Can I rule out celiac?

[AJ]

Hi, I'd really appreciate any advice anyone would be able to give! I've done some googling and reading up on celiac over the last few days, including on the different tests for it, but I'm still unsure when it can be ruled out as a diagnosis, so I hoped someone here might be able to give me some more expert advice!

I'm in my early twenties and I've had gastro-intestinal issues for years, including a lot of the symptoms of celiac/gluten sensitivity. I saw a GP about them 2 1/2 years ago and she did some blood tests which included a test for celiac, which was negative. I don't know what the test was, or what the exact result was, because I just got told over the phone by the GP's receptionist something like I hadn't tested positive for celiac. So my GP suspected a food intolerance or allergy and sent me to a nutritionist. I tried a gluten-free diet, and my symptoms largely improved, so the nutritionist diagnosed gluten as the problem and recommended a gluten-free diet.

I've mostly been better over the last 2 1/2 years, though accidentally eating something with gluten in it tends to upset my digestive system, but over the last few months I've been having GI issues again, including possibly gastritis, and also feeling really, really tired. So I was sent to a GI specialist who ran some blood tests, and is also going to do an ultrasound, endoscopy and biopsies. My diet for the last 2 1/2 years has been broadly gluten-free, although I haven't been careful to avoid low levels of gluten (I've guessed what's in things when I've eaten out, and even ignored low levels of gluten in stuff like ice-creams, sometimes just extracted obviously gluten-containing items from my food and eaten the rest of the meal, and I share a kitchen), so I've probably eaten low levels of gluten every week. All the same, I thought there was no chance I'd test positive on any of the celiac criteria on the blood tests. 

My results were: tTG = 48.3; endomysial antibody = negative; IgA = normal; feratin = 15; haemoglobin = 147; MVC = 86. So the slightly weird ones were the tTG and the ferritin. I eat a largely vegetarian diet, so I guess that could explain the feratin. But the tTG? As far as I know I don't have any other autoimmune disorders (I hope!); but, in the circumstances, can my tTG result possibly indicate celiac, or does it more likely suggest something else?

I've been put back on (deliberately!) eating some gluten every day for the 2 1/2 weeks leading up to my endoscopy and biopsy, and have been eating about one gluten-heavy meal a day. A week in, my digestive system feels pretty bad, and I've also got a nasty cold, but that doesn't indicate anything specific, I know. I'm wondering, if my biopsy is negative for celiac, what do I do then? Does that definitely rule it out, in my case? I'm not sure whether I'll have been eating enough gluten for long enough to make it an accurate biopsy or not. 

I went into these tests thinking I was almost certainly not celiac, because of the test result three years ago. But the tTG + ferritin results have made me a bit more uncertain. I just don't want to decide incorrectly it's gluten-triggered IBS (which is what the GI specialist thought before the blood results), and then find out years down the line it is celiac and that I've done considerable damage to my body by not treating it properly earlier! Of course I'll listen very carefully to the GI specialist's advice, but I'd be grateful if any of you could advise whether there are any further tests I should ask for to rule out celiac?

Thank you so much in advance!!