If you are applying to come to the UK for more than 6 months, from a country which is considered to have a high incidence of tuberculosis (TB), you must provide a valid medical certificate (issued by a Home Office approved medical practitioner) which confirms you have undergone screening and have been diagnosed as free from active TB.
The medical certificate confirming that you are free from TB must be submitted with your application and the Home Office recommends that this is carried in your hand luggage when you come to the UK to present to the immigration officer on arrival if requested.
The list of countries affected and country-specific guidance in relation to the process and testing of dependants (including children) can be found on the Home Office website.
If you are required to undertake TB screening we recommend you do this as early as possible. While the results of the test should be made available quite quickly, if the results are inconclusive additional tests will be required. Where TB is detected a medical certificate will not be issued until after treatment has been received (which can take up to 6 months).