A TB skin test, also known as a Mantoux tuberculin skin test (TST) or purified protein derivative (PPD) test, is a common diagnostic tool used to determine if a person has been infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB). It measures your immune system's response to a purified protein derivative (PPD) solution injected under your skin. PPD is a protein derived from the bacteria that causes TB; it is not live bacteria and will not make you sick.
How a TB Skin Test Works
The principle behind the TB skin test relies on the body's immune memory. If you have been infected with TB bacteria in the past, your immune system will have developed a specific response to its proteins.
- Injection of PPD: A small amount of PPD solution is carefully injected just under the top layer of skin on your forearm, creating a small, pale bump (wheal). This injection is called an intradermal injection.
- Immune System Response: If you have been previously exposed to TB bacteria, your immune system will recognize the PPD proteins and mount a localized immune reaction at the injection site. This reaction typically becomes visible within 48 to 72 hours.
- Safety: It's important to reiterate that the PPD solution does not contain live TB bacteria. Therefore, getting a TB skin test cannot cause a TB infection.
The Administration and Reading Process
Administering and reading a TB skin test requires specific steps to ensure accuracy:
- Administration: A healthcare professional injects the PPD solution into the inner forearm. The injection site should not be touched, scratched, or covered with a bandage.
- Waiting Period: You must return to the healthcare provider within 48 to 72 hours after the injection to have the test site examined.
- Reading the Result: The healthcare professional will visually inspect and carefully feel the injection site to check for a raised, firm area, known as induration. They will measure the diameter of this induration across the forearm, not including any redness.
Interpreting TB Skin Test Results
Interpreting a TB skin test result depends on the size of the induration and the individual's risk factors for TB exposure or progression to active disease. A positive result indicates that the person has been infected with TB bacteria, but it does not necessarily mean they have active TB disease.
| Induration Size | Interpretation |
|---|---|
| ≥ 5 mm | Positive for individuals in high-risk categories, such as: |
| - People with HIV | |
| - Recent contacts of individuals with active TB | |
| - Organ transplant recipients | |
| - People who are immunosuppressed (e.g., taking TNF-α inhibitors, prolonged corticosteroid therapy) | |
| ≥ 10 mm | Positive for individuals in medium-risk categories, such as: |
| - Recent immigrants from high TB prevalence countries | |
| - IV drug users | |
| - Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters) | |
| - Mycobacteriology laboratory personnel | |
| - Children under 5 years of age | |
| - People with certain medical conditions (e.g., diabetes, kidney failure, silicosis, gastric bypass, leukemia, lymphoma) that increase the risk of progressing from latent to active TB. | |
| ≥ 15 mm | Positive for individuals with no known risk factors for TB. In this group, a positive result suggests infection with Mycobacterium tuberculosis. |
| 0 - 4 mm | Negative for all individuals, indicating that the person is unlikely to have been infected with TB bacteria. |
A negative result typically means you have not been infected with TB bacteria. However, it can also occur in individuals who have been recently infected (before the immune system has responded) or in those with very weak immune systems.
Who Should Get a TB Skin Test?
The TB skin test is primarily used for screening and diagnosis in specific populations. Your doctor may recommend a TB skin test if you:
- Have been in close contact with someone known to have active TB disease.
- Are a healthcare worker or work in other settings where TB exposure is possible (e.g., correctional facilities, homeless shelters).
- Were born in or frequently travel to countries with high rates of TB.
- Have a weakened immune system due to conditions like HIV, diabetes, or certain medications.
- Are a child under 5 years of age who has been exposed to an adult at high risk for TB.
Important Considerations
While the TB skin test is a valuable tool, it has certain limitations and requires careful follow-up:
- False Positives: A positive result can sometimes occur in individuals who have received the BCG vaccine (Bacille Calmette-Guérin), a vaccine used in some countries to prevent TB, or those infected with non-tuberculous mycobacteria.
- False Negatives: A negative result does not always rule out TB infection, especially in individuals with severely compromised immune systems or those very recently exposed.
- Alternative Tests: For certain situations, blood tests, known as Interferon-Gamma Release Assays (IGRAs), like QuantiFERON-TB Gold Plus or T-SPOT.TB, are available and may be preferred as they are not affected by prior BCG vaccination.
- Follow-Up: A positive TB skin test result typically requires further evaluation, which may include a chest X-ray, symptom review, and potentially a sputum test, to determine if you have latent TB infection or active TB disease. Early detection and treatment of latent TB can prevent the development of active disease.
