cannot confirm TB, as other lung diseases can appear similar on imaging. They are also more costly than chest X-rays and require specialized equipment. In addition they cannot confirm the presence of TB bacteria in the lungs which is only possible with microbiological tests. Radiation exposure to the child is another concern.
Tuberculin Skin Test (TST)
The Tuberculin Skin Test, or Mantoux test, is one of the oldest and most widely used methods for diagnosing latent TB infection (LTBI). A small amount of purified protein derivative (PPD) is injected just under the skin of the forearm. After 48-72 hours, a healthcare provider measures the reaction at the injection site. If a raised, red bump forms, this indicates the immune system has been exposed to TB bacteria. It is a widely available and low-cost simple procedure. Although it can identify latent TB infection, it cannot differentiate between latent and active TB. False positives may occur, especially in those who have received the BCG vaccine and false negatives are also possible in immunocompromised individuals
Interferon-Gamma Release Assays (IGRAs)
IGRAs, including tests like QuantiFERON-TB Gold and T-SPOT.TB, are blood tests used primarily for diagnosing latent TB infection. The test measures the immune response (release of interferon-gamma) when blood cells are exposed to TB antigens. A positive IGRA result suggests TB exposure, but like the TST, it does not distinguish between latent and active TB. IGRA tests have no cross-reaction with the BCG vaccine, reducing false positives. IGRA’s are more complex and costly.
Choosing the Right Test: Summary
Diagnosing TB accurately and promptly is essential in the global fight against this disease. Each of these tests offers unique advantages that, when used in combination, can help ensure that patients receive timely treatment, reducing transmission and improving health outcomes. The choice of test depends on the suspected type of TB (active or latent), the patient’s history, resource availability, and the setting. Early diagnosis and effective treatment remain the cornerstone of TB control, helping to reduce its spread and impact worldwide.