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megaloblastic anemia AND diagnosis

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Merck Manual megaloblastic macrocytic anemia peripheral smear hypersegmented neutrophils

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https://www.merckmanuals.com/professional/hematology/anemias…

1. Interpretation of the complete blood count

This CBC is most consistent with megaloblastic macrocytic anaemia due to vitamin B12 deficiency, with mild pancytopenia.
FindingInterpretation
Hb 7.4 g/dL, RBC 2.69 million/cummSignificant anaemia
MCV 118 fLMacrocytic anaemia
MCH 39.6 pg, RDW 18%Large red cells with increased variation in size
Reticulocytes 0.4%Inappropriately low marrow response, indicating ineffective erythropoiesis
TLC 3,400/µLMild leukopenia
Platelets 100,000/µLThrombocytopenia
Serum vitamin B12 150 pg/mLLow, supports vitamin B12 deficiency
The overall pattern is macrocytic anaemia with leukopenia and thrombocytopenia, i.e. mild pancytopenia, caused by defective DNA synthesis from B12 deficiency.
Technical note: Some indices shown are mathematically inconsistent with each other. For example, an Hb of 7.4 g/dL with RBC 2.69 million/µL and MCV 118 fL would not give an MCHC of 33.7 g/dL. However, the intended clinical pattern is clear.

2. What is MCV? What does an MCV of 118 fL indicate?

MCV (mean corpuscular volume) is the average volume or size of a red blood cell, expressed in femtolitres (fL).
  • Normal MCV: approximately 80-100 fL
  • MCV 118 fL: macrocytosis, meaning abnormally large red blood cells
An MCV of 118 fL strongly suggests a megaloblastic macrocytic anaemia, particularly due to vitamin B12 or folate deficiency. Fully developed megaloblastic anaemia commonly has an MCV above 115 fL.
Frameworks for Internal Medicine, section “Macrocytic Anemia”

3. Characteristic peripheral blood smear findings in megaloblastic anaemia

The smear typically shows:
  • Macro-ovalocytes: large, oval red blood cells
  • Anisocytosis: variation in red-cell size, reflected by raised RDW
  • Poikilocytosis: variation in red-cell shape, sometimes including teardrop cells
  • Hypersegmented neutrophils
  • Possible Howell-Jolly bodies
  • In more severe disease: reduced white cells and platelets, with occasional large platelets
Peripheral smear in megaloblastic anaemia showing macro-ovalocytes and a hypersegmented neutrophil
The typical picture is macro-ovalocytosis, anisopoikilocytosis, and hypersegmented neutrophils. Frameworks for Internal Medicine, section “Macrocytic Anemia” The Merck Manual summary also lists these findings along with reticulocytopenia.

4. What is a hypersegmented neutrophil?

A hypersegmented neutrophil is a neutrophil with excessive nuclear lobulation.
It is defined as:
  • More than 5% of neutrophils having 5 or more nuclear lobes, or
  • The presence of even occasional neutrophils with 6 or more lobes
It is a classic early morphological clue to megaloblastic anaemia due to vitamin B12 or folate deficiency.

5. Why is the reticulocyte count low in megaloblastic anaemia?

Vitamin B12 deficiency impairs DNA synthesis. Erythroid precursor cells in the bone marrow cannot divide and mature normally. Many abnormal erythroblasts die within the marrow before becoming circulating red cells, called ineffective erythropoiesis or intramedullary cell death.
Therefore, despite severe anaemia, the marrow does not release enough reticulocytes, resulting in reticulocytopenia. Here, reticulocytes are only 0.4%, which is inappropriately low for someone with severe anaemia.

6. Why may total leukocyte and platelet counts also be reduced?

B12 deficiency affects DNA synthesis in all rapidly dividing bone-marrow cell lines, not only erythroid precursors.
This produces defective maturation and premature intramedullary destruction of:
  • Erythroid cells, causing anaemia
  • Myeloid precursors, causing leukopenia
  • Megakaryocytes/platelet precursors, causing thrombocytopenia
Thus severe megaloblastic anaemia can cause pancytopenia. In this case, the low TLC of 3,400/µL and platelet count of 100,000/µL reflect this marrow-wide ineffective haematopoiesis.
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I can see the same CBC and question sheet again. The findings support vitamin B12-deficiency megaloblastic anaemia with mild pancytopenia.
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