The Vitamin B6 test measures the level of vitamin B6 in the blood, most often as pyridoxal 5'-phosphate (PLP), which is the active form of the vitamin. Vitamin B6 is a water-soluble vitamin involved in protein metabolism, red blood cell production, and nervous system function. This test is used to check for a deficiency or, less commonly, an excess of vitamin B6. It may be ordered when a person has symptoms that could be related to low B6 levels, such as certain types of anemia, skin changes, or nerve-related symptoms, or to monitor nutritional status in people at higher risk of deficiency.