One question that's still been plagueing me from Dark Heresy through Black Crusade.

In the Damage sections of the rulebook, it says that certain critical effects force you to take a toughness test or lose a limb permanently. That's easy enough... but then it says that if someone with Medicae is assisting you, you can add a +20 bonus to this test. Master Chirurgeon, the talent, also lets you add an additional (by my understanding) +20 to that test.

Now, my question... every critical effect that presents limb loss asks for an immediate toughness test. How, in this case, could someone with medicae ever be helping someone against this? If it's immediate...

I would just say if it really comes down to it and there is someone with the medicae skill present the gm would have to ask if they intend on helping the player taking the critical. If they don't, then the character gets no bonus.

If they do intend on helping, then you get the bonus to the test, and as soon as possible the player has to move toward the wounded character and attempt to address the situation per the rules. If nothing else, they could be yelling at the character giving them instructions until they are able to reach them.

The toughness bonus in this situation could be seen as optimisim on the character's part (they know there's a medic on hand).

Your solution seems very reasonable, and pretty close with what I thought would work.

I was thinking that if the player got First Aid within a turn or so, they'd get the bonus. The problem facing that was First Aid being a full action, so it would be a rare case that they could receive it within a turn.

It just seems weird that such an oversight could go through 4 main rulebooks and not get addressed.

You could rule that receiving the proper treatment gives a player another saving throw with the proper bonus.