Taking PEP could potentially suppress viral load below detectable levels. 10 days is too soon regardless even if not on PEP. There is not much statistics for those that take PEP. I believe you should test 4 weeks after last dose of PEP for a conclusive result. It is recommended you test 3 and 6 months post exposure, but this is very conservative testing periods. These hiv tests are very sensitive. Was the person confirmed positive?