Fozzy,
Here's the link and pdf that will answer your question: Standards for Assessing Medical Appropriateness Criteria for Admitting Patients to Rehabilitation Hospitals or Units.
http://www.aapmr.org/hpl/legislation/mirc.htm
Unfortunately, the state of the science is low. Predicting functional recovery is largely a crapshoot. So, asking "what makes a good rehab candidate?" is sort of like asking, "what makes a good omelette?"
In general, a patient has to 1) Have functional goals; 2) Be able to participate in an AVERAGE 3 hours of therapy per day over the their length of stay; 3) Require medical supervision; 4) Not be bettered served at a lower level of care.
Okay, let's pick these a part for a little fun...
1) Have functional goals -- Hell, I've got functional goals! Should I be admitted to rehab unit?? Functional goals can fall on a theoretical spectrum ranging from "independent tissue perfusion" to "bench press 240lbs 8 times."
2) Participate in average of 3 hours per therapy -- This is not really a rule per se, but is often thrown around. The problem is **HOW** you count an hour of therapy. If it takes you an hour to sit up to the edge of the bed and tie your shoes is that an hour of therapy well spent?? Your therapists will revolt if you bring patients to rehab who require an hour to do even the simplest of tasks.
3) Require medical supervision -- Okay, so this what I used to tell patients when "it was time to leave." They didn't require a specialist rehab doctor looking after them any more...Nothing makes a little old lady break down and cry more than the dreaded "NH" words ie "Nursing Home." So, after their INR was therapeutic and I tweaked their meds in whatever direction they needed to be tweaked, their requirements for "medical supervision" were really quite low and it was time to go...
4) Not better served at a lower level of care -- Well, again, a subjective call. There are some SNF's with good rehab services and motivated therapists. The food might be better. Maybe the rehab dog is nicer. Who knows...
The point is that some times patients are "too good to come to rehab." They still need services, but those pesky 72 hour admits to acute rehab kind of make auditors a little suspicious and really piss off the housestaff.
Alternatively, maybe they have functional goals, but it will take a long, long time to reach them. The LOS in rehab units is plummetting. In some parts of the country it's down to about 14 days per admission. There are many things you can do in 14 days, but you can't "unboil an egg."
So, there you go: Knock yourself out.