🔬 Vitamin B12 and Folate

Vitamin B12 (cobalamin) and Folate (vitamin B9) are essential nutrients for red blood cell production and nervous system health. Their deficiency leads to macrocytic (megaloblastic) anaemia and, in the case of B12, significant neurological impairment. Testing is common in the workup of anaemia, neuropathy, and cognitive decline. Distinguishing between dietary deficiency, malabsorption, and autoimmune Pernicious Anaemia is essential for correct management. In the UK, B12 is typically replaced via intramuscular injection if malabsorption is suspected.

Tests are indicated for macrocytic anaemia (high MCV), unexplained neurological symptoms (paresthesia, ataxia, confusion), or glossitis (sore, smooth tongue). They are also part of a standard screening panel for dementia and falls in the elderly. Patients with conditions predisposing to malabsorption (gastrectomy, ileal resection, IBD, Coeliac disease) or those on medications like metformin, PPIs, or methotrexate require monitoring. Alcoholics and those with restricted diets (e.g., strict veganism without supplementation) should also be screened.

Both markers are measured via venous blood samples using competitive binding immunoassays. For folate, 'Serum Folate' is most common, representing recent intake. 'Red Cell Folate' measures the folate within erythrocytes and reflects stores over the preceding 3-4 months. B12 assays measure total serum cobalamin. Patients do not strictly need to be fasting, but avoiding vitamin supplements for 24-48 hours prior to the test is advised to ensure a true baseline. Testing for Pernicious Anaemia (anti-IF antibodies) is a secondary step.

Normal B12: >200 ng/L (though 200-300 is often considered 'grey zone'). Normal Serum Folate: >4 μg/L (3-4 μg/L is borderline). Normal results with a high MCV (macrocytosis) should prompt clinicians to look for other causes: excess alcohol consumption, liver disease, hypothyroidism, or medications like hydroxycarbamide or azathioprine. In a healthy individual with a balanced diet, stores of B12 in the liver can last 3-5 years, whereas folate stores last only about 4 months.

Low B12 should be followed by testing for anti-Intrinsic Factor (anti-IF) antibodies to check for Pernicious Anaemia; while highly specific, they are not 100% sensitive. Negative anti-IF does not rule out PA. If B12 is low but folate is normal, think malabsorption or diet. If both are low, think general malabsorption (e.g., Coeliac). Borderline B12 (200-300 ng/L) with symptoms warrants a trial of replacement or further metabolic testing (MMA). Always check B12 levels before starting folate replacement.

B12 deficiency is typically suggested by levels <150-200 ng/L, often accompanied by megaloblastic anaemia (high MCV) and hypersegmented neutrophils. Low Folate (<3 μg/L) similarly causes megaloblastic changes. If B12 is borderline, elevated levels of Methylmalonic Acid (MMA) or Homocysteine can confirm tissue-level deficiency. In Pernicious Anaemia, anti-Intrinsic Factor antibodies are usually present. It is crucial to note that folate deficiency can mask b12 deficiency on a blood film, but treating folate alone in a B12-deficient patient can precipitate subacute combined degeneration of the spinal cord.

B12 and folate are essential for DNA synthesis and neurological function. Deficiency can lead to reversible megaloblastic anaemia but potentially irreversible neurological damage, including peripheral neuropathy, subacute combined degeneration of the cord (SCDC), and cognitive impairment/dementia. Identifying the cause (dietary, malabsorption like Coeliac or Crohn's, or autoimmune Pernicious Anaemia) is vital for determining whether treatment is lifelong (IM injections) or short-term oral replacement. Foetal neural tube defects are linked to maternal folate deficiency.

A common error is diagnosing B12 deficiency solely on a 'borderline' result without clinical correlation; asymptomatic borderline low B12 often requires no treatment. Another pitfall is failing to re-check folate levels after starting B12 injections, as the surge in erythropoiesis can consume folate stores. Pregnancy and oral contraceptive use can falsely lower serum B12 levels without reflecting true tissue deficiency. Don't overlook the fact that Metformin use is a very common cause of B12 malabsorption in Type 2 Diabetics.

Serum B12 is a 'blunt' instrument because it measures total B12, including the fraction bound to haptocorrin which is not available to tissues. Therefore, some patients with 'normal' serum levels may still have intracellular deficiency. Folate levels are highly sensitive to recent dietary intake; a single meal can normalise serum folate even if stores are low. Red cell folate is a more accurate measure of long-term stores but is less commonly used in routine UK practice due to cost and technical requirements. High B12 levels are usually clinically insignificant but can be associated with myeloproliferative disorders or liver disease.

MLA candidates must know that B12 deficiency causes neurological symptoms (ataxia, loss of vibration/proprioception) whereas folate deficiency usually does not. If a patient is deficient in both, start B12 replacement FIRST to prevent subacute combined degeneration of the spinal cord (SCDC). Pernicious anaemia is an autoimmune condition against parietal cells or intrinsic factor, leading to B12 malabsorption in the terminal ileum. Also, remember that hydroxycobalamin is the standard UK replacement for B12.