Why should you even need doctor prescription for something like this? Week ago I just went to nearby lab and asked them to make a blood draw to verify my vitamin D levels and got results same day via email.
US doctors have a strong political presence through the AMA and maintain a monopoly on nearly every medical procedure, test, and many drugs. It's impossible to get anything without a doctor's prescription and it's usually impossible to get that prescription without a (paid) visit to their office.