Which metabolite identifies B6 deficiency, how do we know that somebody is deficient? What is the optimal range of the corresponding metabolite?

Elevated levels of Kynurenic Acid can represent a cellular B6 deficiency. Kynurenic Acid should be very low, ideally below 0.5.

Elevated metabolites that indicate suboptimal production of Flavin Adenine Dinucleotide and Flavin Mononucleotide (such as elevated Oxalic Acid) indirectly suggest a cellular B6 deficiency as well.