So was looking at the Australian government pathology website in Australia, and they do offer blood tests for most b vitamins, and other nutrients. They likely do in other countries too. If someone has been sick a long time it would be a good idea to get them tested.
b1 thiamine- whole blood thiamine pyro phosphate. Better than transketolase test.
b2- whole blood riboflavin
b3- whole blood niacin
b5- not offered in Australia
b6- whole blood pyridoxine
b7 urine organic acids for biotin- just two- 3 HIA, and methyl crotynyl glycine
b9, and b12 are regularly tested anyway.
ADEK- I think it was whole blood for all 4. D is sometimes tested anyway.
From what I recall there was no fasting needed for any of them.
And then there is 13 different minerals some are regularly tested I didn't check if all the others could be tested too.
And now I can't find the website again will find it, then post it here. It might of been covered by medicare in Australia so was free if not was around $25 each.