What’s the problem with diagnosing celiac disease simply by measuring the decrease in antibodies once someone switches to a gluten-free diet?

The con is that you’ll never know if a patient truly has celiac disease versus other causes for elevated antibodies. tTG are thought to be 97-98% specific, but by definition this means that 3 in 100 persons who don’t have celiac disease will have elevated tTG (biological variations, nothing more). If you consider that celiacs are 1% of the general population, it follows that out of 100 persons tested for tTG: 1 has celiac disease and 3 do not. This could mean that only 1 out of 4 of those with positive antibodies will have celiac disease as the cause. This is why you need a biopsy to confirm it.