Vitamin B 6 (Pyridoxine), Plasma

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Turnaround Time: 4 - 6 days
CPT Code:


Test Type: 0.5 mL Plasma (EDTA), protected from light
Stability Time:



Room temperature

3 days


15 days


15 days

Freeze/thaw cycles

Stable x6

Reference Range:

Male: 5.3−46.7 μg/L; female: 2.0−32.8 μg/L


Detect vitamin B6 deficiency.

This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.

Vitamin B6 occurs as an alcohol (pyridoxine), an aldehyde (pyridoxal), and an amine (pyridoxamine). These forms are phosphorylated in the 5'-position to produce the physiologically active coenzymes that are critical to their biological function. Eukaryotes cannot synthesize vitamin B6 molecules from smaller compounds and as a result require dietary B6 for the synthesis of 5'-phosphate vitamins. Pyridoxal 5'Phosphate (PLP), the most clinically significant coenzyme form of vitamin B6, is the form most commonly measured in plasma.1-3

PLP serves as a coenzyme for more than 100 enzymes that catalyze key steps in the metabolism of amino acids, neurotransmitters, nucleic acids, heme, and lipids.1,4,5 Vitamin B6 is a critical cofactor for enzymes involved in energy homeostasis through glycogen degradation and gluconeogenesis.5 Inverse associations have been shown between plasma PLP and chronic or acute disease, including rheumatoid arthritis, cardiovascular disease, deep vein thrombosis, and cancer.4-16 A number of epidemiologic studies have shown reduced concentrations of circulating PLP in association the acute phase marker C-reaction protein13-17 and with inflammatory markers.18-19 Diminished vitamin B6 levels are frequently observed without any indication of a lower dietary intake or excessive catabolism of the vitamin, or congenital defects in its metabolism.4 Research is ongoing to determine if these lower vitamin B6 levels are caused by the mobilization of this coenzyme to the site of inflammation for use by the PLP-dependent enzymes4 or due increased catabolism of vitamin B6 during inflammation.5

PLP serves as a coenzyme for δ-aminolevulinate synthase, which catalyzes the first step in heme biosynthesis.1,5 B6 deficiency can produce a hypochromic form of anemia characterized by the presence of ring sideroblasts (iron positive granules deposited about the nucleus of red cell precursors). Occasionally the anemia may have megaloblastic characteristics. Inherited abnormalities of apoenzymes that bind with pyridoxal phosphate are responsible for newborn conditions characterized by mental retardation, skeletal deformities, thrombotic conditions, osteoporosis, and visual defects. Some inherited abnormalities of vitamin B6 metabolism and transport are associated with aminoacidurias including homocystinuria, hypermethioninemia, cystathioninuria.21 A number of studies have demonstration an inverse association between plasma PLP levels and the risk of developing colorectal cancer.20 A recent meta- analysis indicated that the risk of developing this type of cancer decreased by 49% for every 100-pmol/mL increase in blood PLP level.20

Vitamin B6 deficiency can occur in individuals with a variety of genetic conditions including antiquitin deficiency,21 pyridox(am)ine-5'-phosphate oxidase (PNPO) deficiency22 and hyperprolinemia type II (pyrroline-5- carboxylate dehydrogenase deficiency.23 Vitamin B6 levels can be decreased in malabsorption conditions including inflammatory disease of the small bowel and as a consequence of jejunoileal bypass.4,5 Several drugs, including oral contraceptive agents, levodopa, isoniazid, cycloserine, and pyrazinoic acid may cause B6 depletion.1 B6 levels may be decreased with pregnancy, lactation and alcoholism.1 Infants can develop deficiency when fed formula rendered B6 depleted by excessive heating.

Markedly elevated plasma PLP levels are observed in cases of hypophosphatasia (HPP), an inborn error of metabolism caused by a loss-of-function mutation(s) within the gene for the cell surface enzyme, tissue nonspecific isoenzyme of alkaline phosphatase (TNSALP).24-28 This disorder is characterized by low serum alkaline phosphatase activity and increased plasma levels of TNSALP substrates including inorganic pyrophosphate, phosphatidylethanolamine and PLP. Clinical features can include childhood rickets, adult osteomalacia and dental abnormalities. These symptoms are thought to occur as a result of the accumulation of inorganic pyrophosphate which inhibits hydroxyapatite crystal formation and growth, leading to defective skeletal and dental mineralization. PLP, carried in the plasma on albumin, must be de-phosphorylated by TNSALP for pyridoxal to cross cell membranes. Once inside the cell, the pyridoxal is regenerated as PLP to allow it to function as a coenzyme. The diminished TNSALP of individuals with HPP leads to an accumulation of the PLP substrate in plasma. HPP patients do not typically experience B6 related symptoms. However, the extent of PLP elevation has been related to the disease severity.28

1. Food and Nutrition Board, Institute of Medicine. Vitamin B6. In: Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: National Academy Press; 1998: 150-195. PubMed 23193625

2. Lamers Y. Indicators and methods for folate, vitamin B-12, and vitamin B-6 status assessment in humans. Curr Opin Clin Nutr Metab Care. 2011 Sep;14(5):445-454. PubMed 21832901

3. Morris MS, Picciano MF, Jacques PF, Selhub J. Plasma pyridoxal 5'-phosphate in the US population: the National Health and Nutrition Examination Survey, 2003-2004. Am J Clin Nutr. 2008 May;87(5):1446-1454. PubMed 18469270

4. Paul L, Ueland PM, Selhub J. Mechanistic perspective on the relationship between pyridoxal 5'-phosphate and inflammation. Nutr Rev. 2013 Apr;71(4):239-244. PubMed 23550784

5. Ulvik A, Midttun O, Pedersen ER, Eussen SJ, Nygård O, Ueland PM. Evidence for increased catabolism of vitamin B-6 during systemic inflammation. Am J Clin Nutr. 2014 Jul;100(1):250-255. PubMed 24808485

6. Roubenoff R, Roubenoff RA, Selhub J, et al. Abnormal vitamin B6 status in rheumatoid cachexia. Association with spontaneous tumor necrosis factor alpha production and markers of inflammation. Arthritis Rheum. 1995 Jan; 38(1):105-109. PubMed 7818558

7. Dalery K, Lussier-Cacan S, Selhub J, Davignon J, Latour Y, Genest J Jr. Homocysteine and coronary artery disease in French Canadian subjects: relation with vitamins B12, B6, pyridoxal phosphate, and folate. Am J Cardiol. 1995 Jun 1;75(16):1107-1111. PubMed 7762494

8. Saibeni S, Cattaneo M, Vecchi M, et al. Low vitamin B6 plasma levels, a risk factor for thrombosis in inflammatory bowel disease; role of inflammation and correlation with acute phase reactants low vitamin B6 levels and IBD. Am J Gastroenterol. 2003 Jan; 98(1):112-117. PubMed 12526945

9. Cattaneo M, Lombardi R, Lecchi A, Bucciarelli P, Mannucci PM. Low plasma levels of vitamin B6 are independently associated with a heightened risk of deep-vein thrombosis. Circulation. 2001 Nov 13;104(20):2442-2446. PubMed 11705822

10. Le Marchand L, White KK, Nomura AM, et al. Plasma levels of B vitamins and colorectal cancer risk: the multiethnic cohort study. Cancer Epidemiol Biomarkers Prev. 2009 Aug;18(8):2195-2201. PubMed 19661077

11. Wei EK, Giovannucci E, Selhub J, Fuchs CS, Hankinson SE, Ma J. Plasma vitamin B6 and the risk of colorectal cancer and adenoma in women. J Natl Cancer Inst. 2005 May 4;97(9):684-692. PubMed 15870439

12. Rimm EB, Willett WC, Hu FB, et al. Folate and

Collection Details:

Collection Instructions:

Lavender-top (EDTA) tube; amber plastic transport tube with amber-top. (If amber tubes are unavailable, cover standard transport tube completely, top and bottom, with aluminum foil. Identify specimen with patient's name directly on the container and on the outside of the aluminum foil. Secure with tape.) For amber plastic transport tube and amber-top, order LabCorp item No. 23594.

Collect blood by venipuncture into a lavender-top tube containing EDTA and mixed immediately by gentle inversion at least six times to ensure adequate mixing. The specimen must be separated and protected from light in an amber transport tube with amber stopper. Specimens should be stored refrigerated or frozen immediately and maintained at temperature during shipping and at the testing facility. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.

Refrigerate or freeze and protect from light.