Vitamin B6 is a group of related compounds, with pyridoxal-5-phosphate (PLP) being the biologically active form the body uses. B6 is involved in over 100 enzymatic reactions, primarily related to protein metabolism, neurotransmitter synthesis (including serotonin, dopamine, and GABA), haemoglobin production, and immune function.
This test measures PLP in plasma, which is the most reliable marker of vitamin B6 status. Unlike some other B vitamin tests, B6 testing is relevant for both deficiency and excess, because high-dose supplementation can cause nerve damage.
Deficiency can occur in people with alcohol dependence, those taking certain medications (including isoniazid for tuberculosis and some anticonvulsants), people with inflammatory conditions such as rheumatoid arthritis, those with malabsorption conditions, and individuals with poor dietary variety. Symptoms of deficiency include irritability, low mood, poor concentration, anaemia, skin changes around the mouth and nose, and in severe cases, peripheral neuropathy and seizures.
On the other end of the spectrum, B6 toxicity from high-dose supplements is a recognised clinical issue. Prolonged daily intake above 50mg has been associated with sensory peripheral neuropathy, characterised by numbness, tingling, and burning in the hands and feet. This can occur even at lower supplemental doses in some individuals. The neuropathy is generally reversible when supplementation is reduced or stopped, but it takes time.
B6 is found in a wide range of foods including poultry, fish, potatoes, bananas, and legumes, though absorption from plant sources can be lower than from animal sources. People following plant-based diets who are experiencing fatigue, mood changes, or nerve symptoms may find it useful to check B6 alongside B12 and folate.
To measure pyridoxal-5-phosphate (PLP) and determine whether B6 levels are too low (deficiency) or, in people taking high-dose supplements, too high (which can cause nerve damage).
