There has been some discussion of the recent case in which the Children's Hospital of Philadelphia was accused of denying a three-year-old girl a kidney transplant solely on the basis of mental retardation. (The Hospital has apologized, and the parents have since suggested that it was perhaps merely one doctor rather than the hospital as a whole.) One thing I have noticed is that several people have raised the issue of triage as a defense of the original decision; some of these are interesting arguments, but unfortunately most of these appeals to triage principles don't show an understanding of what triage actually is.
The entire point of triage is that only need is considered. Triage systems were originally developed in a military context when field doctors started giving medical treatment not on the basis of rank but on the basis of need, as determined by purely medical criteria. This is what genuine triage is: it is a system, operating under a scarcity of resources significant enough to require careful discrimination of who actually receives those resources (most clearly in emergency or disaster, but resources do not necessarily have to be anywhere near that scarce to become an issue), where distribution of those resources is done purely on the basis of actual medical need according to established principles that only consider medical issues. Remember, it has always been the case that doctors have had to make hard choices based on scarce resources. Actual triage systems only developed when the principles governing those choices were no longer official rank, social status, subjective assessment, or any other nonmedical criterion. We can call those other resource-management systems 'triage' in a loose sense, but they are radically different for moral purposes, and cannot all be lumped together as if the justification for one were justification for another. Just as genuine triage management cannot, by its nature, be indiscriminate in the use of medical resources, so it cannot, by its nature, take into account anything other than medical need. And precisely the reason why triage is an important ethical as well as medical concept is that it operates in conditions of necessity according to principles wholly geared to dealing with the necessity; it's the medical necessity, and the proportion of means to the end of dealing with that particular necessity, that justifies triage decisions.
Precisely one of the worries in this case -- and whether it is right or not, the issue needs to be taken seriously -- is that we here have a case in which the discriminatory principles were not purely concerned with medical need, but involved making subjective or merely pseudo-objective judgments about something like future quality of life. If this were in fact true, it would mean that the system was not a genuine triage system, and that triage principles could not justify the action. Mere scarcity of resources and the need to make hard decisions is not enough; and you can only defend an action on the basis of triage principles if the principles involved were real triage principles. This is something that requires investigation, and cannot be merely assumed.