Resistor Ohm Meter

[Gusse]

So your current sense capability is limited to ~0.5nA with 1Tera ohm measurement equipment. Definetely it would be much easier to try lower sensing capability than increase test voltage.
Most sensible would be increase test voltage by 2x (or 4x) and try to improve current sensing (what ever it might need).

Pico amper range current sensing is very demanding and any noise from environment will affect your results (even measuring at DC). Most probably you have to do your measurements in shielded rooms, etc..

Even if you could build up measurement system that could measure up to 2 Peta ohm then limiting factor for accuracy would be ADC (even 16bit). One step would be ~30.5G ohm. Probably you should consider some better ADC than in PICs. (This is valid only if you have just one range for input, full range).

BR,
-Gusse-