B12 and Folate Deficiency: Symptoms, Tests & Treatment

Why B12 and Folate Matter

Vitamin B12 (cobalamin) and folate (vitamin B9) are essential B vitamins that work together in DNA synthesis, red blood cell production, and nervous system function. Deficiency in either vitamin can lead to megaloblastic anaemia, where red blood cells become abnormally large and fail to function properly. B12 deficiency carries the additional risk of neurological damage, which can become permanent if treatment is delayed.

In the UK, B12 deficiency affects an estimated 6% of adults under 60 and up to 20% of those over 60. Folate deficiency is less common since the body stores less folate and symptoms tend to develop within a few months of inadequate intake, prompting earlier investigation.

Serum B12

The serum B12 test measures the total amount of vitamin B12 in your blood. In most UK laboratories, the normal reference range is approximately 200–900 ng/L (or 148–664 pmol/L). A level below 200 ng/L is generally considered deficient, while levels between 200–300 ng/Lare considered borderline or “indeterminate” and may require further investigation.

Serum B12 has limitations. It measures both active and inactive forms of the vitamin, meaning your total level may appear normal even when the biologically active fraction (holotranscobalamin) is low. If clinical suspicion is high despite a normal serum B12, your doctor may request additional tests.

Serum Folate

Serum folate measures the level of folate circulating in your blood. The normal range is typically 3.0–20.0 µg/L. A level below 3.0 µg/L indicates folate deficiency. Red cell folate can also be measured and is a better indicator of long-term folate status, though it is requested less frequently in practice.

It is important to check both B12 and folate together. Treating folate deficiency without addressing concurrent B12 deficiency can worsen neurological symptoms — a situation known as “masking” B12 deficiency.

Methylmalonic Acid and Homocysteine

When serum B12 results are borderline, two additional tests can help clarify the picture:

• Methylmalonic acid (MMA) — elevated in B12 deficiency (but not folate deficiency). This is the most specific test for functional B12 deficiency.
• Homocysteine — elevated in both B12 and folate deficiency. It is sensitive but less specific, as levels can also rise with kidney disease, hypothyroidism, and certain medications.

If both MMA and homocysteine are elevated with a borderline serum B12, functional B12 deficiency is very likely. If only homocysteine is raised, folate deficiency is the more probable cause.

Pernicious Anaemia

Pernicious anaemia is an autoimmune condition in which the body produces antibodies against intrinsic factor, a protein produced by the stomach lining that is essential for B12 absorption. It is the most common cause of B12 deficiency in the UK. Testing involves:

• Intrinsic factor antibodies (IFAb) — positive in approximately 50–60% of patients with pernicious anaemia. A positive result is highly specific.
• Gastric parietal cell antibodies (GPC) — present in around 90% of patients, but also found in 10% of the healthy population, making it less specific.

Patients diagnosed with pernicious anaemia require lifelong B12 replacement, typically via intramuscular hydroxocobalamin injections. The standard NHS regimen is an injection every 2–3 months after an initial loading course.

Neurological Symptoms of B12 Deficiency

While both B12 and folate deficiency cause fatigue, breathlessness, and pallor (signs of anaemia), B12 deficiency can also cause neurological symptoms that folate deficiency does not:

• Pins and needles (paraesthesia) in hands and feet
• Numbness or reduced sensation
• Difficulty walking and balance problems
• Cognitive changes including poor memory and confusion
• Mood disturbance, irritability, and depression
• Visual disturbances

NICE and the British Society for Haematology recommend that patients with neurological symptoms should be treated urgently with B12 injections, even before blood test results are available, to prevent irreversible nerve damage.

Who Is at Risk?

• Vegans and vegetarians — B12 is found almost exclusively in animal products.
• Older adults — reduced stomach acid production impairs B12 absorption.
• People taking metformin — long-term use reduces B12 absorption.
• People taking proton pump inhibitors (PPIs) — long-term acid suppression can reduce B12 absorption.
• People with coeliac disease or Crohn’s disease — malabsorption of both B12 and folate.
• Pregnant and breastfeeding women — increased folate requirements; B12 demands also rise.

Treatment and Monitoring

B12 deficiency with neurological symptoms: hydroxocobalamin injections on alternate days until symptoms stop improving, then every 2 months. B12 deficiency without neurological symptoms: three injections per week for 2 weeks, then every 3 months. Dietary deficiency may be managed with oral cyanocobalamin (1 mg daily) or dietary changes if the patient can absorb B12 normally.

Folate deficiency: folic acid 5 mg daily for 4 months. The underlying cause should be addressed. Ensure B12 status is checked and corrected first.

Monitoring typically involves a repeat FBC at 8–10 weeks to confirm that haemoglobin is rising and the MCV is normalising. Serum B12 levels are not useful for monitoring once injections have started, as they will be elevated from the treatment itself.

Getting Tested at Home

If you are experiencing fatigue, tingling, or cognitive changes, getting your B12 and folate levels checked is a sensible first step. A mobile phlebotomist can take your blood at home and send it to an accredited laboratory for analysis. This is particularly helpful for elderly patients or those with mobility difficulties who find it hard to attend a GP surgery. Visit our home blood test page to learn more about the tests available.