If you need blood tests, you need to go to the laboratory of Certe. Please check their website. For most tests you can visit them without an appointment, although in times of corona they ask you to make an appointment online.
For some tests ( for example a sementest) you need to call them first to make an appointment as they do these tests only on certain times/days.
Certe Groningen main location: Damsterdiep 191
Please visit their website for other locations.