You COULD just call the doctor and/or clinic where you had your test done and ask them this question.
Since you HAVE your negative results, you are not coming to us for an assessment of your risk. We are a very busy forum and to save time on giving tutorials for the sake of curiosity, I have sent you a link that I trust will answer your questions. It is highly informative.
http://www.capitolregiontelehealth.org/documents/AETC-Webinar-2.pdf