juniemarie,
You're right, testing is a bucket of worms, and not just for newbies. Kati's thoughts are good ones, that most PCPs should be familiar with. Also, it wouldn't hurt to have Vitamin D checked - though not at all specific for CFS, like most tests, it's commonly low and easily treatable.
The testing you have done would depend on your symptoms and what has already been tested for and excluded. For example, if you have symptoms that might suggest autoimmune disease, most doctors will want to do some testing. An ANA test is often done as a first step. The problem is that by itself it doesn't tell you much. It's often mildly positive in CFS patients who don't test positive on the more specific autoimmune tests. It does hellp fill in a picture, but it's not a key indicator.
Most CFS experts' protocols seem to be geared towards treating the patient's symptoms (like sleep meds and hygiene for sleep problems) or certain identified abnormalities like thyroid or infections. PCPs can actually be helpful at this - what can stump them is that there are so many symptoms, but they just have to start somewhere, with the most important things. I think Charles Lapp and Nancy Klimas both have written about the first things they generally try to address in CFS patients, with sleep being at or near the top of the list if that is a problem. I'd probably search their names with "treatment" and maybe the term "step-wise".
The natural killer cell tests that Kati mentioned are among the most consistently abnormal in CFS - function especially, and or number. But I'm not sure a PCP could interpret them, and I'm not sure how good the standard lab companies are about following the protocol to make them meaningful. I think I read somewhere that testing within a certain time after the draw is particularly important for this test.
Of blood tests, the only one I recall offhand that the Social Security Administration mentioned in its ruling on CFS and disability is certain EBV titers. But, I believe the levels it referred to are high even for most people whose tests indicate viral reactivation. And it's not an issue for everyone. I would just say, if doing viral testing makes sense for you, make sure you get the right EBV tests done, for example, not just IgG VCA. And maybe even check if the lab used the titers of the type SSA used.
I think the SSA ruling also mentioned tilt table, and other non-blood tests. Not cheap and readily available.
Functional testing (physical like VO2 max and / or cognitive) is probably most persuasive for disabilty purposes, since you need to establish disability not just illness. Having the right type of testing, and the right person administering, are absolutely important if you go down that route. There's a group called disability insurance issues at yahoo that is a good resource for disability-related questions.
Sorry, I've gone on a bit here and I'm not sure how helpful it's been - there's not an easy answer, and I'm sure others would have different suggestions.
Anika