I've always thought it simple to eliminate potential trouble sources in donors so long as you keep an open eye towards physical or mental illness. One needn't be a doctor to sense that something is amiss.
For example, on the psychological end, it's important to remember that vampirism is an intimate act. Because this is so it has the potential to become a deeply positive or negative experience depending upon how up-front or elusive both parties choose to be. Neediness, paranoia, nervous tics, aggressiveness, depression, should be treated as red flags and you should move on without question. That last bit is important not just because an emotionally unstable person can be quite persistent about maintaining a connection, even when it is explained that this could be detrimental ..but also because it is quite common for a desperate vampire to take unsafe or irresponsible short cuts to meet their goals. Instinct is as much your friend here as common sense, but it is surely advisable to match them on the scale as best you can. In feeding psychically, the quality of prana you bring in will be compromised and cause negative effects to your energy system, regardless of how great a filter you believe you've set up. It is not uncommon to suffer empathic echo effects either, so choose wisely.
In contrast though, some vampires believe that the act of energy drawing is more balancing than depleting. It has indeed been suggested by some that it can be beneficial in this respect, provided that limits are placed on the withdrawal. It's all a matter of opinion and preference really, but I tend to err on the side of caution.
Physically, the issue of blood-borne pathogens is probably the most serious concern. This is especially true since many people whom are host to such diseases, like AIDS, may not be aware that they have them. In that, you can't merely rely on their candor to give you a green light. It's important to not just have a single test for such elements, but continue them regularly throughout the course of your association. That being said, while it's obviously a good idea, such testing will likely not be engaged in by most blood feeders for reasons of inconvenience, financial or otherwise. Not much else can be said about that, because short of interviewing the donor and having regular doctors check-ups, you're pretty much on your own far as screening goes. I like to stick with the rule that if they seem unhealthy in any way, shape, or form, they are off limits. No one expects you to diagnose the person, but you should look for more common signs of sickness. Fatigue, a malnourished look, being overweight or underweight, recreational or non-recreational drug use - this includes cigarette smoking and regular drinking, deep ridges in the finger nails, unhealthy diet, or any of a hundred more apparent signs that something - anything - is, or could be wrong. This caution, while sometimes potentially offensive to the donor keeps the both of you safer than not. You should probably also avoid those with fresh piercings or those with fresh tattoo work.
These are just a couple ideas, and it's usually never just one or the other. You are very unlikely to find people that are completely, 100% healthy, so we all just have to do the best we can. Essentially though, you are just looking for signs of weakness. Since it is the basic idea of vampirism, and nourishment in general, that one should consume quality life sources to insure strength and integrity, I would say it's best to keep your sources balanced and not to go for the quick fix. Bad habits, after all, develop so easily.
- D
