Tuberculosis (TB) Testing
Nov 25
Tuberculosis (TB) Testing: Importance, Methods, and Interpretation
Tuberculosis (TB) remains a major global health concern, and early detection through testing is key to preventing its spread. TB is a bacterial infection caused by Mycobacterium tuberculosis, primarily affecting the lungs but potentially involving other organs. Testing is critical for identifying both active TB and latent TB infection (LTBI), which can remain dormant in the body for years without symptoms.
Why is TB Testing Important?
TB testing helps detect both latent and active infections. People with active TB can transmit the bacteria to others through coughing, sneezing, or speaking. Latent TB, although not infectious, can progress to active TB if left untreated. Early identification and treatment can prevent this progression and reduce transmission.
Who Should Get Tested?
Testing is especially important for individuals at higher risk of TB, including:
- People who have close contact with someone with active TB.
- Healthcare workers who may be exposed to TB patients.
- Individuals with weakened immune systems, such as those with HIV or diabetes.
- Immigrants from regions with high TB prevalence.
- People who have been in settings with increased risk, like prisons or shelters.
Methods of TB Testing
There are two primary tests used to diagnose TB:
- Tuberculin Skin Test (TST)
- Also known as the Mantoux test, the TST is the most common method for screening latent TB infection. A small amount of purified protein derivative (PPD) from the TB bacteria is injected just under the skin on the forearm.
- After 48 to 72 hours, a healthcare provider checks for a raised, hard area at the injection site. A positive result indicates that a person has been exposed to the TB bacteria, but it does not necessarily mean they have active TB. Further testing, such as a chest X-ray, may be required to confirm whether the infection is active.
- Interferon-Gamma Release Assays (IGRAs)
- IGRAs are blood tests that measure the immune system’s response to TB bacteria. They are often used for individuals who have received the BCG vaccine (common in countries with high TB rates) since the TST may produce a false positive in vaccinated individuals. IGRAs provide more accurate results in such cases and are becoming more widely used in TB screening.
Interpreting TB Test Results
- A positive TST or IGRA result indicates exposure to TB, but further tests (such as a chest X-ray or sputum test) are needed to determine if the infection is active or latent. Active TB usually presents with symptoms such as a persistent cough, weight loss, fever, and night sweats.
- A negative result means the person has not been exposed to TB or is not infected at the time of testing. However, individuals with weakened immune systems, such as those on immunosuppressive therapy, may have false-negative results.
What Happens After a Positive Test?
If TB infection is suspected, a chest X-ray or sputum culture is typically ordered. Active TB is diagnosed if bacteria are found in sputum samples or X-ray images suggestive of TB. Treatment usually involves a lengthy course of antibiotics, often for 6-9 months, to ensure complete eradication of the bacteria. Latent TB can also be treated with medication to prevent future development of active TB.
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