Another group is showing that GDF15 is normal in ME/CFS,
@Jonathan Edwards. The testing I was part of showed that the most severe patients had the lowest GDF15. Explaining how this could be would go a long way, in my opinion.
I still think it's plausible that, due to poor microvascular diffusion and reduced gradients, blood values might not reflect tissue values. Something similar is seen in mitochondrial diseases, where lactate levels in the blood can be normal.
There could be many other reasons, of course.