Vitamin B12 (Cobalamin) and Folate Panel, Serum
The Vitamin B12 and Folate panel measures serum cobalamin and serum folate together to investigate megaloblastic anaemia, peripheral neuropathy, and malabsorption. The two are paired because their deficiencies overlap haematologically, and folate repletion can mask B12 deficiency while neurological damage progresses. Indeterminate results are clarified with methylmalonic acid and homocysteine.
2
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — however, practitioners should document recent B12 or folate supplementation, as both significantly affect serum levels
Fasting
What is the B12 & Folate (Quest) test?
The Vitamin B12 and Folate Panel measures serum cobalamin and serum folate together, providing a combined assessment of the two micronutrients most commonly implicated in megaloblastic anaemia and one-carbon metabolism disorders. Pairing the two analytes is clinically deliberate: both deficiencies produce overlapping haematological pictures, and distinguishing them is essential because management and the risk of irreversible neurological harm differ substantially between them. In the diagnostic landscape, this paired panel is the standard first-line investigation for macrocytic anaemia, unexplained neuropathy, and suspected malabsorption. Serum B12 and serum folate reflect circulating status and recent intake respectively; where results fall in indeterminate zones, practitioners escalate to functional markers — methylmalonic acid for B12 and homocysteine for both — or to red cell folate for a more stable measure of tissue stores. The combined panel is favoured over single-analyte testing whenever the clinical picture cannot reliably separate the two deficiencies on presentation alone.
What does the B12 & Folate (Quest) test measure?
Cobalamin is a water-soluble cobalt-containing vitamin obtained almost exclusively from animal-derived foods. Dietary B12 is liberated by gastric acid and pepsin, bound to haptocorrin, then transferred to intrinsic factor (secreted by gastric parietal cells) for receptor-mediated absorption in the terminal ileum. In circulation it is carried by transcobalamin (the metabolically active holotranscobalamin fraction) and haptocorrin, with substantial hepatic storage giving a body half-life measured in years. It functions as a cofactor for methionine synthase and methylmalonyl-CoA mutase.
Serum B12 reflects total circulating cobalamin and is the standard first-line marker for deficiency assessment, which manifests as megaloblastic anaemia, peripheral neuropathy, subacute combined degeneration, and cognitive change. Total B12 has recognised limitations in the indeterminate zone (200–400 pg/mL), where functional markers — methylmalonic acid and homocysteine — provide clarification. Practitioners use it to investigate macrocytosis, malabsorption, and neurological presentations.
Folate is a water-soluble B-group vitamin (B9) absorbed primarily in the jejunum after deconjugation, then reduced to tetrahydrofolate derivatives. It serves as the central one-carbon donor for purine and thymidylate synthesis and for the remethylation of homocysteine to methionine. Serum folate reflects recent dietary intake and fluctuates rapidly, whereas red cell folate is a more stable index of tissue stores over the preceding months.
Serum folate is used to investigate megaloblastic anaemia and to assess folate status in malabsorption, alcohol use, and inadequate intake. It is interpreted alongside B12 because folate supplementation can correct the anaemia of B12 deficiency while allowing neurological damage to progress unchecked. Folate status is also clinically relevant in the preconception and periconceptional period for neural tube defect risk reduction.
Why is the B12 & Folate (Quest) test ordered?
- Investigation of macrocytic or megaloblastic anaemia identified on full blood count with raised mean cell volume
- Peripheral neuropathy, paraesthesia, or suspected subacute combined degeneration of the cord where B12 deficiency is in the differential
- Cognitive change, unexplained fatigue, or mood disturbance in older adults where micronutrient deficiency is being excluded
- Assessment of malabsorptive conditions — coeliac disease, Crohn's disease, pernicious anaemia, or post-bariatric and post-gastrectomy states
- Patients on long-term metformin or acid-suppressing therapy (proton pump inhibitors, H2 antagonists) where B12 depletion is a recognised concern
- Vegan and strict vegetarian patients with limited dietary cobalamin intake
- Preconception and periconceptional folate status assessment as part of neural tube defect risk reduction planning
- Investigation of glossitis, angular stomatitis, or unexplained raised homocysteine prompting micronutrient evaluation
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — however, practitioners should document recent B12 or folate supplementation, as both significantly affect serum levels
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Protect from light, as folate is photosensitive. Separate serum promptly and avoid haemolysis, which artificially elevates folate through release of intra-erythrocyte stores. Where baseline status rather than peak levels is required, advise withholding supplementation for an appropriate interval prior to collection. # REVIEWER FLAG: Confirm exact tube type, light-protection, and stability protocol with Quest specimen handling guidelines before publication.
Specimen requirements
Minimum 1.0 mL serum. Separate from cells promptly and protect from light; avoid haemolysed specimens.
What can affect B12 & Folate (Quest) results?
Oral, sublingual, or parenteral cobalamin raises serum B12 rapidly and can mask underlying deficiency. Practitioners should document supplementation type and timing relative to specimen collection.
Serum folate reflects recent intake and rises quickly after supplementation or fortified food, potentially normalising a result despite depleted tissue stores. Red cell folate is less affected.
Long-term metformin reduces ileal calcium-dependent B12 absorption, lowering serum B12 over time independent of intake. # REVIEWER FLAG: Drug-interaction claim based on established but probabilistic clinical knowledge — reviewer to verify against pharmacological literature.
Acid suppression impairs the gastric release of protein-bound B12 required for intrinsic factor binding, reducing absorption with prolonged use. # REVIEWER FLAG: Confirm magnitude and clinical significance against current literature.
Physiological haemodilution and altered binding protein levels in pregnancy can lower measured B12 without true deficiency; oral contraceptives may modestly reduce both analytes. Interpret with trimester and clinical context.
Alcohol impairs folate absorption, hepatic storage, and enterohepatic recycling, and reduces dietary intake, commonly producing low serum folate.
What do B12 & Folate (Quest) results mean?
Elevated
Elevated B12 most often reflects recent supplementation or injection, but can also occur in hepatic disease, myeloproliferative disorders, and renal impairment, and warrants clinical context rather than reassurance alone. Elevated folate is typically dietary or supplemental and is not clinically concerning in isolation, though it can mask the haematological signs of concurrent B12 deficiency.
Normal
Normal results for both analytes make combined deficiency unlikely, but recent supplementation can produce falsely reassuring values. Where clinical suspicion is high despite normal serum levels, methylmalonic acid and homocysteine provide functional confirmation. Normal folate with low B12 still requires B12-directed management before any folate repletion.
Reduced
Reduced B12 supports cobalamin deficiency — consider pernicious anaemia, malabsorption, dietary inadequacy, and drug effects (metformin, acid suppression); confirm with methylmalonic acid in the indeterminate zone. Reduced folate supports folate deficiency from inadequate intake, malabsorption, alcohol use, or increased demand; red cell folate clarifies chronicity.
Note: Folate must never be repleted in isolation when B12 deficiency is possible, as it can correct the anaemia while permitting progression of irreversible neurological damage. Serum folate fluctuates with recent meals and is a poor index of tissue stores compared with red cell folate. Haemolysis of the specimen artificially raises folate by releasing the much higher intra-erythrocyte concentration. Total B12 does not distinguish the metabolically active holotranscobalamin fraction.
Frequently asked questions
Both deficiencies cause overlapping megaloblastic anaemia, so they are paired to distinguish the cause. The distinction matters clinically because folate repletion can correct the anaemia of B12 deficiency while allowing irreversible neurological damage to progress.
How to access the B12 & Folate (Quest) test
The B12 & Folate (Quest) reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: June 3, 2026
Last reviewed: June 3, 2026
- Devalia V, Hamilton MS, Molloy AM. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. (2014)
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. (2013)
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. (2017)
- Allen LH. Causes of vitamin B12 and folate deficiency. Food Nutr Bull. (2008)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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The Vitamin B12 test measures serum cobalamin, the front-line investigation for suspected deficiency presenting with macrocytic anaemia, neuropathy, or cognitive symptoms. Low results reflect dietary insufficiency, malabsorption (pernicious anaemia, ileal disease, post-surgical states), or drug interference (metformin, acid suppression).
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