It depends on where the current is flowing. At picoamp levels, shielding is always needed. This means that the sample and all connections must be shielded. If the current signal is not at system ground, use a triaxial input type electrometer, such as the Model 6517A, and triaxial shielding around the sample, connecting the inner shield to circuit common, and the outer shield to system ground (mains ground). More information is provided in Keithley's handbook, Low Level Measurements.