HSV Tests

Hi all.. 
I'm wondering if anyone has experience with HSV tests. I went to my OB this week with, what I can only best describe as a pencil-eraser sized single canker sore on my labia. I've had this before, and both times I've seen a doctor about it it's been cultured for HSV, and it came back negative. 
When I went this week, my OB cultured the spot and I just got the call that it, again, had come back negative. 
Because I'm pregnant they want to make sure, if it's HSV-2, I'm taking antivirals the last month of my pregnancy. So, they want me to have a blood test. 
I'm wondering if anyone has had negative cultures, especially multiples, has then gone on to have a positive blood test? I know that false negatives are common for cultures, especially if not caught early- but this is driving me nuts. 
This whole experience is really worrying me, and I've been crying off and on about it since I saw my doctor. I want to be able to have a vaginal birth but am so anxious over the possibility of not being able to, if the test is positive and I'm showing symptoms in labor. 
Any anecdotes would be appreciated...