Are diabetes tests always reliable?

Hello
First, many thanks for any help given.
How likely is it that diabetes is NOT detected in tests?
I am writing re: my 91 year old aunt. She has been tested for diabetes but her results say she is fine. However I had undiagnosed thyroid disease for years for which I was tested regularly, due to my symptoms, but all my tests said I was 'normal'. I was extremely ill before I managed to find a doctor who would treat me despite the test results. The long period undiagnosed has left me with permanent heart damage for example.
My aunt has SOME diabetes symptoms but not excessive thirst or urination which I believe are high on the list. She suffers terribly from psoriasis and there is a history of Type 2 diabetes (her Mum and brother.) My aunt is very thin (not as thin now since she hardly moves from her chair but when she was still slightly active there was nothing of her and she eats very well.) Her main symptoms that I know of are extreme tiredness, macular degeneration and burning feet at night – there are likely to be more but she is a very private person and it is difficult to know what to include since she is very old and has a level of dementia - well we think this must be the case because she has memory issues and her behaviour has become very self-centred, attention-seeking and inconsiderate - quite child-like in many ways. She can do more than she lets on but demands that her 92 year old sister do everything for her (they live 360 miles from me and there are no family who can help.)
I can probably find out more if you tell me that diabetes is not always detected with routine screening and that not all people have all the symptoms. Her life has become so awful she is just waiting to be taken out in her box.
ANY HELP GRATEFULLY RECEIVED especially, if it IS possible she DOES have diabetes, what should my aunts do next?
Thanks again in advance, Louise

Thanks so much for that reply.
I should imagine that whatever tests have been done will have been whatever is the most basic - I don't think they'll know - they don't ask questions of doctors. I will pass on your recommendations (had to look up the abbreviations - a whole new world for me!) I will just have to pray that they DO ask the doctor - they are of the age where one doesn't bother the GP unless on death's door.
Overall I have been VERY disappointed with the support and care my aunts have been receiving but I don't know how much of that is down to them saying they don't need any help or the fact that my potentially diabetic aunt is very difficult nowadays.
All very worrying. I only just thought of asking on a forum since my concerns have been brushed aside by my aunts (who belief doctors are always right) and I do everything I can to avoid adding to the pile. It just seems to have got worse again though she has been exhibiting symptoms (IMHO) for years.
Thanks again.