Fringe field effect: calibrating vs guard electrode
When you have very low conductivity or high impedance, the measurements are prone to low levels or magnetic fields on the loop area or electric fields between the electrodes.  These fringe fields must be considered when making amplitude measurements.  

So depending on the frequency of interest, sensitivity, geometry,  size of sensor or dielectric constant,  one can consider any method or none to reject common mode noise.

Guarding
For EKG, the conductance is low but enough to measure internal impulses but they still use a right leg guard or RLG electrode to reduce EMI from fringe line frequency.

Reference method
If you have a standard dielectric to compare ( with all factors exactly the same), and in the same fringe field then this can be used for Common Mode Rejection as a bridge to compare conductivity dielectrics in biometrics.  

-a resistor is used with a thermistor in a "bridge" to support a comparison for differential resistance vs temperature measurements. 
   - there are probably lots of low conductivity examples here as well.

No method

Testing a capacitor for conductance leakage with a probe needs just a good INA or Instrument Amp configured (tri-Op Amp) for high Common Mode Rejection ratio with a low current source to measure conductance voltage at some frequency and then convert this to Ω, MΩ etc.