I did a 4th generation DUO ELFA test at 35 days and 54 days post exposure with an initial 28 days of Viropil PEP. The lab reported non reactive antibodies, i.e 0.12 and 0.11 (reference value 0.25) and in both the tests they said P24 antigen - 'Non determinable'. Upon checking with the lab they told me that the values were too low to be detected and it was a standard reporting protocol. What does this 'Non determinable' mean and is this common in ELFA tests ?