Anemia

Macrocytic Anemia

Distinguish megaloblastic from nonmegaloblastic macrocytosis, interpret vitamin B12, methylmalonic acid and homocysteine, identify pernicious anemia and malabsorption, and recognize medications, alcohol, liver disease, reticulocytosis and marrow disorders.

Audience: internal medicine residents Last reviewed: July 2026

Bedside approach

Answer these questions first

Is the macrocytosis genuine? Repeat the CBC when unexpected and review prior MCV values, analyzer flags, transfusion history and the peripheral smear.
Is the marrow responding? Reticulocytosis itself raises the MCV and may indicate hemolysis, blood loss or marrow recovery.
Is the pattern megaloblastic? Macro-ovalocytes and hypersegmented neutrophils support impaired DNA synthesis from B12 or folate deficiency.
Are neurologic symptoms present? Paresthesias, gait disturbance, impaired proprioception, weakness, cognitive change or optic symptoms increase the urgency of B12 assessment and treatment.
Is there a reversible exposure? Review alcohol, medications, nitrous oxide, dietary restriction, gastrointestinal surgery and malabsorptive disease.
Could this be a marrow disorder? Progressive cytopenias, dysplasia, circulating blasts or an unexplained persistent abnormality may require marrow evaluation.

Urgent escalation

  • New or progressive neurologic deficits suggesting B12 deficiency
  • Severe symptomatic anemia or cardiovascular compromise
  • Pancytopenia with infection or clinically important bleeding
  • Schistocytes, thrombocytopenia and extreme LDH elevation requiring distinction between pseudo-TMA and true TTP
  • Circulating blasts or suspected acute leukemia
  • Rapidly progressive cytopenias or severe marrow failure
Do not delay vitamin B12 replacement while waiting for test results when megaloblastic anemia with neurologic symptoms is strongly suspected. [1]

1. Initial evaluation

  1. Confirm the CBC abnormality and review previous hemoglobin, MCV, RDW, WBC and platelet values.
  2. Obtain an absolute reticulocyte count and inspect the peripheral smear.
  3. Check vitamin B12 and folate, while considering iron studies because mixed deficiency may normalize the MCV.
  4. Review alcohol, liver disease, hypothyroidism, medications, gastrointestinal disease, surgery, diet and nitrous-oxide exposure.
  5. Use MMA when the B12 result is indeterminate and the diagnosis remains uncertain.
  6. Escalate to hematology or bone-marrow evaluation when the abnormality remains unexplained or the smear suggests marrow disease.

Common initial tests

Core evaluation

  • CBC with differential and prior-count review
  • Absolute reticulocyte count
  • Peripheral smear
  • Vitamin B12
  • Folate
  • Iron studies when mixed deficiency is possible
  • Creatinine
  • AST, ALT, bilirubin and albumin
  • TSH when clinically appropriate

Cause-directed tests

  • Methylmalonic acid
  • Homocysteine
  • Anti-intrinsic-factor antibody
  • Celiac testing
  • LDH, haptoglobin and bilirubin
  • Copper and zinc when indicated
  • HIV testing when clinically appropriate
  • Bone-marrow aspirate and biopsy

2. Peripheral smear and pattern recognition

Finding Important associations Practical interpretation
Macro-ovalocytes Vitamin B12 deficiency, folate deficiency and other megaloblastic states Support impaired DNA synthesis, particularly when accompanied by hypersegmented neutrophils.
Hypersegmented neutrophils Megaloblastic hematopoiesis Strongly supports B12 or folate deficiency but is not completely specific.
Round macrocytes Alcohol, liver disease and other nonmegaloblastic causes Interpret with liver tests, medication history and alcohol exposure.
Polychromasia Reticulocytosis Consider hemolysis, blood loss or recovery after marrow suppression.
Target cells Liver disease, hemoglobinopathy and postsplenectomy states Review liver disease, splenic status and hemoglobin studies.
Dysplastic neutrophils or platelets Myelodysplastic neoplasm or other marrow disease Persistent cytopenias and dysplasia warrant hematology review and possible marrow evaluation.
Schistocytes TMA, DIC, mechanical hemolysis or severe B12 deficiency Integrate platelet count, reticulocyte response, LDH, neurologic and renal findings, and the entire clinical syndrome.

3. Vitamin B12 deficiency

Do not exclude vitamin B12 deficiency solely because anemia or macrocytosis is absent. Neurologic, psychiatric and cognitive manifestations can occur without classic hematologic findings. [1]

Common manifestations

Hematologic

  • Macrocytosis or macrocytic anemia
  • Hypersegmented neutrophils
  • Leukopenia or thrombocytopenia
  • Pancytopenia
  • Ineffective erythropoiesis
  • Indirect bilirubin and LDH elevation
  • Low reticulocyte response

Neurologic and systemic

  • Paresthesias or numbness
  • Gait disturbance
  • Impaired vibration or proprioception
  • Weakness
  • Cognitive or psychiatric change
  • Glossitis
  • Fatigue and reduced exercise tolerance

Risk factors

  • Autoimmune gastritis
  • Total or partial gastrectomy
  • Terminal ileal resection or ileal disease
  • Bariatric surgery
  • Celiac disease or other malabsorption
  • Vegan or highly restricted diet
  • Metformin exposure
  • Long-term acid suppression
  • Nitrous-oxide exposure
  • Older age and impaired nutritional intake
Metformin-associated reduction in B12 is considered a common adverse effect, particularly with higher doses, longer treatment and existing risk factors. [4]

4. MMA and homocysteine

Test pattern B12 deficiency Folate deficiency Important caveats
MMA elevated Supports functional B12 deficiency Usually not elevated from folate deficiency alone Renal dysfunction and other factors can increase MMA. Interpret against the laboratory range and kidney function.
Homocysteine elevated May be elevated May be elevated Less specific than MMA and affected by renal function, nutrition, medications and other conditions.
Both elevated Supports B12 deficiency Consider combined B12 and folate deficiency if other findings support it Interpret with serum B12, folate, renal function and treatment already received.
MMA normal, homocysteine elevated B12 deficiency becomes less likely Compatible with folate deficiency Also consider vitamin B6 status, kidney disease, medications and other causes of hyperhomocysteinemia.
Draw confirmatory metabolic testing before replacement when feasible, but do not delay treatment for a patient with neurologic symptoms or a strongly convincing presentation.

Interactive tool

B12, MMA and homocysteine interpretation

This tool organizes the laboratory pattern. It does not replace the local laboratory reference range, kidney-function assessment or clinical judgment.

Enter the B12 value and select the accompanying findings.

This tool uses broad NICE total-B12 categories: below 180 pg/mL is compatible with confirmed deficiency, 180–350 pg/mL is indeterminate, and above 350 pg/mL makes deficiency less likely. Local assays and clinical context remain controlling.

5. Autoimmune gastritis and pernicious anemia

Autoimmune gastritis can impair intrinsic-factor production and vitamin B12 absorption. Anti-intrinsic-factor antibody is useful when autoimmune gastritis is suspected, but a negative result does not reliably exclude it. [1]

Features that raise suspicion

  • Vitamin B12 deficiency without a clear dietary explanation
  • Personal or family history of autoimmune disease
  • Autoimmune thyroid disease
  • Vitiligo
  • Iron deficiency preceding or accompanying B12 deficiency
  • Gastric atrophy or prior gastric pathology
  • Persistent deficiency despite adequate oral intake

Testing approach

  • Anti-intrinsic-factor antibody
  • Consider additional evaluation for autoimmune gastritis when suspicion remains despite a negative result
  • Assess iron deficiency and other nutritional deficiencies
  • Consider gastroenterology evaluation based on symptoms, age, local practice and concern for gastric pathology
The term “pernicious anemia” traditionally describes the hematologic consequence of autoimmune intrinsic-factor failure. Current guidance often uses the broader term autoimmune gastritis because deficiency may exist before anemia develops.

6. Vitamin B12 treatment

When neurologic symptoms or severe megaloblastic disease are present, begin replacement promptly rather than waiting for every diagnostic result. [1]
Clinical setting Preferred framework Important considerations
Dietary deficiency Oral replacement is often appropriate with dietary counseling Confirm there is not another risk factor suggesting malabsorption or an irreversible cause.
Possible malabsorption Oral or intramuscular treatment may be used depending on severity, adherence and cause When oral therapy is used for suspected malabsorption, NICE recommends a dose of at least 1 mg daily. [1]
Autoimmune gastritis Lifelong intramuscular replacement is recommended by NICE Ongoing therapy is required because the cause is irreversible.
Total gastrectomy or complete terminal ileal resection Lifelong intramuscular replacement Absorption is permanently impaired.
Neurologic disease or severe presentation Intramuscular therapy is commonly favored for rapid, dependable replacement Do not delay treatment while awaiting confirmatory tests.
Medicine-induced deficiency Replace B12 and reassess the implicated medication Continue, modify or stop the medication based on clinical need and the responsible prescriber’s assessment.

Expected response

  • Symptoms may begin improving within days to weeks.
  • Reticulocytosis should occur after effective treatment.
  • Hemoglobin and cytopenias should progressively recover.
  • Neurologic recovery may be slower and can remain incomplete.
  • Lack of response should prompt review of adherence, diagnosis, ongoing malabsorption and coexisting iron, folate or marrow disease.
Iron deficiency can become apparent after B12 replacement as effective erythropoiesis resumes. Recheck iron studies when the hematologic response is incomplete.

7. Folate deficiency

Common causes

  • Reduced dietary intake
  • Alcohol use
  • Malabsorption
  • Pregnancy or increased physiologic demand
  • Chronic hemolysis
  • Dialysis
  • Medications affecting folate metabolism, including methotrexate, trimethoprim and selected antiseizure medications

Typical laboratory pattern

  • Macrocytosis or macrocytic anemia
  • Macro-ovalocytes and hypersegmented neutrophils
  • Low folate measurement
  • Elevated homocysteine
  • Normal MMA when isolated folate deficiency is present

Do not give folate alone until B12 deficiency is addressed

Folic acid may improve the anemia while allowing B12-related neurologic injury to continue. Assess for B12 deficiency and replace B12 first or concurrently when uncertainty remains.

Folate replacement should be directed by the severity and cause of deficiency, medication interactions, pregnancy status and local protocols. Continue long enough to restore stores and correct the underlying cause.

8. Other causes of macrocytosis

Cause Supporting clues Next steps
Alcohol Exposure history, liver abnormalities, nutritional deficiency and round macrocytes Address alcohol use, nutrition, liver disease and coexisting folate or B12 deficiency.
Liver disease Target cells, abnormal liver tests, portal hypertension and splenomegaly Evaluate liver disease and avoid assuming that all cytopenias are from hypersplenism.
Hypothyroidism Compatible symptoms and elevated TSH Treat thyroid disease and reassess the CBC response.
Reticulocytosis Increased absolute reticulocytes, polychromasia, hemolysis or recent blood loss Evaluate the underlying hemolytic or bleeding process.
Medication effect Hydroxyurea, antimetabolites, selected antiretrovirals, antiseizure medications and other marrow-active drugs Review timing, dose, expected pharmacologic effect and alternative causes.
Myelodysplastic neoplasm Persistent macrocytosis, progressive cytopenias, dysplasia and older age Hematology evaluation, marrow aspirate and biopsy, cytogenetics and molecular testing when appropriate.
Copper deficiency Anemia, neutropenia, neurologic findings, gastrointestinal surgery, malabsorption or excess zinc Check copper and zinc and evaluate the underlying exposure or malabsorption.
Spurious macrocytosis Marked hyperglycemia, cold agglutination, leukocytosis or specimen-related artifact Review analyzer flags, smear and repeat testing under corrected conditions.

9. Severe B12 deficiency and pseudo-TMA

Severe vitamin B12 deficiency can produce anemia, thrombocytopenia, schistocytes, biochemical hemolysis and extreme LDH elevation, mimicking thrombotic thrombocytopenic purpura. [2] [3]

Feature Severe B12 pseudo-TMA True TTP
Reticulocyte response Often inappropriately low because erythropoiesis is ineffective Usually regenerative unless another process suppresses marrow response
LDH May be strikingly elevated from intramedullary cell destruction Elevated from microangiopathic hemolysis
MCV and smear Macro-ovalocytes and hypersegmented neutrophils may coexist with schistocytes Schistocytes predominate; classic megaloblastic morphology is not expected
White blood cells Pancytopenia or leukopenia may occur Leukopenia is less characteristic
ADAMTS13 Severe deficiency is not expected Severe ADAMTS13 deficiency supports immune TTP

Do not dismiss possible TTP

When genuine TTP remains clinically plausible, obtain ADAMTS13 testing before plasma therapy when feasible and involve hematology urgently. B12 deficiency and TTP are not mutually exclusive, and treatment of strongly suspected TTP should not be delayed solely because the B12 level is low.

10. When to consider bone-marrow evaluation

  • Persistent unexplained macrocytosis
  • Progressive or multilineage cytopenias
  • Peripheral-blood dysplasia
  • Circulating blasts
  • Failure to respond to appropriate nutrient replacement
  • Concern for MDS, leukemia, aplastic anemia or marrow infiltration
Do not perform marrow biopsy before completing a focused evaluation for common reversible causes when the patient is stable and the smear does not suggest an urgent marrow disorder.

11. Resident summary

  1. Confirm macrocytosis and review the reticulocyte count and smear.
  2. Macro-ovalocytes and hypersegmented neutrophils suggest megaloblastic hematopoiesis.
  3. Normal MCV and hemoglobin do not exclude neurologically important B12 deficiency.
  4. MMA supports B12 deficiency but may be elevated in renal dysfunction.
  5. Homocysteine may rise in either B12 or folate deficiency and is less specific.
  6. Test for anti-intrinsic-factor antibody when autoimmune gastritis is suspected, but recognize that a negative result does not exclude it.
  7. Do not delay B12 replacement when neurologic disease or severe megaloblastic anemia is strongly suspected.
  8. Avoid folate-only treatment until B12 deficiency has been addressed.
  9. Consider alcohol, liver disease, hypothyroidism, medications, reticulocytosis, copper deficiency and MDS.

12. Knowledge check

Question 1

A patient has paresthesias, gait instability and impaired vibration sensation, but the MCV is normal. Which statement is most accurate?

Question 2

Vitamin B12 is indeterminate, MMA is elevated and the patient has chronic kidney disease. What is the best interpretation?

Question 3

Which laboratory pattern most strongly supports isolated folate deficiency rather than B12 deficiency?

Question 4

A patient has macrocytic anemia, low B12 and new gait disturbance. Confirmatory antibody testing will take several days. What is the best next step?

Question 5

A patient has anemia, thrombocytopenia, schistocytes, an extremely elevated LDH and an inappropriately low reticulocyte response. Which alternative to TTP should be considered?

References

  1. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE guideline NG239. Published March 2024. NICE guideline
  2. American Society of Hematology. A 60-year-old woman with severe macrocytic anemia, thrombocytopenia and schistocytes. ASH case
  3. George JN, et al. Medical consult: aHUS, TTP? How to distinguish and what to do. ASH Education Program. 2023. ASH review
  4. Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. 2022. MHRA guidance
  5. Medicines and Healthcare products Regulatory Agency. Vitamin B12: advise patients with known cobalt allergy to be vigilant for sensitivity reactions. 2023. MHRA safety update

Educational disclaimer

This guide is intended for clinician education and does not replace local laboratory reference ranges, institutional protocols, pharmacy guidance, hematology consultation, neurology evaluation or individualized clinical judgment. New neurologic deficits, severe anemia, pancytopenia and possible TTP require urgent local escalation.