London says "
When scheduling the appointment, please note that it takes a minimum of 5 work days for the results of the medical examination to be couriered to the Embassy.
If you find that you are unable to attend a medical examination prior to your visa interview, you should reschedule your visa interview for at least 5 working days after your medical appointment. Please schedule the earliest available medical examination."
But as @Lil bear said, sometimes they just place your case in administrative processing until the medical results are sent.