There is a gotcha: quantitative thiamine tests are false negative most of the times. But other blood markers can be revealing: homocysteine, HOMA-IR, blood pH,

There is a gotcha: quantitative thiamine tests are false negative most of the times. But other blood markers can be revealing: homocysteine, HOMA-IR, blood pH, lactic acid. And it's not always exclusively a thiamine, for some people it's anemia, B3, or B12 deficiencies, or a combination of those.