You can also find a lab in a neighboring state to draw the blood and send it off from there.
ADHD could be associated with so many different dietary factors it can make my head spin. Additional things to consider is not enough Omega 3s in the diet, or Vitamin D deficiency.
One more test to consider is LEAP/MRT. http://www.nowleap.com
Some of the affiliated dietitians will do consultations over the internet or phone.
Renata
ETA:
If Celiac is suspected, then it is better to be tested for that first. Taking the test when on a GF diet can lead to a false negative. Ruling out Celiac will determine the extent you need to avoid gluten exposure.