PA15.1-3 | Peripheral Smear of Macrocytic Anaemia — Practical — Summary & Reflection

REFLECT

Before completing this module, take 2 minutes to consolidate:

1. Close your eyes and mentally reconstruct what you would see scanning a megaloblastic smear at 40× from left to right across the monolayer zone. What three findings would you call out aloud?

1. A colleague says: 'MCV is only 99 fL — that's nearly normal, so B12 deficiency is unlikely.' How would you challenge this using what you learned today?

1. In your practical examination, you have 90 seconds per slide. List the three things you would look for first on a macrocytic smear and why you would prioritise them in that order.

KEY TAKEAWAYS

Peripheral Smear of Macrocytic Anaemia — Key Points

• Read in the monolayer zone — 10× to orient, 40× for morphology, 100× oil for neutrophil lobe count and inclusions.

• Macro-ovalocytes are the hallmark RBC change of megaloblastic anaemia — large, oval, normochromic, no central pallor. Round macrocytes suggest a non-megaloblastic cause.

• Hypersegmented neutrophils (≥6 lobes in any one neutrophil, OR ≥5% with ≥5 lobes) are the earliest and most specific finding — present before significant anaemia.

• Anisopoikilocytosis and high RDW accompany the macrocytosis. Leucopenia + thrombocytopenia occur in severe disease (pancytopenia).

• Howell-Jolly bodies and Cabot rings appear in severe cases — Cabot rings are most specific for megaloblastic change.

• Distinguish three patterns: megaloblastic (oval macrocytes + hypersegmentation) vs non-megaloblastic (round macrocytes, target cells, no hypersegmentation) vs reticulocytosis (polychromatic macrocytes, left shift).

• A dimorphic smear (micro-hypochromic + macro-ovalocytic cells) with hypersegmented neutrophils and normal MCV = co-existent iron + B12/folate deficiency — do not miss this by relying on MCV alone.