Anion Gap calculation
The Anion Gap is the (sum of the cations) - (the sum of the anions). Both sodium and potassium are cations and CO2 and Chloride are the anions. Given this, the Anion Gap should be calculated like this (from the data you sent through):
(136 + 4.1) - (103 + 29) = 8.1
It looks as if your lab is leaving out the Potassium in the calculation, which would give us the following: (136) - (132) = 4
Both are considered Anion Gap calculations but I believe ODX's way of doing this, which is the way we were taught in med school, is the more accurate because Potassium is definitely a cation, albeit an intracellular one, and should be used in the calculation.
That being said, please use the ODX calculation as it is just as accurate and relevant as the one from the lab. The reference ranges take the addition or the subtraction of sodium into effect.