What they are

These tests involve injecting small amounts of protein from the mycobacteria that cause tuberculosis into the skin. PPD stands for 'purified protein derivative' which tells you that this is completely non-infectious. It is not a vaccine and it should be completely harmless, although there may be an immune reaction after a couple of days, seen as reddening and hardening of the area where the injection was made.

What it means

The aim of the test is to identify people who have recently been exposed to TB or who have active disease. It may be offered to household contacts of anyone identified as having been exposed to active TB. After further tests, treatment may be offered to anyone who shows a strong positive reaction.

Unfortunately, a negative reaction on this test does not prove the absence of TB. Especially among people with HIV, it may instead show that the ability to respond to TB has diminished, even though TB may be present or even causing disease. A further limitation of the test is that some people have some positive reaction as a result of previous BCG vaccination. There is therefore some difference of views among doctors as to the usefulness of this test and practice varies from country to country, just as there are differences in national policies on the use of BCG.