Tuberculosis Skin Test: What to Expect, Results, and Why It Matters
If you have recently started a new job in a healthcare centre, planned a trip to a high-risk area, or been in close contact with someone who has a persistent cough, you might have been asked to get a tuberculosis skin test. While the idea of a medical test can feel daunting, this common diagnostic tool is a simple, effective way to ensure your lungs and your loved ones stay healthy.
Tuberculosis (TB) remains a significant global health concern, but with early TB screening, it is highly manageable. In this guide, we will break down the procedure, how to read your results, and what happens next if your test comes back positive.
What is a Tuberculosis Skin Test?
The tuberculosis skin test, also frequently referred to as the Mantoux test or the PPD skin test (Purified Protein Derivative), is used to determine if you have been infected with Mycobacterium tuberculosis, the bacterium that causes TB. It does not measure whether you have active TB disease, but rather if your immune response recognises the bacteria.
According to the World Health Organization, millions of people carry the bacteria in a state known as latent TB infection. In this state, you aren’t contagious and don’t feel ill, but the bacteria are “sleeping” in your body and could wake up later.
How the TST Test is Performed
The TST test is a two-stage process that requires two separate visits to a healthcare professional. It is a very routine procedure, often required for any healthcare worker or individuals travelling to specific regions.
- The Injection: A clinician will utilise a tiny needle to inject a small amount of PPD fluid just under the top layer of skin on your internal forearm. This usually creates a small, pale bump (often called a wheal).
- The Waiting Period: You must wait 48 to 72 hours for a reaction to occur. During this time, you must avoid scratching or covering the site with a bandage.
- The Reading: You return to the clinic where a professional looks for a hard, raised area called an induration. They measure the diameter of this swelling, not the colour or redness.
According to the NHS, failing to return for the reading within 72 hours means you will likely have to start the process all over again.
Understanding Your Results
Interpreting a tuberculosis skin test is more complex than a simple “yes” or “no.” The measurement of the induration is interpreted based on your specific risk factors. For example, a 5mm swelling might be considered a positive result in someone with an impaired immune system, whereas a 15mm swelling is the threshold for someone with no known risk factors.
If you have previously received the BCG vaccine, which is common in many countries outside the UK and US, it can sometimes cause a “false positive” on a skin test. In these cases, doctors often prefer a TB blood test, also known as an IGRA.
Comparing TB Testing Methods
To help you understand your options, here is a comparison between the traditional skin test and the newer blood tests:
| Feature | Tuberculosis Skin Test (Mantoux) | TB Blood Test (IGRA) |
|---|---|---|
| Number of Visits | Two visits required | One visit required |
| Results Time | 48–72 hours | 24–48 hours |
| BCG Vaccine Impact | Can cause false positives | Not affected by BCG vaccine |
| Cost | Generally lower | Generally higher |
| Primary Use | Routine screening/workplace | Confirmation/BCG-vaccinated individuals |
What Happens if You Have a Positive Result?
A positive result does not automatically mean you have pulmonary tuberculosis. It simply means your body has encountered the TB bacteria before. To determine if the infection is active or latent, your doctor will likely order further tests, such as:
- A chest X-ray to look for changes in the lungs.
- Sputum tests to see if bacteria are present in your cough.
- A physical examination to check for symptoms like night sweats, weight loss, or persistent fever.
If the infectious disease specialist confirms you have latent TB infection, they may recommend a course of preventative antibiotics to ensure it never turns into active TB disease. You can find more about treatment protocols from the Mayo Clinic.
Factors That Can Affect Your Test
Several factors can influence how your body reacts to the tuberculosis skin test. It is essential to share your full medical history with your provider. For more information on immune health, visit Nature’s Immunology section.
- Recent Vaccinations: Certain live-virus vaccines (like MMR) can temporarily suppress your reaction to the PPD test.
- Immune Suppression: Conditions like HIV or medications like chemotherapy can lead to a “false negative” because the body cannot mount a visible immune response.
- Previous Infection: Once you have had a positive TB skin test, you will likely always test positive, even after successful treatment.
Resources like NICE Guidelines provide extensive details for clinicians on how to manage these variables.
Maintaining Lung Health
While the tuberculosis skin test is a vital tool, general respiratory health is equally important. Organizations like the American Lung Association and the British Lung Foundation offer resources on keeping your lungs clear and identifying symptoms early. If you are a healthcare worker, staying up to date with your TB screening is not just a workplace requirement but a critical step in public health safety.
If you are concerned about your exposure risk, you can check the UK Government’s TB by country rates to see if your recent travels warrant a check-up. Reliable health information can also be found on WebMD or Patient.info.
Frequently Asked Questions (FAQs)
Can I shower after a tuberculosis skin test?
Yes, you can shower and wash your arm normally. However, you should avoid rubbing or scrubbing the injection site. Pat the area dry gently with a towel. Do not apply lotions, creams, or bandages to the site, as this could irritate the skin and interfere with the measurement of the induration.
What does a “negative” result mean?
A negative result usually means your body has not been infected with the TB bacteria. However, it can take up to 10 weeks after exposure for the immune system to react to the test. If you were tested very recently after being exposed, you might need a follow-up test to be certain. For more on testing windows, see MedlinePlus.
Why is the skin test done in two steps sometimes?
The “two-step” tuberculosis skin test is often used for adults who will be tested periodically, such as healthcare workers. Sometimes, an old infection “fades” and doesn’t show up on the first test. The first injection stimulates the immune system, and the second test (done 1–3 weeks later) provides a more accurate diagnostic tool baseline. Detailed protocols are available via the CDC.
Is the test safe during pregnancy?
Yes, the tuberculosis skin test is considered safe for pregnant individuals. The PPD solution does not contain live bacteria and will not harm the baby. In fact, identifying TB during pregnancy is crucial for the health of both the parent and the child. You can find guidance on this from Johns Hopkins Medicine.
Where can I get a tuberculosis skin test?
Most local GP surgeries, occupational health clinics, and travel clinics provide the Mantoux test. If you are a student or volunteer, organisations like the Red Cross or your university health centre may offer screening programmes. For specialised advice, consult Cleveland Clinic.
