I love Vietnam. I've been lucky enough to visit a couple of times - one of my very favourite places.
I may be wrong, but this is my guess as to what is happening:
PCR tests are excellent at identifying CML in tiny amounts. It can see the needle in the haystack. It's not so useful when there is lots of CML in the body.
FISH is good when the is lots of CML. So it's very useful in the months after diagnosis, but usually not so useful in the long term.
I studied the doctor wants to see a negative FISH result, which would indicate the medication is working, before moving to PCR tests. A negative FISH test usually equates to (about) a 1% PCR at the highest.
Why isn't the doctor ordering the PCR test earlier? They are more expensive, so may wait to use PCR until it is the most useful testing tool.
This is just all speculation.