Any sufficiently low-calorie diet, whether it's low-carb or not, will cause
ketosis.  This includes fasting.  Ketosis is simply the burning of fat as a
primary fuel source, and if you're not eating enough your body will start
to do that.  Ketosis when brought about for dietary reasons is simply the
body switching over from burning glucose as its primary fuel to ketones.
Ketones are unburned fat which your body circulates through your
bloodstream so that the cells of the body may use it as fuel instead of
glucose; when starving, or when in very-very-low carbohydrate intake
situations, the body will do this so it can use fat as its main fuel
instead of glucose, because in the absense of carbohydrates to make
glucose, it would have to use protein.  If you're eating a lot of protein
this is no problem, but if you're fasting or on a very low caloric intake,
your body will turn to its muscles to make glucose instead.  Ketosis allows
it to slow this process down by making free floating fat bodies available
instead.

Of course if you are fasting, you have a problem because even while in
ketosis your body still must have SOME glucose, because some tissues simply
cannot use ketones as fuel.  Most of the body can (and some parts, like the
heart and kidneys, actually prefer ketones over glucose if they can get it)
but those parts that can't MUST have some glucose.  So when you're in
ketosis you'll have less total glucose in your body and ketones will be
making up the difference, but unless you're eating enough protein, your
body will start using its muscle tissue to make the smaller but still
signifcant amount of glucose it must have.

Excess ketones will be excreted through the bladder, bowels, and lungs.
Some people will tend to excrete more ketones by lung action than others so
some will get worse or more frequent bad breath than others but it's all
the same in the end.

If you've fasted for extended periods, you've been in ketosis.  If you've
been on a very low caloric intake for a significant period of time, you've
been in ketosis.  If you've eaten a very low carbohydrate, high fat diet
you've been in ketosis.  There is even some reason to suspect that a lot of
the side-effects of fasting are actually caused by the ketosis that fasting
brings on rather than the cleansing involved, though I suspect it's a
multitude of factors.  (Still, fasting people lose appetite after a while,
usually, and so do people on low-carbohydrate diets.  They also feel quite
energetic, another common side-effect of ketosis for low-carbers.)

In any case, if you're on a reasonable number of calories but are in
ketosis because you've gone very low carb, a doctor will not know you're in
ketosis because there is no reasonable way to tell and no reason for him to
look for it.  Possibly he might notice bad breath and ask you but it's just
as easy to shrug and say, "I get that sometimes" as it is to try to explain
you're in ketosis.  No doctor is going to launch a full-scale investigation
because you have a little halitosis.  Believe me, there's no way he's going
to sniff your breath and say, "Hey that smells like ketones!  What the hell
have you been doing?"

If you're really worried about it, eat a couple of oranges or apples or
other sugary fruit an hour or two before you go in.  You'll be knocked out
of ketosis pretty quickly.  You'll spend a day or two trying to get back
in, but...

Really this is a lot of worry over something you're very, very unlikely to
have a problem with.

 -=-=-

Once in a while you get shown the light/
 In the strangest of places if you look at it right   ---Robert Hunter

http://www.syndicomm.com/esmay