In my readings about Vitamin D stability exposure to UV radiation does process the vitamin D on into substances the body doesn't use. This happens with both UVB and UVA light.
So draw blood (containing vitamin D3) place in a clear tube by a UVA light source (window or flourescent light tube) and you have the chance of a lower reading than you thought.
I wonder if the reports of poor response from Vitamin D2 arise from pharmacies buying in bulk, keeping the bulk supply on a high open shelf near a fluorescent light tube and then when the Ergocalciferol is dispensed it's even more worthless than when it was manufactured.

It's mainly light that is the problem. You can freeze D3 in ice cream and bake D3 in cakes/bread/biscuits and it's fine but put it under UVA or UVB and the conversion to suprasterols the body can't use will happen.

The Grassroots D Action (ZRT) 25(OH)D test requires just 2 drops of blood. These go directly onto the sample tissue which you allow to dry for a few minutes and then return in an envelope. I suppose if you put the sample tissue paper to dry by the window or under a strong light you could harm it's D3 content but the results I've had have been in line with my expectations.