The Tuberculosis (TB) skin test, also known as the Mantoux test, is a common procedure to determine if you have been infected with TB bacteria. If you’ve recently had a TB skin test, understanding what the results mean, especially a positive result, is crucial for your health and the health of those around you. This article will explain what a positive TB skin test looks like, what it signifies, and the next steps to take.
Decoding Latent TB Infection
To understand a positive TB test, it’s important to first understand latent TB infection. TB exists in two phases: latent TB infection and active TB disease. Both are treatable with medication. Latent TB infection occurs when TB germs enter your body and become dormant. Without treatment, this latent infection can progress into active TB disease. TB is an airborne infection, meaning anyone can contract it.
| Phase 1 – Latent TB Infection | Phase 2 – Active TB Disease |
|---|---|
| TB germs are inactive within your body. This phase can be long-lasting, even years. | TB germs become active and multiply, causing tissue damage. |
| You are asymptomatic and appear healthy. Chest X-rays usually appear normal. | You typically experience illness. Further tests are required to determine where TB is affecting your body. |
| You cannot transmit TB to others. | If TB is in your lungs, you can spread it through coughing, sneezing, speaking, or singing. |
| Usually treated with a single medication for 9 months. | Requires a combination of 3 or 4 medications for at least 6 months. |
Identifying Latent TB Infection: The Role of the Skin Test
The Mantoux TB skin test is designed to detect latent TB infection. You might be at risk if you’ve been in close proximity to someone with active TB, even if you were unaware of their illness. During the test, a healthcare provider injects a small amount of tuberculin fluid beneath the skin of your forearm using a fine needle.
It is essential to return to your healthcare provider 48 to 72 hours after the test for them to assess the reaction, regardless of how your arm looks to you.
A reaction to the tuberculin will manifest as a raised bump at the injection site. The healthcare provider will measure the diameter of this induration (raised bump). If a bump forms, it will typically subside within a few weeks.
Understanding a Negative TB Skin Test
A TB skin test is considered “negative” when there is no bump or only a very minimal bump at the injection site. Generally, a negative result indicates that you are likely not infected with TB.
However, in certain situations, a follow-up TB skin test may be necessary, especially if you have been recently exposed to TB or are at high risk.
What a Positive TB Skin Test Looks Like: Recognizing the Signs
A TB skin test is deemed “positive” if a raised bump of a specific size develops at the injection site. The size threshold for a positive test varies depending on individual risk factors, such as immune status and exposure history. A positive result suggests that you have likely been infected with TB bacteria at some point in your life.
Visually, a positive TB skin test is characterized by:
- Induration: This is the key feature. Induration refers to a palpable, raised, hardened area at the injection site. It’s not simply redness, but a swelling you can feel.
- Size: The size of the induration is critical. Healthcare providers measure the diameter of the raised, firm area in millimeters. The cut-off for a positive test varies:
- ≥5 mm: Considered positive in HIV-infected persons, recent contacts of TB cases, persons with fibrotic changes on chest radiograph consistent with prior TB, patients with organ transplants, and persons who are immunosuppressed for other reasons.
- ≥10 mm: Considered positive in recent arrivals (< 5 years) from high-prevalence countries, injection drug users, residents and employees of high-risk congregate settings, mycobacteriology lab personnel, persons with clinical conditions that place them at high risk, children < 4 years of age, and infants, children, and adolescents exposed to adults in high-risk categories.
- ≥15 mm: Considered positive in any person, including persons with no known risk factors for TB.
- Appearance: The bump is typically red, but the redness itself is not measured. The induration is the firm, raised area underneath the skin. It may resemble a wheal, similar to a mosquito bite, but firmer and more defined.
It’s important to note that a positive skin test only indicates TB infection; it does not differentiate between latent TB infection and active TB disease. Further evaluation is necessary to determine if you have active TB disease. This usually involves a chest X-ray and possibly sputum tests.
If you have a positive TB skin test, it’s crucial to consult your doctor for further evaluation and appropriate treatment.
Next Steps After a Positive TB Skin Test
A positive TB skin test is not a diagnosis of active TB disease, but it does mean further investigation is needed. Your doctor will typically order a chest X-ray to look for signs of active TB in your lungs. You may also need to provide sputum samples to test for TB bacteria.
Most individuals with a positive TB skin test have latent TB infection. Preventive treatment is usually recommended for latent TB infection to reduce the risk of developing active TB disease in the future. Treatment typically involves a course of antibiotics.
Who Should Get a TB Skin Test?
You should consider getting a TB skin test if you fall into any of these categories:
- Close Contact: If you have been in close contact with someone known to have active TB disease.
- Travel History: If you have lived in or traveled to countries with a high prevalence of TB.
- High-Risk Environments: If you work or reside in settings like nursing homes, hospitals, prisons, or homeless shelters.
- Compromised Immune System: If you have HIV infection or other health conditions that weaken your immune system.
TB Skin Test and BCG Vaccination
Having received the Bacille Calmette-Guérin (BCG) vaccine in the past does not preclude you from getting a TB skin test.
- BCG vaccinated individuals can still develop both latent TB infection and active TB disease.
- BCG vaccine offers some protection to young children against severe forms of TB, but this protection diminishes with age.
- BCG vaccination can sometimes cause a false-positive TB skin test reaction. However, in most cases, a positive reaction in a BCG-vaccinated person is more likely due to TB infection rather than the vaccine itself.
It is still important to get a TB skin test, even if you have been vaccinated with BCG.
Protect yourself and your community – get tested for TB if you are at risk! Contact your healthcare provider or local health department to schedule a TB skin test.
