I don’t actually think we need to ration health care (the first world is treating the edge cases that affect people in their 70th year, while people in the third world are dying of vitamin deficiencies; healthcare is poorly distributed, but there’s probably enough to go around). But it is conceivable that there would be situations where there is actually less ability to treat illness and injury than there are illness and injuries to be treated – say, a Mars colony, where supplies are limited and injuries common and relatively new (not many low-gravity related impairments cropping up on Earth).
In such a situation, it would be necessary to prioritize not only diseases, but patients: in a case where doctors are the bottleneck, it would be very important to treat any ill doctor first, and to expend the most resources to ensure a quick recovery. But that is an easy case. What should the policy over the longer term?
Each human life should be considered valuable. A more able life should be more valuable, but with diminishing returns; most of the value of a life should be recouped by increasing wellness from 0% to 10%, while significantly less value comes from increasing wellness from 90% to 100%.
A similar principle should apply between individuals: the most valuable individual is not significantly more valuable than average (especially where a group is small, extreme outliers in terms of social value are unlikely). As such, saving the life of an average person (0-10%) is likely to be much more valuable than solving minor issues (90-100%) for the most valuable individuals.
But there are harder cases. While there are diminishing returns, the shape of the curve is not easy to specify. It might even be that the curve is non-linear; large stems in value seems to occur between alive and dead, comatose and conscious, delirious and coherent, immobile and mobile, etc.
But if value is quantifiable, the value of all such improvements should also be quantifiable. And in such a situation, the values should be used to ration healthcare to maximize social value. Sometimes, valuable people should get preferential treatment, because by definition their improvements are more beneficial to all of society. But that does not justify unlimited preference. In nearly all cases, the severely ill should be treated first, to prevent as many deaths as possible, before treating mere inconveniences such as pain, fatigue, or arthritis.