Your GP contacts you, explains what the result means and sets out the next step. Some infections can be managed straightforwardly in primary care. Others, including HIV, syphilis, gonorrhoea and hepatitis B or C, are managed by specialist NHS sexual health services, both because that is where the expertise sits and because of antibiotic resistance monitoring and follow-up testing.
Where a specialist service is the right route, you are referred there with confirmed laboratory results, which makes that appointment considerably more productive than arriving with a question. Your GP will also talk through partner notification, which matters and is more manageable than most people expect.