Pernicious anaemia: what it is, how it's diagnosed, why it's lifelong

What it is

Pernicious anaemia (PA) is an autoimmune disease characterised by:

• Antibodies against parietal cells (parietal cell antibodies, PCA).
• Antibodies against intrinsic factor (intrinsic factor antibodies, IFAB).
• Atrophic gastritis of the gastric body and fundus (the acid-and-intrinsic-factor-producing zone).
• Loss of intrinsic factor production.
• Resulting inability to absorb dietary B12.
• Eventual B12 deficiency with anaemia and neurological symptoms.

The disease typically progresses slowly over years. Patients in early stages may be asymptomatic with positive antibodies and normal B12 (because of liver stores). As stores deplete, symptoms emerge.

Why it matters

Without intrinsic factor, oral B12 supplementation — even at high doses — cannot work efficiently. The body has a small passive-diffusion absorption pathway that handles roughly 1% of an oral dose, but this is rarely enough to maintain serum B12 in pernicious anaemia. Lifelong injection-based supplementation is the standard.

Untreated, pernicious anaemia progresses to severe B12 deficiency with potentially irreversible neurological damage. Subacute combined degeneration of the spinal cord (SCD) is the classical advanced presentation — progressive numbness, weakness, gait ataxia, and (in extreme cases) loss of vibration and proprioception. Early treatment prevents progression; treatment started after SCD often only partially reverses the changes.

Symptoms

The early picture is non-specific:

• Fatigue, often described as exhaustion rather than tiredness.
• Glossitis — a smooth, beefy-red, painful tongue.
• Cognitive change — brain fog, slowed thinking, poor concentration.
• Pallor with mild yellow tinge (megaloblastic anaemia + mild haemolysis).
• Peripheral neuropathy — tingling, numbness in feet and hands, classically symmetric.
• Mood changes — low mood, irritability.
• Loss of appetite, weight loss.
• Palpitations and breathlessness on exertion (from anaemia).

For wider symptom context see B12 deficiency symptoms explained.

Diagnosis

The standard diagnostic workup:

• Serum B12 — typically low (<150 pmol/L) but can be borderline in early disease.
• Full blood count (FBC) — macrocytic anaemia (raised MCV), but MCV can be masked by concurrent iron deficiency. Hypersegmented neutrophils on the blood film support megaloblastic anaemia.
• Intrinsic factor antibodies (IFAB) — specific (>95%) but not sensitive (40–60%). A positive IFAB essentially confirms PA. A negative does not exclude it.
• Parietal cell antibodies (PCA) — more sensitive (~85% in PA) but less specific (also positive in autoimmune thyroid disease and other autoimmunity). A positive PCA with a negative IFAB still supports PA in the right clinical context.
• Methylmalonic acid (MMA) and homocysteine — elevated in true cellular B12 deficiency. Useful when serum B12 is borderline and the clinical picture is suggestive.
• Folate — always measured alongside B12; folate deficiency can also cause macrocytic anaemia, and folate replacement without B12 in a deficient patient can precipitate neurological damage.

The Schilling test, historically the gold standard, is now obsolete. Modern serology plus clinical context replaces it.

Treatment

The standard UK BNF regime is hydroxocobalamin 1 mg intramuscular, with the loading and maintenance schedule depending on whether neurological symptoms are present:

• No neurological symptoms: Loading — hydroxocobalamin 1 mg IM three times a week for 2 weeks (six injections). Maintenance — hydroxocobalamin 1 mg IM every 3 months for life.
• Neurological symptoms: Loading — hydroxocobalamin 1 mg IM on alternate days until no further improvement, often over several weeks. Maintenance — hydroxocobalamin 1 mg IM every 2 months for life.

Hydroxocobalamin is preferred over cyanocobalamin in the UK because it has a longer plasma half-life, allowing the 2–3-monthly maintenance interval. See hydroxocobalamin vs cyanocobalamin.

Why it is lifelong

The autoimmune process does not resolve. The parietal cells continue to be destroyed; intrinsic factor production does not recover. Stopping treatment will lead to recurrence of deficiency over months to years (depending on residual liver stores).

Patient stories of "my B12 levels are normal now so I don't need injections" are misleading. The serum B12 is normal because of the injections. Stop the injections and the serum B12 will fall as stores deplete and absorption fails.

Associated autoimmune conditions

Pernicious anaemia clusters with other autoimmune disorders. We screen at diagnosis and periodically for:

• Autoimmune thyroid disease — Hashimoto's hypothyroidism most commonly. See thyroid panel explained.
• Type 1 diabetes.
• Addison's disease (autoimmune adrenal insufficiency).
• Vitiligo.
• Coeliac disease.

Family history matters too — first-degree relatives of patients with PA have roughly 3 times the population risk and should consider screening if they develop suggestive symptoms.

Risk of gastric malignancy

Chronic autoimmune atrophic gastritis is associated with an increased risk of gastric adenocarcinoma and gastric carcinoid tumours. The absolute risk is small but real. UK practice is not routine endoscopic surveillance, but any new dyspeptic symptoms in a patient with PA warrants gastroscopy.

Living with pernicious anaemia

Once stabilised, most patients on a 3-monthly injection regime live entirely normal lives. Practical considerations:

• Travel — carry your prescription and a brief medical letter if you need an injection while abroad. We can issue these.
• Some patients feel their energy dips in the 3rd month before the next injection. Discuss with us — occasionally a 2-monthly interval is more comfortable.
• NHS vs private maintenance — most patients get their injections free on NHS prescription via the GP nurse. Private is an option for patients who prefer more flexible timing or experience access difficulties.

Booking

We provide hydroxocobalamin injections at all our locations — Manchester, Old Trafford, Sale, Altrincham and Chorlton. Our brand vitamin B12 page covers the wider context.

For related conditions see metformin and B12 deficiency, PPIs and B12 deficiency and vegan B12. For initial blood testing see our blood test clinic.