Hi J.
Sorry for your trouble!!! Sorry also for my long answer!!!
Hope something helps and / or encourages you.
Wow!!! I see 3 or 4 issues that you are addressing. Are you wondering if they are all related? They could be but not necessarily.
First the breath test. Maybe others have heard of it, but I just had a lactose tolerance test and I was so sick just from drinking the mixture they stopped taking my blood, which was to be taken every 15 minutes for 3 hours. I had serious diarreah, and began vomiting. It was horrible. I was so sick but they also told me that I should start to feel better so I left the hospital but it was 3 days before I was better. The test proved beyond a shadow of a doubt that I was lactose intolerant, but I already knew I could not drink milk, MD needed test not me. OK so do they take her blood too or are they going to go by her breath & not take her blood? Is she having breathing issues? You don't say so. I assume not.
3 out of my 4 children were declared lactose intolerant or allergic to milk at a young age, all were given alternate formulas without testing. What is gastro trying to prove?
Does this test prove a need for scans? or other tests?
Do you have family who are lactose intolerant?
Underweight could be a related issue, but my lactose intolerant children bloated, therefore looked heavy. My only underweight child was borderline malnourished at that age, and no cause has ever been found. At least none MD's could agree on. Of course they all have their opinions.
My most allergic child followed a routine but I would call it when I feed him, he will scream and continue to scream for 2 hours after that he will whimper and fuss, and about the time it is time to feed him again he will be an angel.
I am an elimination/substitution diet person. Try Silk Soy, my favorite, but there is also rice milk and others at any health food store. Goat's milk work for some. I am also a person who would go to the health food store and ask for their help giving them the MD's diagnosis. I don't know if kids can take "pearls" which I get there, or "Digestive Advantage ---lactose intolerance therapy"(drug store) which I used on the start, but either or both of these worked far better than anything the gastro did once we had a diagnosis. We eliminated lactose from my children's diet, and some needed other things eliminated as well.
God bless you and give you wisdom as you make some major decisions. Can I pray for you to seek wise counsel?
K. --- SAHM married 38 years === adult children 37, coach, at 4 mos he was in hospital and records show they thought he would be dead in 24 hours--miracle; 32, lawyer, at 4 mos he was hospitalize 28 days, later SIDS symptoms, later hives, ADHD at 12 mo, and ADD later, all allergic related(used elimination diet), married with 6 mo son---miracle; twin girls 18, born after 19 weeks of bed rest for premature labor at 18 wks, weighing in at 5'11" & 6'8", in college after homeschooling 3.8 and 3.7 GPA and one is overweight and one underweight(at a very young age I learned she was a snacker and would eat it if I left it to be eaten, so I did, when she didn't do meals I accepted that; when she went into the hospital at 12 weighing 72#, I was worried but in 3 days she was down to 53, and I was furious, saying I did better than they were. They said she ate everything, but was loosing. I knew that is why we were there, but I kept her weight up. Why couldn't they? Because she was a grazer. She still is) due to allergies.
She must eat about 4-5 thousand calories a day to keep the 100 pounds on her body, if she must exercise(walking around the college campus) she must eat more. No one knows why.