Because the symptoms of TB can resemble those of other illnesses seen in people with HIV, doctors will often carry out a number of tests to see if it is TB or something else causing illness. Also, symptoms of TB can come on very slowly, often over a period of months, and it can be hard for either the person with TB or their doctor to recognise them.

Chest X-rays are the standard test. TB can show up in a number of ways:

• Active TB can cause white patches to show up on the X-ray. These can have holes or cavities in the middle of them.
• Something called a ‘pleural effusion’ may form. This is fluid on the lung and it shows up as a block of white at the bottom of the lung.

Samples of phlegm can be checked for TB germs. If these are present it shows that a person has active TB and that they could potentially infect other people with TB.

Sometimes the lung is examined with a tiny camera that is put down the nose. This is done under local anaesthetic. It is called a bronchoscopy and will only be conducted if doctors are uncertain what is causing the illness.

Samples of tissue (a biopsy) from the place of infection may be taken under anaesthetic for examination under a microscope. Again, this should only be performed if doctors cannot diagnose the cause of illness using simpler methods.

When TB germs are found they will be checked in the laboratory to see which anti-TB drugs work against them. This will help doctors to make sure that you're taking the right TB treatment.

Tests are also available to see if a person has latent TB - TB that is present in the body, but not causing illness.

Chest X-rays can be used to diagnose latent TB. The scar tissue, which often contains ‘chalk’ (or calcium), around the TB shows as a ‘shadow’.

Another test is called a Mantoux or tuberculin skin test (sometimes called a purified protein derivative or PPD test). This involves injecting a small amount of dead, purified TB protein into the skin. After a few days, the area of injection may show a reaction by reddening or hardening. The larger the size of this reaction, the more likely it is that a person has been infected with TB in the past and has either active TB or latent infection.

However, a lack of reaction does not prove that a person does not have TB, and this is particularly the case in people who have very weakened immune systems. What’s more, the Mantoux test does not provide very accurate results in people who have had the BCG TB vaccination (there’s more about this in the next section). This used to be given to most school children in the UK.

A newer, more reliable and quicker blood test has been developed called the T SPOT-TB test. It looks for key immune cells called T cells that the body produces in response to infection with TB. There is some evidence to show that it can be better at detecting TB in people with weakened immunity due to HIV than the Mantoux test.