Yes, a PCR DNA test, if done after 4 weeks of exposure, if negative indicates that you do not have HIV DNA in your blood.
But the need for retesting will depend on the type of exposure and the risk, that you have. Since that has not been provided, I would advice that you need not do a PCR test again. But instead, you should do a HIV Antibody test, at the end of 12 weeks post exposure. In case that is negative, then it is conclusive that you do not have HIV infection.
You may consult with your doctor or counselor for proper advice and help.
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