Your bias current is usually measured across a 1 ohm resistor so that mV=mA. The meter measures volts but because the resistance is 1, then the number is the same. So a custom meter will measure volts, but it will say amps.
if your maker didn't order a special meter dial, it may say mV, but is actually reading mA.
There is a post above that contradicts this and I'm not sure what the poster meant.
Jerry