It is essential that an individual is able to give their informed consent to treatment, test or examination prior to it commencing.
This is an important part of medical ethics and human rights legislation (see the Human Rights Act 1998).
For individuals with learning disabilities, there may not be immediate certainty that they are able to give consent as they may not have the capacity to make such a decision. Having capacity means that an individual is able to understand the information given to them and then use this information to make an informed decision.
The other two pillars of consent are being informed (the individual is given all the information they need to make the decision) and the choice being voluntary (the individual makes the choice of their own free will and without pressure from others).
Having said that, individuals with learning disabilities should always be considered as having capacity unless there is reason think otherwise. It should not be presumed that just because someone has a learning disability, they lack capacity.