At my local hospital, the lowest they test to is 0.1, so <0.1 is classed as undetectable. I had one test at the Royal Marsden where their lower limit is 0.04, and I tested <0.04 there.
My surgeon and oncologist said the 0.1 threshold is perfectly good, and another oncologist said lower limit testing could be of use if there were consistent rises over many tests and years.
All five tests over the nine months following my surgery have been undetectable, so happy days.
Keep calm and carry on.
Cheers, John.