Blood tests questions

[Whatcouldpossiblygowrong]

Hi
Don’t know if anyone can help but here goes anyway.
Did that thing of having dr alignment and not asking the Qs I should have asked!
Long story short I have long term symptoms of fatigue, raised glands, pain in knees, sore throat, bad skin and stinging eyes.
I’ve put it down to extreme stress- which was the case for some time for various reasons. However- the symptoms don’t get better even when I sleep eat well and avoid stress. I don’t feel depressed just totally exhausted and unwell.
The gp did blood tests a year ago and the full blood count was a little low and liver not working that well- otherwise all good.
Went back today and they’ve asked me to repeat bloods for b12, CRP, Ferritin, Glucose, LFT, rheumatoid factor, TSH, urea-creat, electrolytes.
So here are my Qs-
I’m not sure what any of these are- what are they ruling out/ looking for?
Do I need to worry that what I eat before hand might skew the test?
What will happen if nothing shows up? I’m worried they won’t be able to help me further if everything is clear although obv that’s ideal outcome.
Sorry for ramble!!

Aww thank you OP x I don't think all Drs take autoimmune diseases seriously but it might be worth googling it a bit yourself and if you think there us a possibility, ask to be referred to a rheumatologist

Thank you luckymarmitelover. I can’t go private- but my gp is quite nice and would probably do the test if I asked and explained why. What would you say the benefit of these ones would be?
Mynameisarthur they did test rheumatoid factor and it was

As below:

The antinuclear antibody test (ANA) is often the first test that doctors use when symptoms suggest an autoimmune disease. ... Your doctor might also do tests to check for the inflammation these diseases produce in the body. BOTTOM LINE: A positive ANA blood test can show that you have an autoimmune disease.

Thanks - maybe I will ask for that too. Or at least asks if he thinks there’s any way symptoms could be auto immune related