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PA14.1-2 | Peripheral Smear in Microcytic Anaemia: Hands-on Interpretation — Part 1
CLINICAL SCENARIO
A 28-year-old woman from Puducherry walks into the medicine OPD with fatigue and breathlessness on exertion. Hb is 8.2 g/dL. MCV 62 fL. The junior resident immediately orders a serum ferritin and HPLC. But the CP registrar looks at the smear first — and in 90 seconds, without any chemistry, she says: 'This isn't IDA. Look at the target cells. Run the HPLC, but I'm betting on thalassaemia trait.' She was right.
The smear told her. It will tell you too — if you know how to read it.
WHY THIS MATTERS
PA14.2 (SH): 'Identify and describe the peripheral smear in microcytic anaemia' — this competency is assessed in your Clinical Pathology (CP) practicals, in long and short cases in final MBBS, and — more importantly — in every ward and OPD posting where you are the first person to look at a CBC report and decide what to do next. The smear is a witness statement from the patient's own blood. Your job is to read it correctly.
RECALL
Before we look at slides: pull up what you already know.
From SDL 2 (IDA pathogenesis to diagnosis): Iron deficiency depletes the iron stores → marrow makes smaller, paler RBCs → MCV falls → smear shows microcytic hypochromic cells. Serum ferritin is the gold standard for storage iron depletion.
From SDL 3 (Microcytic differentials): The four major differentials are IDA, thalassaemia trait, anaemia of chronic disease (ACD), and sideroblastic anaemia. Each has a distinct smear pattern — and a distinct RDW signature (IDA: high RDW; thal trait: low RDW; ACD: normal or mildly elevated; sideroblastic: very high RDW due to dimorphic population).
From H2 SDL 2 (Smear technique): You already know how to prepare and stain a smear (Leishman stain, push technique). Here we focus on what you find — not how you make the slide.
Step 1 — The Systematic Scanning Approach
The most common smear-reading error: going straight to 100× oil.
The correct sequence:
1. 10× (low power) — distribution scan: Identify the monolayer (feathered edge area where RBCs are touching but not overlapping). Assess overall cellularity, RBC distribution, any rouleaux, platelet clumping. Don't characterise individual cells yet.
2. 40× (high dry) — field selection and initial morphology: Choose 4 representative fields in the monolayer. Now begin characterising RBC size, colour, and shape. Note any abnormal populations.
3. 100× oil — definitive morphology: Confirm the findings from 40×. Identify specific inclusions (basophilic stippling, Pappenheimer bodies). Count your observations across at least 4 oil fields before drawing a conclusion — the '4-field rule'.
The logic: most diagnoses (IDA, thalassaemia trait, ACD) are visible at 10-40×. Oil immersion confirms; it rarely discovers. Training your 10× eye is the high-yield skill.
Systematic Scanning Approach: Progressive Magnification Analysis of Iron Deficiency Anemia
Step 2 — The Four-Parameter RBC Framework: Size, Colour, Shape, Content
Every time you look at a smear, run through these four parameters systematically. Don't skip ahead.
SIZE — Microcytosis:
RBCs should be approximately the same diameter as the nucleus of a small lymphocyte (~ 7 µm). Microcytic RBCs are clearly smaller than this reference nucleus. In IDA and thalassaemia trait, the microcytosis can be severe (MCV 55–70 fL).
COLOUR — Hypochromia:
Normal RBCs have a central pallor that occupies approximately 1/3 of the cell diameter (the biconcave disc's thin centre). Hypochromic RBCs have central pallor >1/3 — in severe IDA, the cell looks like a pale ring with only a thin rim of haemoglobin at the periphery ('pessary cells'). This is reduced haemoglobin per cell (MCH ↓, MCHC ↓).
SHAPE — Anisopoikilocytosis:
Anisocytosis = variation in size (quantified by RDW). Poikilocytosis = variation in shape. In microcytic anaemias, specific shapes carry diagnostic weight:
- Pencil cells (elliptocytes/elongated microcytes): Classic for IDA
- Target cells (codocytes): Thalassaemia > IDA; also in liver disease, HbC
- Teardrop cells (dacrocytes): Advanced IDA, myelofibrosis
CONTENT — Inclusions:
Inclusions are intracytoplasmic structures visible on Leishman stain:
- Basophilic stippling: Coarse dark blue dots = precipitated ribosomes. Seen in thalassaemia, lead poisoning, sideroblastic anaemia
- Pappenheimer bodies: Iron-containing granules (siderosomes). Stain with Perl's Prussian blue. Visible on Leishman as faint dots. Classic for sideroblastic anaemia.
- Howell-Jolly bodies: Nuclear remnants (asplenic states) — not specific to microcytic anaemia but may coexist.
Four-Parameter RBC Framework for Microcytic Anemias
CLINICAL PEARL
The smear is the cheapest, fastest, highest-yield investigation in microcytic anaemia — review it BEFORE ordering iron studies or HPLC.
Reason: smear morphology can redirect your entire workup. A dimorphic RBC population (two distinct crowds of cells) means sideroblastic anaemia — and your next step is bone marrow aspiration with Perls' stain, not serum ferritin. An HPLC would miss this. A smear takes 3 minutes; recalling a patient for bone marrow takes 3 days. Read the slide first.
IDA Smear — Pattern Recognition
Iron deficiency anaemia produces the most 'textbook' microcytic hypochromic picture, but the classic severe picture takes months of depletion to develop. Know what early and late IDA look like.
Classic IDA smear features (in order of appearance as severity increases):
1. Microcytosis — RBCs smaller than small lymphocyte nucleus (earliest feature: MCV drops before MCHC)
2. Hypochromia — central pallor >1/3 diameter
3. Anisocytosis — HIGH RDW (>14%) — the mix of new small cells and older normal-sized cells creates size variability. HIGH RDW is a hallmark — distinguishes IDA from thalassaemia trait (where RDW is normal or low)
4. Pencil cells (elongated, cigar-shaped microcytes) — near-pathognomonic for IDA. These are not elliptocytes; they're elongated microcytes under iron-starved haemoglobin synthesis
5. Target cells — mild and scattered (a few per field). Much less prominent than in thalassaemia
6. Teardrop cells (dacrocytes) — advanced/severe IDA
7. Thrombocytosis — reactive platelet elevation (Fe deficiency stimulates thrombopoiesis) — check platelet count on the smear
What you will NOT see in IDA:
- Coarse basophilic stippling (that's lead/thal/sideroblastic)
- A dimorphic population (that's sideroblastic or post-transfusion)
- Spherocytes (that's haemolysis)
Iron Deficiency Anemia: Blood Smear Pattern Recognition
SELF-CHECK
You are examining a smear at 40× from a 25-year-old woman with Hb 7.8, MCV 63, RDW 18.5%. The dominant RBC population is microcytic and hypochromic with >1/3 central pallor. You notice elongated, cigar-shaped cells scattered throughout the field — approximately 1 in every 8–10 RBCs. Which of the following best describes these elongated cells and their diagnostic significance?
A. Spherocytes — indicate haemolytic anaemia, order Coombs test
B. Pencil cells (elongated microcytes) — near-pathognomonic for iron deficiency anaemia
C. Elliptocytes — indicate hereditary elliptocytosis, order osmotic fragility
D. Schistocytes — indicate microangiopathic haemolytic anaemia, order LDH and peripheral film review
Reveal Answer
Answer: B. Pencil cells (elongated microcytes) — near-pathognomonic for iron deficiency anaemia
Pencil cells (also called cigar cells or elongated microcytes) are narrow, elongated red cells with tapered ends — distinct from true elliptocytes which are oval but not as narrow. They arise when iron-deficient haemoglobin synthesis produces abnormally malleable red cells that deform under shear stress. Combined with high RDW (18.5% here), microcytosis, and hypochromia, this picture is near-pathognomonic for IDA. Spherocytes are small but ROUND and densely haemoglobinised (not hypochromic). Schistocytes are fragmented cells — helmet-shaped or triangular — and indicate a haemolytic process.