What is a TB skin test or blood test and how accurate is it?
Tuberculosis testing is a fundamental part of public health screening in the United Kingdom, particularly for individuals who may have been exposed to the bacteria or those arriving from countries with high infection rates. There are two primary methods used to detect tuberculosis infection: the Mantoux skin test and the Interferon-Gamma Release Assay, which is a specialised blood test. Both tests are designed to identify whether the body’s immune system has encountered the tuberculosis bacteria, but they do so in different ways. Understanding the differences between these diagnostic tools is essential for interpreting results accurately and determining the next steps for clinical care or preventative treatment.
What We’ll Discuss in This Article
- The procedure and purpose of the Mantoux skin test.
- How the Interferon-Gamma Release Assay blood test functions.
- Comparison of accuracy between skin and blood testing methods.
- The impact of the BCG vaccine on tuberculosis test results.
- Why these tests cannot distinguish between latent and active infection.
- NHS protocols for follow up investigations after a positive result.
The Mantoux skin test procedure
The Mantoux test is the traditional method used by the NHS to screen for tuberculosis infection by checking the skin’s sensitivity to tuberculin. During the procedure, a clinician injects a tiny amount of purified protein derivative, known as tuberculin, just under the surface of the skin on the forearm. This injection typically creates a small, pale bump that disappears quickly. The patient must then return to the clinic between 48 and 72 hours later to have the site examined by a healthcare professional. The Mantoux test is the standard method used in the UK to screen for tuberculosis. The clinician looks for a raised, hardened area called an induration, and the diameter of this hardened area is measured in millimetres to determine if the result is positive.
Understanding the tuberculosis blood test
The Interferon-Gamma Release Assay, commonly known as an IGRA, is a modern blood test used to detect the immune system’s response to the tuberculosis bacteria. Unlike the skin test, it only requires a single blood sample and does not require the patient to return for a manual reading of the skin. The test works by measuring the amount of interferon gamma, a protein produced by the immune system, when it is exposed to specific tuberculosis antigens in a laboratory setting. The IGRA is a blood test for tuberculosis that is not affected by the BCG vaccine. Because it focuses on proteins that are specific to the tuberculosis bacteria, it is often more precise than the skin test for individuals who have previously received the tuberculosis vaccination.
Accuracy and the impact of the BCG vaccine
The accuracy of a tuberculosis test depends heavily on the individual’s vaccination history and the specific type of test used. The Mantoux skin test has a high specificity in people who have not been vaccinated, meaning it rarely gives a positive result if the person is truly uninfected. However, because the tuberculin used in the skin test is similar to the bacteria used in the BCG vaccine, the test often produces “false positive” results in people who have had the vaccine. In these cases, the skin reacts not because of a tuberculosis infection, but because of the prior immunisation. The blood test is considered more accurate in these populations because it uses antigens that do not cross-react with the BCG vaccine, significantly reducing the likelihood of a false positive.
Clinical interpretation of positive results
It is important to understand that neither the skin test nor the blood test can confirm whether a person has active tuberculosis disease. Both tests identify a tuberculosis infection, but they cannot tell the difference between “latent” tuberculosis, where the bacteria are dormant in the body, and “active” tuberculosis, where the person is ill and potentially infectious. A positive result on either test simply indicates that the immune system has recognised the bacteria at some point. If a result is positive, the NHS typically follows a protocol that includes a chest X-ray and a review of symptoms to determine if the infection is active or latent. This distinction is vital because latent tuberculosis is not contagious and is treated differently than the active form of the disease.
Factors that may affect test reliability
Several factors can influence the reliability of tuberculosis testing, potentially leading to inaccurate results. In individuals with weakened immune systems, such as those with HIV or those taking immunosuppressant medications, the body may not be able to mount a visible reaction to the skin test or produce enough interferon gamma for the blood test. This can result in a “false negative,” where the test shows no infection even though the bacteria are present. Additionally, the skin test requires precise administration and an accurate reading within the 72 hour window. If the test is read too early or too late, the measurement may not reflect the true status of the infection. Because of these variables, clinicians often consider a patient’s full medical history and risk factors rather than relying solely on a single test result.
Conclusion
The Mantoux skin test and the IGRA blood test are both effective tools for identifying tuberculosis exposure, though their accuracy varies based on an individual’s vaccination history. While the skin test is a long standing screening method, the blood test offers higher specificity for those who have received the BCG vaccine. Neither test can diagnose active disease on its own, and a positive result will always require further medical investigation through imaging and symptom assessment.
If you experience severe, sudden, or worsening symptoms, such as coughing up blood or a high fever with a persistent cough, call 999 immediately.
Can a TB skin test give a false positive?
Yes, the skin test often gives a false positive result in people who have had the BCG vaccine because the test reacts to the vaccine’s components.
How long does it take to get TB blood test results?
While the blood is drawn in one visit, it usually takes several days to a week for the laboratory to process the sample and provide the results.
What happens if my Mantoux test is positive?
If your test is positive, you will usually be referred for a chest X-ray and a clinical assessment to check for signs of active tuberculosis.
Is the TB blood test better than the skin test?
The blood test is often preferred for people who have been vaccinated with the BCG, as it is less likely to produce a false positive result.
Does a negative TB test mean I am definitely not infected?
A negative result is highly reliable, but it can be inaccurate in people with very recently acquired infections or severely weakened immune systems.
Can I have a TB test if I am pregnant?
Yes, both the skin test and the blood test are considered safe during pregnancy and do not pose a risk to the developing baby.
What is the difference between latent and active TB?
Latent TB means you have the bacteria but are not ill and cannot spread it, while active TB means the bacteria are making you sick and you can infect others.
Authority Snapshot (E-E-A-T Block)
This article is developed to provide clear, factual information about the diagnostic processes for tuberculosis in the UK. The content is strictly aligned with the clinical pathways established by the NHS and the National Institute for Health and Care Excellence (NICE). It has been authored by a medical content team and reviewed by Dr. Stefan Petrov, a UK-trained physician, to ensure it meets the highest standards of medical accuracy and safety.
