Nursing care
Iron deficiency vs pernicious anaemia: cell size, nerves and lifelong treatment
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
Red cell size and nerve involvement separate them. Iron deficiency produces small, pale cells and usually reflects blood loss. Pernicious anaemia is B12 deficiency from absent intrinsic factor, producing large megaloblastic cells and nerve damage such as numbness and poor balance. Iron needs short-term oral teaching; pernicious anaemia needs B12 replacement for life.
Small pale cells or large cells with nerve signs
Iron deficiency is the most common cause of anaemia and typically produces microcytic, hypochromic red cells. Ferritin is low, serum iron falls and iron-binding capacity rises. Because the usual cause is blood loss, the finding should prompt a search for the source, especially in men and postmenopausal women, rather than supplements alone.
Pernicious anaemia is a specific cause of vitamin B12 deficiency in which autoimmune damage to the stomach stops production of intrinsic factor, the protein needed to absorb B12. The result is megaloblastic, macrocytic anaemia. Testing includes B12 levels and antibodies to intrinsic factor. The single biggest differentiator for nursing is that B12 deficiency damages nerves. Because the absorption problem is permanent, increasing dietary B12 alone does not correct it.
Neurological symptoms and the tongue
B12 deficiency can cause numbness and tingling in the hands and feet, loss of position and vibration sense, weakness, unsteady gait and, later, confusion or paranoia. Severe neurological disease can occur even without anaemia or large cells. Nerve damage may become permanent if treatment is delayed, so new paraesthesia or a fall in an older adult deserves attention.
Both conditions share fatigue, weakness, pallor and breathlessness on exertion, and both can affect the mouth, so a sore tongue does not settle the diagnosis alone. Pernicious anaemia is classically linked with a swollen, red, smooth tongue. Iron deficiency may bring pica, cracks at the mouth corners and spoon-shaped nails. A full blood count with red cell size, iron studies and B12 levels together provide the answer, not a single symptom.
Teaching oral iron safely
Oral iron is best absorbed when taken before meals, and taking it with vitamin C or orange juice can improve uptake. Warn the patient that dark stools are expected, and that constipation is a common side effect to manage with fluids, fibre and activity. Report black, tarry stools with other bleeding signs rather than assuming they are from iron.
Treatment usually continues for months after haemoglobin returns to normal to rebuild stores, so stopping when energy improves is a common error. Reinforce follow-up blood tests and any investigation of blood loss the provider has ordered. Keep iron out of reach of children, because accidental overdose in young children is dangerous.
Lifelong vitamin B12 and the folate trap
Because the problem in pernicious anaemia is absorption, B12 is commonly replaced by injection, though high-dose oral B12 is used in some patients. Treatment continues for life unless the underlying mechanism is corrected. Teach that feeling better is not a reason to stop, and set up a reliable way to receive regular doses. Missed doses can allow symptoms, including neurological ones, to return gradually.
Folic acid alone can improve the blood count in B12 deficiency while nerve damage continues or worsens. If a patient is taking folate without confirmed B12 status, raise it with the prescriber. People with pernicious anaemia also have a higher risk of gastric cancer, so ongoing follow-up and reporting of new stomach symptoms are worth teaching.
Worked scenario: two tired patients
A hypothetical 68-year-old has fatigue, tingling feet, an unsteady gait and a large mean cell volume. Options include teaching her to take iron with orange juice, explaining that she will need B12 replacement long term, recommending folic acid as a simple fix, or advising her to stop treatment once the tingling resolves. Long-term B12 is correct.
A second hypothetical patient has heavy periods, small pale cells, low ferritin and cravings for ice. Here oral iron teaching fits: take it before meals, expect dark stools, manage constipation and continue for the prescribed period. The exam is testing whether cell size and nerve findings change the teaching, not whether you remember that anaemia causes fatigue.
Sources and further reading
MSD Manual Professional: Iron Deficiency Anemia. Microcytic hypochromic anaemia, ferritin and iron studies, pica and koilonychia, oral iron timing, vitamin C, dark stools, constipation and investigating blood loss.
MSD Manual Professional: Vitamin B12 Deficiency. Megaloblastic anaemia, neurological signs without anaemia, intrinsic factor loss in pernicious anaemia, lifelong treatment and folate masking.
MedlinePlus: Pernicious anemia. Swollen red tongue, numbness and balance problems, intrinsic factor antibody testing, B12 injections, gastric cancer risk and permanent nerve damage if treatment is delayed.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our med-surg practice questions are the closest set to what this page covers.
One question from the med-surg set
A client with chronic obstructive pulmonary disease has an oxygen saturation of 88% on 2 L/min via nasal cannula and is alert with no distress. What should the nurse do first?
Rationale
In COPD a saturation of 88–92% is the therapeutic target, not an emergency, and this client is alert with no distress. The first action is the independent nursing intervention that is least invasive and most likely to help: sit them up and reassess. Turning the oxygen up to 6 L/min risks blunting the hypoxic drive, and calling rapid response or drawing an ABG escalates ahead of an assessment you have not finished.
Answer: B
Common questions
Why does pernicious anaemia need injections rather than diet changes?
The stomach no longer makes intrinsic factor, so dietary B12 is poorly absorbed. Replacement bypasses that problem and usually continues for life.
Are dark stools a reason to stop oral iron?
Dark stools are expected with oral iron. Black, tarry stools with dizziness, abdominal pain or other bleeding signs should still be reported, because iron deficiency often reflects blood loss.
Can nerve symptoms appear before anaemia in B12 deficiency?
Yes. Neurological damage can occur without anaemia or large red cells, so tingling, poor balance or memory change can be early clues that need assessment.
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