Causes and diagnoses of tuberculosis

TB or Tuberculosis is caused by a bacterium called Mycobacterium tuberculosis. The condition is spread when a person with an active TB infection in the lungs, coughs or sneezes and someone else inhales the expelled droplets, which contain TB bacteria. Although it is spread in a similar way to a cold or the flu, TB is not as transmittable. One would usually have to spend prolonged periods in close contact with an infected person to catch the infection. For example, TB infections usually spread between family members who live in the same house. It would be highly unlikely to become infected by sitting next to an infected person on a bus or train. Not everyone with TB is infectious. Generally, children with TB or people with TB that occurs outside the lungs (extra-pulmonary TB) do not spread the infection.

 

Latent or active TB

Usually immune system of the healthy people is able to destroy the bacteria that cause TB. However in some cases the bacteria infect the body but don't cause any symptoms (latent TB), or the infection begins to cause symptoms within weeks or months (active TB). Up to 10% of people with latent TB eventually develop active TB years after the initial infection. This usually happens when the immune system is weakened – for example, during chemotherapy.

Who's most at risk?

Anyone can catch TB, but people particularly at risk include those:

• Who live in, come from, or have spent time in a country or area with high levels of TB. For example around three in every four TB cases in UK, affect people born outside UK
• In prolonged close contact with someone who is infected
• Living in crowded conditions
• With a condition that weakens their immune system, such as HIV
• Having treatments that weaken the immune system, such as corticosteroids, chemotherapy or tumor necrosis factor (TNF) inhibitors (used to treat some types of arthritis and certain gut conditions)
• Who are very young or very old – the immune systems of people who are young or elderly tend to be weaker than those of healthy adults
• In poor health or with a poor diet because of lifestyle and other problems, such as drug misuse, alcohol misuse, or homelessness

Diagnosing tuberculosis 

Several tests are used to diagnose tuberculosis (TB), depending on the type of TB suspected. One’s GP may refer him/her to a TB specialist for testing and treatment if they think that the suspected person have TB.

Pulmonary TB

Diagnosing pulmonary TB (TB that affects the lungs) can be difficult and several tests are usually needed. This will include a chest X-ray to look for changes in the appearance of your lungs that are suggestive of TB. Samples of phlegm will also often be taken and checked for the presence of TB bacteria. These tests are important in helping to decide on the most effective treatment.

Extra-pulmonary TB

If someone is suspected the extra-pulmonary TB, (a kind of TB, that occurs outside the lungs), several tests can be used to confirm the diagnosis. These may include:

• A computerized tomography (CT) scan, magnetic resonance imaging (MRI) scan, or ultrasound scan of the affected part of the body
• Urine and blood tests
• Biopsy – a small sample of tissue or fluid is taken from the affected area and tested for TB bacteria

Patient may also have a lumbar puncture. This involves taking a small sample of cerebrospinal fluid (CSF) from the base of your spine. CSF is fluid that surrounds the brain. It can be checked to see whether TB has infected the patient’s central nervous system (brain and spinal cord).

 

Screening for latent TB

In some circumstances, the suspected person may need to be tested to check for latent TB (when someone have been infected with TB bacteria but do not have any symptoms). For example, the sufferer may need to be screened if he/she have been in close contact with someone known to have an active TB infection (an infection that causes symptoms), or if he/she have recently spent time in a country where TB levels are high. If he/she have just moved to any other country from a country where TB is common, the suspected may be screened when he/she arrives or his/her GP may suggest screening when he/she is registered as a patient.

Mantoux test

Mantoux test is a widely used test for latent TB. It involves injecting a substance called PPD tuberculin into the skin of one’s forearm. It's also called the tuberculin skin test (TST). If the suspected person have a latent TB infection, his/her skin will be sensitive to PPD tuberculin and a hard red bump will develop at the site of the injection, usually within 48 to 72 hours of having the test. If the person has a very strong skin reaction, he/ she may need a chest X-ray to confirm whether the suspected have an active TB infection.

If the person does not have a latent infection, the skin will not react to the Mantoux test. However, as TB can take a long time to develop, one may need to be screened again at a later stage. If the suspected person has had the BCG vaccination, he/ she may have a mild skin reaction to the Mantoux test. This does not necessarily mean that the suspected one have latent TB.

Interferon gamma release assay (IGRA)

It is a newer type of blood test for TB that is becoming more widely available. The IGRA may be used to help diagnose latent TB:

• If some patient have a positive Mantoux test
• If the patient previously had the BCG vaccination (as the Mantoux test may not be reliable in these cases)
• As part of one’s TB screening if the person have just moved to the UK from a country where TB is common
• As part of a health check when the person is registered with a GP
• If one is about to have treatment that will suppress his/her immune system
• If someone is a healthcare worker