Tuberculosis (TB) represents a serious bacterial infection that can impact various parts of the body, most commonly the lungs. This illness is caused by Mycobacterium tuberculosis bacteria, which spread through the air when an infected individual coughs or sneezes. Testing for TB is a relevant public health measure, enabling early detection and prevention of further transmission.
Types of TB Tests
Two primary methods detect TB infection: the tuberculin skin test and blood tests. While the skin test is a traditional approach, blood tests offer an alternative, especially for individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine, which can cause false positive results with the skin test. The choice between tests depends on individual risk factors and clinical considerations.
The Tuberculin Skin Test Procedure
The tuberculin skin test (PPD test) involves injecting a small amount of purified protein derivative (PPD) tuberculin. A healthcare professional administers this solution into the top layer of skin, typically on the inner forearm, using a small needle to create a small, pale bump or wheal.
A waiting period of 48 to 72 hours is necessary before the test can be accurately read. During this time, the injection site should not be scratched or rubbed. A trained healthcare worker then examines the site, measuring the induration, which is the raised, hardened area, rather than just any redness. The measurement of this induration, in millimeters, is crucial for interpreting the test results.
Blood Test Procedure
TB blood tests (IGRAs) offer an alternative way to detect TB infection. This procedure involves a standard blood draw. The collected blood sample is then sent to a laboratory for analysis. A key advantage of blood tests is that they typically require only one visit for sample collection, eliminating the need for a return visit to read a reaction, unlike the skin test. These tests measure the immune system’s response to TB-specific antigens in the blood.
Understanding Test Results
The interpretation of TB test results depends on the specific test and individual risk factors. For the tuberculin skin test, a positive result is determined by the size of the induration in millimeters, alongside the person’s risk of infection. For example, an induration of 5 millimeters or more is considered positive for individuals with HIV or recent contact with someone with infectious TB, while 15 millimeters or more may be positive for those with no known risk factors. A positive skin test indicates that the person has been infected with TB bacteria, but not necessarily that they have active disease.
For blood tests, results are typically reported as positive or negative. A positive IGRA result means an immune reaction to TB bacteria was detected, indicating a likely infection. Conversely, a negative result suggests TB infection is not likely.
What Happens After a Positive Result
A positive TB test result indicates infection with Mycobacterium tuberculosis bacteria. Further medical evaluation is necessary to determine the type of infection and the appropriate next steps. This typically involves additional diagnostic tests, such as a chest X-ray to check for signs of active TB in the lungs. Sputum samples may also be collected and tested to detect active TB bacteria.
If active TB disease is ruled out, the individual is diagnosed with latent TB infection, which often requires treatment to prevent progression to active disease. Treatment for latent TB involves antibiotics for several months, while active TB disease requires a combination of multiple antibiotics for a longer duration. Consulting a healthcare provider is important for proper diagnosis and a personalized treatment plan.