What is the circuit arrangement, not enough details to give any sort of precise suggestion.
To do this sort of thing, you would normally put a current sense resistor into the circuit, and use an amplifier to get a usable signal. At 10nA, you would need 1K to give you 10uV, but that requires a very good no drift amp, so you would probably need to go higher. Voltage offset and offset drift are the two parameters you need to watch out for if you need good precision.
Linear Technology have parts, see app note, 105, which may give you some useful pointers, and Texas Instruments have one or two precision amps for this sort of application in the INA series.
Once you have done a current to voltage conversion with gain, a window comparator could sort out the range you require, and from there a mosfet switch could be used to disable the output.
Depending on the circuit, you could end up locking the current off it it is outside the range you require, so it may mean bypassing it so that the measurement is continuous.
Without further details, can't add much more,
cheers,
Richard
