Vitamin B6 (pyridoxal 5-phosphate)

Ranges vary between laboratories (for example 35 to 110 or 20 to 120 nmol/L) and depend on whether plasma or whole blood was measured, so the printed range is the one to use.

Vitamin B6 is a water-soluble vitamin involved in making neurotransmitters, haemoglobin and in handling amino acids. The blood test measures its active form, pyridoxal 5-phosphate. Both too little and too much can affect the nerves, and the high values seen nowadays nearly always come from over-the-counter supplements.

It is requested when unexplained nerve symptoms such as tingling or numbness are being looked into, whether from shortage or from taking too much in supplements, in people with poor absorption, kidney failure or heavy alcohol use, in some rare seizure and anaemia conditions, and on some private nutrition panels.

• Supplements, including many multivitamins, energy drinks and 'B complex' products, where high values are common
• Recent B-vitamin injections
• Some inherited conditions affecting the enzyme that breaks it down

• Heavy alcohol use
• Some medicines, including isoniazid, some anti-epileptics, the combined pill and long-term high-strength theophylline
• Poor absorption, including coeliac disease and after weight-loss surgery
• Kidney failure and dialysis
• Any inflammation or infection, which lowers it for a few weeks
• Poor diet in older age

• A supplement taken in the past day or two raises it, often above the range
• Inflammation on the day of the test lowers it
• The sample must be protected from light or the value falls
• Different labs measure plasma or whole blood, with different ranges

A fasting morning sample protected from light, ideally after stopping supplements for a few days so the body's own level is measured. Sent to a specialist laboratory.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.