- Author: Ashwin Pai, MBBS; Chief Editor: Eric B Staros, MD more...
Vitamin B6 is a complex of 6 vitamers: pyridoxal, pyridoxol, pyridoxamine, and their 5'-phosphate esters. Vitamin B6 deficiency causes blood, skin, and nerve changes.
The reference range for pyridoxal phosphate (PLP), the biologically active form of vitamin B6, is 5-50 µg/L.[1, 2, 3]
Conditions associated with vitamin B6 deficiency include the following:
- Early myocardial infarction
- Early stroke
- Recurrent thromboembolism
- Peripheral neuropathy
- Generalized seizures
Factors that may increase the risk of vitamin B6 deficiency include the following:
- Sickle cell disease
- Inflammatory conditions
- Rheumatoid arthritis
- Hyperoxaluria types I and II
- Excessive alcohol ingestion
- High serum alkaline phosphatase (eg, in cirrhosis, tissue injury)
- Catabolic state
- Peritoneal dialysis
- Phototherapy for hyperbilirubinemia
- Certain medications (cycloserine, hydralazine, isoniazid, D-penicillamine, pyrazinamide)
Marked increases in serum pyridoxal phosphate (PLP) levels are seen in hypophosphatasia.
Collection and Panels
Specifics for collection and panels are as follows:
- Specimen type: Blood plasma
- Container: Vacutainer, green top (heparin)
- Collection method: Venipuncture
- Specimen volume: 1 mL
Other instructions include the following:
- Overnight fasting specimen
- No vitamin supplements in preceding 24 hours
- Transport in light-protective container
- Centrifuge at 4°C; refrigerate immediately
- Transfer plasma to dark-brown polypropylene or polyethylene transport tubes to protect from light
Panels include vitamin B complex.
Related tests include alkaline phosphatase and pyridoxic acid.
Vitamin B6 is a complex of 6 vitamers: pyridoxal, pyridoxol, pyridoxamine, and their 5'-phosphate esters. Pyridoxine 5'-phosphate (PLP) is an essential cofactor in various transamination, decarboxylation, and synthesis pathways involving carbohydrates, sphingolipids, sulfur-containing amino acids, heme, and neurotransmitters. Vitamin B6 deficiency causes blood, skin, and nerve changes. This vitamin is unique in that either deficiency or excess can cause peripheral neuropathy.
Dietary sources of vitamin B6 include cereals, beans, vegetables, liver, meat, and eggs. After absorption, pyridoxine, pyridoxamine, and pyridoxal are transported into hepatic cells by facilitated diffusion. Pyridoxal kinase phosphorylates pyridoxine and pyridoxamine, after which they are converted to PLP, a coenzyme in tryptophan and methionine metabolism. PLP is the primary active pyridoxal form, and serum PLP is used as the primary index of whole-body pyridoxal levels.
In methionine deficiency, S -adenosylmethionine accumulates, resulting in the inhibition of sphingolipid and myelin synthesis. Tryptophan is a precursor to several neurotransmitters and is required for niacin production. Thus, pyridoxine deficiency can cause a syndrome indistinguishable from pellagra. The neurotransmitters dopamine, serotonin, epinephrine, norepinephrine, glycine, glutamate, and gamma aminobutyric acid (GABA) also require PLP for their production. Homocystine metabolism is dependent on pyridoxine, and high homocystine levels can result from pyridoxine deficiency.
Hypophosphatasia is a rare inborn error of metabolism caused by low activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Alterations in the TNSALP gene lead to rickets, osteomalacia, or both. Serum PLP levels are typically elevated and are used in determining the diagnosis.
Laboratory methods to detect deficiency are as follows:
- Pyridoxine level in blood
- Xanthurenic-acid level in urine
- Erythrocyte glutamic oxalo-acetic transaminase
- Lymphocytic growth response
Methods to make a quantitative assessment are as follows:
- High-performance liquid chromatography (HPLC) method, using cyanide derivatization: Method to determine the plasma pyridoxal-5-phosphate (PLP) concentrations as an indicator of vitamin B6 adequacy
- Tandem mass spectrometry
Serum PLP analysis is indicated for the following:
- Evaluation of suspected vitamin B6 deficiency
- Diagnosis of hypophosphatasia
Other manifestations of vitamin B6 deficiency may include the following:
- Scaling of the skin
- Severe gingivitis
Manifestations of vitamin B6 toxicity may include tachypnea and/or the following neurologic effects:
- Sensory neuropathy (eg, burning pains, paresthesias, perioral numbness)
- Progressive sensory ataxia
- Impaired proprioception
- Impaired vibration sense
- Diminished tendon reflexes
Burris CA, Ashwood ER, Burns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. St. Louis: Elsevier Saunders; 2006.
McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia: Elsevier Saunders; 2011.
Wallach J. Interpretation of Diagnostic Tests. 6th ed. New York: Little, Brown; 1996.