B Vitamins Blood Test Guide: B1, B5, B6, B7, B9, B12 and What Your Levels Mean

Why Test B Vitamins?

B vitamins are essential cofactors in hundreds of enzymatic reactions—from converting carbohydrates into usable energy to synthesising neurotransmitters and replicating DNA. Because they are water-soluble, the body cannot store large reserves, making dietary adequacy and absorption critical. Deficiencies are more common than many clinicians expect, particularly in older adults, vegetarians, people with gastrointestinal conditions, and those taking certain medications. A targeted blood panel can detect shortfalls months before classic deficiency symptoms appear.[1]

Vitamin B1 (Thiamine)

Thiamine is required for the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes, which sit at the gateway between glycolysis and the citric acid cycle. Without adequate B1, cells cannot efficiently extract energy from glucose—an especially serious problem for the brain and heart.

Testing uses whole-blood thiamine diphosphate (the active coenzyme form), reported in nmol/L. Normal range is approximately 70–180 nmol/L. Deficiency below 70 nmol/L is associated with Wernicke encephalopathy (confusion, ataxia, ophthalmoplegia) and beriberi (cardiovascular or peripheral neuropathy forms). At-risk groups include heavy alcohol users, patients receiving parenteral nutrition, and people who rely heavily on refined carbohydrates.[2] Learn more about the thiamine blood test.

Vitamin B5 (Pantothenic Acid)

Pantothenic acid is the precursor to coenzyme A (CoA), the universal acyl-group carrier that participates in fatty acid synthesis, the citric acid cycle, and acetylcholine production. Isolated B5 deficiency is rare because it is widely distributed in food, but it can appear during severe malnutrition or in patients with pantothenate kinase-associated neurodegeneration (PKAN).

Plasma pantothenic acid is measured in micromol/L, with a typical reference range of 1.6–4.3 µmol/L. Symptoms of deficiency include burning feet syndrome, fatigue, irritability, and gastrointestinal distress. Routine testing is rarely ordered unless malnutrition or a rare metabolic disorder is suspected. Explore the pantothenic acid biomarker page for clinical context.

Vitamin B6 (Pyridoxine)

Vitamin B6 encompasses three natural forms—pyridoxine, pyridoxal, and pyridoxamine—all of which are converted to the active coenzyme pyridoxal-5-phosphate (PLP). PLP participates in over 100 enzymatic reactions, most notably transamination (amino acid metabolism) and the synthesis of serotonin, dopamine, and GABA. It also participates in the conversion of homocysteine to cystathionine, making B6 status a direct determinant of homocysteine levels.[3]

Plasma PLP is the preferred measurement, with a normal range of roughly 20–125 nmol/L. Deficiency is linked to seborrheic dermatitis, glossitis, peripheral neuropathy, and elevated cardiovascular risk via homocysteine accumulation. Paradoxically, chronic supplementation above 200 mg/day can cause sensory neuropathy. Visit the vitamin B6 (PLP) biomarker page for dosing considerations.

Vitamin B7 (Biotin)

Biotin serves as the prosthetic group for five carboxylase enzymes involved in gluconeogenesis, fatty acid synthesis, and amino acid catabolism. It also plays a role in gene expression regulation. Deficiency is uncommon in healthy adults but can follow prolonged raw egg-white consumption (avidin binds biotin), biotinidase deficiency, or extended parenteral nutrition.

Serum biotin is measured in pmol/L, with a broadly accepted normal range of 133–1329 pmol/L. Deficiency presents as alopecia, scaly dermatitis, conjunctivitis, and neurological symptoms including lethargy and paresthesias. A critical clinical note: high-dose biotin supplements (often taken for hair and nail health) can interfere with immunoassays for troponin and thyroid hormones, potentially masking serious diagnoses.[4] See the biotin test page for interference warnings.

Vitamin B9 (Folate)

Folate is essential for one-carbon metabolism, which underpins nucleotide synthesis (particularly thymine), DNA methylation, and the remethylation of homocysteine back to methionine. Adequate folate is critical during early pregnancy to prevent neural tube defects.[5]

Two measurements are available: serum folate (reflects recent intake, normal approximately 7–45 nmol/L) and red blood cell (RBC) folate (reflects long-term tissue status, normal approximately 340–1020 nmol/L). RBC folate is the preferred marker for assessing true body stores because it is less susceptible to short-term dietary fluctuations. Low folate causes megaloblastic anemia, glossitis, and elevated homocysteine. The RBC folate biomarker page explains when to order each test.

Vitamin B12 (Cobalamin)

Vitamin B12 is required for two enzymes: methionine synthase (which remethylates homocysteine using folate-derived methyl groups) and methylmalonyl-CoA mutase (which metabolises odd-chain fatty acids and certain amino acids). B12 is found almost exclusively in animal products, making vegans and vegetarians at elevated risk without supplementation. Absorption requires intrinsic factor secreted by gastric parietal cells, so autoimmune gastritis and gastric surgery are major causes of deficiency.

Serum B12 normal range is approximately 148–740 pmol/L. Deficiency causes megaloblastic anemia, subacute combined degeneration of the spinal cord, and irreversible neurological damage if untreated. Methylmalonic acid (MMA) and holotranscobalamin (active B12) are more sensitive functional markers useful when borderline serum B12 levels are ambiguous.[6] Explore the full vitamin B12 biomarker reference.

The Homocysteine Connection

Homocysteine is an intermediate amino acid generated during methionine metabolism. Its safe clearance depends on three B vitamins: B6 (transsulfuration pathway), B9 (remethylation), and B12 (remethylation cofactor). Deficiency in any of these elevates plasma homocysteine, which is itself an independent cardiovascular and cognitive risk factor. Normal fasting homocysteine is below 15 µmol/L; levels above 30 µmol/L are classified as severe hyperhomocysteinaemia.[7]

An elevated homocysteine result is therefore a functional signal that prompts measurement of B6, B9, and B12. Treating the underlying deficiency typically normalises homocysteine within weeks. See the homocysteine biomarker page for cardiovascular risk interpretation.

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