Do you mean a vaginal culture? If so, it's probably to check the bacteria ratio.
Thank you for your answer.
Do you know if this cold be done during your period?
I'm not sure. It probably depends on the reason they're doing the culture. The best thing to do is probably to just call the doctor's office and ask. They usually have a nurse-line where you can leave a message and then they'll call you back with an answer. Good luck!