Tuberculin skin test (Mantoux): how it works and how it is read

Tuberculosis (TB) is caused by Mycobacterium tuberculosis. The NHS says it usually affects the lungs, can be treated with antibiotics and can be serious if untreated, and that some people carry the bacteria without symptoms, which is called latent TB, that cannot be spread to others but can turn into active TB in future. Lab Tests Online UK adds that about 90 per cent of people who become infected control the bacteria in a few cells, while the remainder can develop active disease.

The skin test uses purified protein derivative (PPD), a solution of TB antigens without live bacteria. If you have met TB before, your immune system reacts at the injection site with a red, raised bump within a day or two. Because TB is slow to culture, up to 42 days, the skin test can suggest infection within 48 to 72 hours.

Lab Tests Online UK lists screening of people at higher risk: those with weakened immunity such as HIV, those in confined living conditions, healthcare workers, close contacts of someone with active TB, and people who have lived in a country where TB is more common, sometimes before starting a new job or course. It is not used as a general population screen.

The NHS says people with no symptoms but at risk, for example close contacts, may have a Mantoux test or blood tests, and that anyone with a cough lasting more than 3 weeks, night sweats, weight loss or unexplained tiredness should see a GP.

No preparation is needed. A healthcare professional cleans your inner forearm with alcohol and injects 0.1 mL of PPD just under the first layer of skin, forming a small blister-like bubble. The site should be left uncovered and undisturbed, with no perfume or cosmetics applied, and examined at 48 and/or 72 hours, so you must return for the reading.

You may feel mild pain, itching or redness. If the test is not done according to guidelines it needs repeating.

A positive result is a red, swollen circle at the injection site, and the diameter of the raised area determines significance. Lab Tests Online UK says the size considered positive varies with age and health status, and that children, the elderly and severely immunocompromised patients may have smaller, delayed or even negative reactions.

A negative result may mean you have not been exposed or that it is too early, as it takes about 6 weeks after infection before the test can be positive; it may be repeated. A positive result may reflect latent or active infection or occasionally a false positive, and may also occur after BCG vaccination. Positive results must be followed up with other tests such as a chest X-ray to look for active disease, with sputum cultures if active TB is suspected.

Interferon gamma release assays measure T-cell release of interferon gamma in response to TB-specific antigens. Lab Tests Online UK says they are more specific than the skin test because these antigens are not found in BCG vaccine strains or many non-tuberculous mycobacteria, give a quantitative value rather than a subjective visual reaction, and need only a single laboratory visit; they are the preferred method and are often used after a positive or inconclusive skin test. A positive IGRA means your body is likely mounting an immune response to M. tuberculosis and leads to further tests such as sputum culture.

With a positive test, a doctor looks for symptoms and arranges a chest X-ray. The NHS says latent TB is usually treated with antibiotics for 3 to 6 months and active TB for at least 6 months, and treatment must be completed even if you feel better. Do not repeat the skin test after a previous positive. Call 999 for stiff neck with severe headache, seizures or sudden confusion, and seek urgent advice if you cough up blood.