A Vitamin B6 plasma blood test measures the levels of vitamin B6 (pyridoxine) in the plasma, a liquid component of blood. Vitamin B6 is a water-soluble vitamin that is essential for various bodily functions, including protein metabolism, neurotransmitter synthesis, and the formation of red blood cells. This blood test helps assess the individual's vitamin B6 status and can indicate if there is a deficiency or excess of vitamin B6 in the body.