The combo tests simultaneously for antibody to HIV and p24 antigen, a protein component of the virus. The RNA test looks for the virus's genetic material. In other words, both p24 antigen and RNA are direct tests for the virus itself, whereas antibody measures the immune system's response to the virus.
It is not possible to have HIV for 12 months and have a negative antibody tests -- regardless of the RNA test result.
Having also looked at your thread on the HIV community forum, I agree with the advice there -- i.e. you could not have acquired HIV from the exposure described, even if your girlfriend had HIV, which probably she did not.
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