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Macrocytic anemias

15 min

  • Tell vitamin B12 from folate deficiency with methylmalonic acid and homocysteine
  • Recognize a raised MCV caused by a high reticulocyte count
  • Recognize a vitamin B12 deficiency pattern from the CBC, film, and B12 result
  • Recognize polychromatophilic cells as young red cells on a Wright-stained film

Read the full reference

Try first

Try first

A patient's homocysteine is elevated and methylmalonic acid is elevated. Which deficiency does this support?

The next section explains it.

Right. The next section explains why.

The next section explains it.

The next section explains it.

Get the idea

Sort megaloblastic from nonmegaloblastic macrocytosis

Megaloblastic anemia comes from impaired DNA synthesis and delayed nuclear maturation. It often produces an MCV from about 100 to 150 fL, oval macrocytes, marked anisocytosis, and hypersegmented neutrophils.1 The International Council for Standardization in Haematology defines hypersegmentation as at least one neutrophil with six or more lobes, or more than 3% of neutrophils with five lobes, out of 100 examined.2 Nonmegaloblastic causes include reticulocytosis, liver disease, hypothyroidism, marrow disease and medication effect. They more often produce round macrocytes without hypersegmentation and an MCV nearer 100 to 110 fL.1

Reticulocytes are large red cells too

Reticulocytes are larger than mature red cells, so hemolysis, recovery after hemorrhage, or a response to effective treatment can raise the MCV on its own.1 Round macrocytes and a raised reticulocyte count without hypersegmented neutrophils separate this regenerative pattern from a megaloblastic one.1

Polychromasia signals the response, without measuring it

Polychromatophilic cells on a Wright-stained film are young reticulocytes, slate blue-gray from residual ribosomal RNA.3 They show that marrow output has reached the blood. A film impression cannot measure how many there are. The reticulocyte count and its production index quantify the response.3

B12 and folate need a biochemical answer

Macro-ovalocytes and hypersegmented neutrophils support megaloblastic change but do not name its cause, since folate deficiency and myelodysplasia can produce the same morphology.4 A low vitamin B12, with methylmalonic acid when B12 is indeterminate, supports B12 deficiency. Homocysteine rises with either B12 or folate deficiency and cannot separate them.4 Many laboratories treat a total B12 below about 200 to 250 pg/mL as subnormal, though the exact decision limit is assay-dependent.4

References
  1. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed September 26, 2026. https://www.us.elsevierhealth.com/rodaks-hematology-9780323936507.html
  2. Palmer L, Briggs C, McFadden S, et al. ICSH recommendations for the standardization of nomenclature and grading of peripheral blood cell morphological features. Int J Lab Hematol. 2015;37(3):287-303. doi:10.1111/ijlh.12327
  3. College of American Pathologists. 2026 Hematology, Clinical Microscopy, and Body Fluids Glossary. 2026. Accessed September 26, 2026. https://www.cap.org/wp-content/uploads/documents/2026-Heme-Glossary-FINAL.pdf
  4. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated April 2, 2026. Accessed September 26, 2026. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/

Watch one

A patient's CBC shows hemoglobin 8.4 g/dL, MCV 118 fL, WBC 3.1 × 10³/µL, and platelets 98 × 10³/µL. The film shows oval macrocytes and a neutrophil with 6 nuclear lobes. The reticulocyte count is 0.6%, below the reference interval. Vitamin B12 is 150 pg/mL, and methylmalonic acid is elevated. Interpret the pattern.

  1. Read the MCV: 118 fL is macrocytic, within the range megaloblastic anemia often produces.

    MCV sorts the anemia into a category before finer testing.

  2. Read the film: oval macrocytes and a neutrophil with 6 lobes both support megaloblastic change.

    Hypersegmentation and oval macrocytes together support the megaloblastic pattern.

  3. Read the other lines: the WBC count and platelet count are both reduced, consistent with megaloblastic hematopoiesis affecting more than one lineage.

    Megaloblastic change can lower more than one cell line, and the count shows whether it has.

  4. Read the reticulocyte count: 0.6% is inappropriately low for this degree of anemia, a result consistent with impaired DNA synthesis.

    An adequate marrow response would show reticulocytosis, and this pattern shows the opposite.

  5. Read the B12 and methylmalonic acid: B12 is low and methylmalonic acid is elevated, supporting B12 deficiency as the cause.

    Morphology alone cannot name the cause. The biochemical result names it.

The pattern is megaloblastic macrocytic anemia from vitamin B12 deficiency: macro-ovalocytes, a hypersegmented neutrophil, reduced WBC and platelet counts, an inappropriately low reticulocyte count, a low B12, and an elevated methylmalonic acid all fit together.

Your turn

Problem 1 of 3

Which paired blood-film findings most strongly support megaloblastic hematopoiesis?

Incorrect. These findings suggest membrane or immune processes and do not define the megaloblastic pattern.

Incorrect. These findings support impaired hemoglobin assembly, including iron restriction or thalassemia.

Correct. The paired findings support delayed nuclear maturation from impaired DNA synthesis. Biochemical confirmation is still needed.

Hint
  1. Weigh both listed findings together as a pair.
  2. A finding that supports impaired hemoglobin assembly is a microcytic clue.
  3. The megaloblastic pattern needs biochemical confirmation even when the film fits it.

Review Macrocytic anemia patterns

Problem 2 of 3

A patient recovering from acute blood loss has MCV 106 fL, a reticulocyte count of 8%, and a markedly elevated absolute reticulocyte count. The film shows round macrocytes and no hypersegmented neutrophils. What explains the raised MCV?

Megaloblastic change produces oval macrocytes and hypersegmented neutrophils, and the reticulocyte count would be inappropriately low. Neither fits here.

Called reticulocyte macrocytosis megaloblastic

Reticulocytes are larger than mature red cells, so a brisk response after bleeding or hemolysis raises the MCV. Polychromasia, a raised reticulocyte count, and round macrocytes without hypersegmented neutrophils separate it from megaloblastic change, which shows oval macrocytes and an inappropriately low reticulocyte count.

Reticulocytes are larger than mature red cells. A brisk response after bleeding raises the MCV, and round macrocytes without hypersegmentation fit this regenerative pattern.

Cold agglutinins falsely raise the MCV by clumping red cells, but the reticulocyte count and film here already explain the macrocytosis without invoking interference.

Hint
  1. Compare the size of a reticulocyte with a mature red cell.
  2. Check the film for the specific findings that support megaloblastic change.

Review Nonmegaloblastic macrocytosis and spurious results

Problem 3 of 3

A patient's homocysteine is mildly elevated and methylmalonic acid is within its reference interval. Which deficiency does this best support?

Homocysteine rises with either vitamin B12 or folate deficiency, so an elevated homocysteine alone cannot point to B12 deficiency specifically.

Used homocysteine to separate B12 from folate deficiency

Homocysteine rises in both vitamin B12 and folate deficiency, so it cannot separate them. Methylmalonic acid rises in B12 deficiency and is the more specific marker, although kidney impairment and age also raise it.

Methylmalonic acid is the more specific marker for B12 deficiency and is normal here. A normal methylmalonic acid with an elevated homocysteine favors folate deficiency.

Serum folate and B12 testing, read with these two markers, already narrow the cause. A marrow biopsy is not the routine next step for this question.

Review Vitamin B12 and folate

Use it

  • A man on the medical ward is recovering 5 days after a large upper gastrointestinal bleed, now hemodynamically stable.
  • His hemoglobin has risen from 7.2 to 9.8 g/dL since the day of the bleed, without transfusion.
  • The film shows round macrocytes and frequent polychromatophilic cells, with no hypersegmented neutrophils.
  • Homocysteine is mildly elevated. Methylmalonic acid is within its reference interval.
  • He has no history of alcohol use, liver disease, or thyroid disease.
TestResultPreviousReference intervalFlag
Hemoglobin9.8 g/dL7.2 g/dL5 days ago13.5–17.5 g/dLLow
MCV101 fL89 fL5 days ago76–100 fLHigh
WBC11.0 × 10³/µL9.8 × 10³/µL5 days ago3.6–10.6 × 10³/µLHigh
Platelets480 × 10³/µL260 × 10³/µL5 days ago150–450 × 10³/µLHigh
Reticulocytes, relative6.5 %1.2 %5 days ago0.5–2.5 %High
Reticulocytes, absolute260 × 10³/µL55 × 10³/µL5 days ago20–115 × 10³/µLHigh

Specimen: H 6, L 10, I 2. Serum and EDTA specimens drawn together this morning.

Decision 1 of 3

What do the polychromatophilic cells on the film show, and can their number on the film substitute for the reticulocyte count?

Polychromatophilic cells are young reticulocytes, but a film impression cannot measure how many there are. The reticulocyte count and its production index quantify the response.

Treated polychromasia as a direct reticulocyte count

Polychromatophilic cells are young reticulocytes that stain slate blue-gray. A film impression cannot measure how many there are. Estimating the response from the film misstates marrow output, which the reticulocyte count and production index quantify.

Polychromasia shows that marrow output has reached the blood. The reticulocyte count, 6.5% and markedly elevated in absolute terms, is what quantifies how large that response is.

Polychromasia signals marrow response to any cause of anemia, including this recent hemorrhage. It does not by itself distinguish hemolysis from a brisk response to blood loss.

Review Red-cell morphology on the stained film

Decision 2 of 3

What explains the raised MCV in this case?

Megaloblastic change would show oval macrocytes, hypersegmented neutrophils, and an inappropriately low reticulocyte count. This film and count show the opposite.

Called reticulocyte macrocytosis megaloblastic

Reticulocytes are larger than mature red cells, so a brisk response after bleeding or hemolysis raises the MCV. Polychromasia, a raised reticulocyte count, and round macrocytes without hypersegmented neutrophils separate it from megaloblastic change, which shows oval macrocytes and an inappropriately low reticulocyte count.

Reticulocytes are larger than mature red cells. The markedly elevated absolute reticulocyte count and round macrocytes without hypersegmentation fit a regenerative response to the recent bleed.

Nothing in the history or results points to liver disease, and the reticulocyte response already explains the macrocytosis.

Review Nonmegaloblastic macrocytosis and spurious results

Decision 3 of 3

Given the mildly elevated homocysteine and the normal methylmalonic acid, does this patient need further vitamin B12 workup right now?

Homocysteine rises with either B12 or folate deficiency and cannot point to B12 deficiency specifically. The normal methylmalonic acid argues against it.

Used homocysteine to separate B12 from folate deficiency

Homocysteine rises in both vitamin B12 and folate deficiency, so it cannot separate them. Methylmalonic acid rises in B12 deficiency and is the more specific marker, although kidney impairment and age also raise it.

The film and reticulocyte count already explain the macrocytosis as regenerative. A normal methylmalonic acid argues against B12 deficiency as an additional cause.

The CBC, film, and reticulocyte count here already point to a single, sufficient explanation. Testing follows the pattern each case shows.

Review Vitamin B12 and folate

The clue that settles this case is the reticulocyte response itself. A markedly elevated absolute reticulocyte count and round macrocytes without hypersegmentation explain the raised MCV as regenerative, and a normal methylmalonic acid, the specific marker, is what argues against vitamin B12 deficiency.

Keep

Sources checked