Tuberculosis (TB) is a serious bacterial infection primarily affecting the lungs, though it can impact other body parts like the brain or kidneys. This infection spreads through the air when an infected person coughs or sneezes, making screening a crucial step in public health efforts.
Understanding the Tuberculosis Skin Test
The tuberculosis skin test, often known as the Mantoux tuberculin skin test or PPD test, is a common method for detecting TB infection. This procedure involves a healthcare provider injecting a small amount of purified protein derivative (PPD) tuberculin just beneath the skin, typically on the inner forearm. The injection creates a small, pale bump called a wheal, which indicates correct administration. The test measures the immune system’s reaction to TB proteins, indicating if a person has been exposed to the TB bacterium.
After the injection, the test site must be examined by a healthcare professional within 48 to 72 hours. This specific timeframe is important, as results read outside this window may not be accurate and could require retesting. The test does not contain live bacteria, ensuring it cannot cause a TB infection.
Safety and Purpose During Pregnancy
The tuberculosis skin test is generally considered safe and reliable throughout all stages of pregnancy. It poses no known adverse effects on the pregnant individual or the developing fetus. This safety stems from the nature of the test itself; it is a simple skin injection of inactivated proteins, not a live vaccine, and it does not involve radiation.
Pregnant individuals might need a TB skin test under specific circumstances. Healthcare providers often recommend testing for those with recent exposure to someone with active TB disease, or if they reside in or have traveled to areas with high rates of TB. Testing is also advised for pregnant individuals with certain medical conditions that can weaken the immune system, such as HIV, diabetes, or those on immunosuppressive medications.
Interpreting Test Results
Interpreting the results of a TB skin test involves carefully measuring the induration, which is the hardened, raised bump at the injection site. Redness around the site is not measured, as it does not indicate a positive reaction. A healthcare provider uses a ruler to measure the diameter of this induration in millimeters.
A negative test result typically shows no induration or a very small bump, usually less than 5 millimeters. This generally means that the individual has not been infected with TB bacteria. A positive result, indicated by a significant induration, suggests exposure to TB bacteria. The size of induration considered positive can vary depending on individual risk factors; for example, an induration of 5 millimeters or more may be positive for those with HIV, while 15 millimeters or more is considered positive for individuals with no known risk factors. A positive skin test does not necessarily mean active TB disease, but it prompts further evaluation, which may include a chest X-ray with abdominal shielding or additional blood tests.
Alternative Screening Options
Beyond the skin test, Interferon-Gamma Release Assays (IGRAs) offer another method for TB screening. These are blood tests, such as QuantiFERON-TB Gold or T-SPOT.TB, which also measure the immune system’s response to TB proteins. Like the skin test, IGRAs are considered safe for use during pregnancy.
One advantage of IGRAs is that they typically require only a single visit for a blood draw, unlike the skin test which requires a return visit for reading. Additionally, IGRAs are not affected by prior Bacille Calmette-Guérin (BCG) vaccination, which can sometimes cause a false-positive result on the TB skin test. While both testing methods are effective, the choice between a skin test and an IGRA may depend on individual circumstances, such as a history of BCG vaccination or specific clinical guidelines.