It's possible the prior tests were wrong: http://consults.blogs.nytimes.com/2010/08/27/getting-tested-for-herpes/. Why did she have prior tests? I would find out what tests were done since the older ones were know to be unreliable. Why second guess yourself? Have her doctor perform the same blood test on you. You may not have it and want to protect yourself from getting it. The two of you should sit down with her doctor and discuss what steps should be taken based on your blood results. Relationships require honesty and this is a good way to take the guessing out of it.
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