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PA14.1-2 | Peripheral Smear in Microcytic Anaemia: Hands-on Interpretation — Part 2
Thalassaemia Trait Smear — Pattern Recognition
Thalassaemia trait (beta or alpha) is the great mimicker of IDA — same MCV, same haemoglobin range, but a DIFFERENT smear and a fundamentally different biology.
The critical distinguishing feature: DISPROPORTIONATE MICROCYTOSIS WITH LOW/NORMAL RDW
In thalassaemia trait, the marrow makes fewer alpha or beta globin chains, but the chains it makes are normal. The result: all RBCs are small (microcytic), but they are uniformly small — low RBC size variation (RDW normal or low). Contrast with IDA where the iron deficiency creates a mixed population (high RDW).
Thalassaemia trait smear features:
1. Marked microcytosis — often disproportionate to the degree of anaemia. MCV 60–65 fL with Hb only mildly reduced (10.5–12 g/dL) — 'mild anaemia, severe microcytosis'
2. RBC COUNT HIGH NORMAL or ELEVATED (>5 million/µL in females) — the marrow compensates by making MORE cells; each is smaller. A female patient with Hb 11, MCV 64 but RBC count 5.8 million — suspect thalassaemia trait
3. Target cells PROMINENT — much more than in IDA. Multiple target cells per field
4. Hypochromia mild — central pallor present but less severe than IDA
5. Basophilic stippling — may be present, particularly in beta-thalassaemia trait; coarser in haemoglobin H disease (alpha-thalassaemia)
6. Low RDW (or normal) — uniform microcytosis, not anisocytosis
The Mentzer Index (quick bedside calculation):
MCV ÷ RBC count (millions)
- <13 → thalassaemia trait
- >13 → IDA
Limitations: not definitive, overlaps exist — confirm with HPLC. But the smear will already have told you: target cells = thalassaemia.
Thalassaemia Trait vs Iron Deficiency Anemia: Pattern Recognition
CLINICAL PEARL
Disproportionate microcytosis (MCV very low, anaemia mild) — think thalassaemia trait first.
The classic teaching scenario: a 22-year-old Sindhi or Gujarati student with Hb 11.0, MCV 63, RBC count 5.6 million, RDW 12.8%, and target cells on the smear. Ferritin is normal. This is beta-thalassaemia trait until proven otherwise. Order HPLC — look for elevated HbA2 (>3.5%).
The danger of missing this: if you treat with iron (for 'IDA'), the patient's haemoglobin may not respond and you have wasted 3 months. The screening implication: if both partners carry thalassaemia trait, 25% of children will have thalassaemia major — a devastating, preventable outcome. The smear flags the carrier.
Sideroblastic Anaemia Smear — Pattern Recognition
Sideroblastic anaemia is the rarest of the four differentials but has the most distinctive — and unmistakable — smear pattern.
The pathognomonic finding: DIMORPHIC RBC POPULATION
In sideroblastic anaemia, the marrow simultaneously produces:
- A population of small, hypochromic RBCs (the defective iron-loaded cells that cannot form haem properly)
- A population of normocytic normochromic RBCs (the surviving normal cells)
On the smear, you see two distinct crowds of cells side by side in the same field — some pale and small, some normal-sized and pink. This 'two populations' appearance (dimorphism) is the defining finding.
Full smear picture in sideroblastic anaemia:
1. Dimorphic RBC population — the dominant, diagnostic feature. Very high RDW (often >20%) because of the two distinct size/colour populations
2. Basophilic stippling — especially prominent in lead-induced sideroblastic anaemia (lead inhibits multiple enzymes in the haem synthesis pathway)
3. Pappenheimer bodies — siderosomes (iron-containing vacuoles). Visible as faint dark dots in the cytoplasm of hypochromic cells. Confirmed with Perls' Prussian blue stain.
4. MCV may be misleadingly 'normal' — the average of a microcytic and a normocytic population can produce a numerically normal MCV. The RDW flags the heterogeneity.
Causes to keep in mind (for next steps after smear):
- Acquired: alcohol (most common), lead poisoning, drugs (isoniazid, chloramphenicol, pyridoxine antagonists), myelodysplastic syndrome
- Congenital: X-linked (ALAS2 gene mutation) — rare
Sideroblastic Anaemia: Blood Smear Pattern Recognition
SELF-CHECK
A 42-year-old male mill worker presents with fatigue and abdominal colic. Hb 9.1 g/dL, MCV 72 fL, RDW 22.4%. The smear at 100× oil shows two distinct RBC populations — some cells pale and small, others normal-sized and pink — coexisting in the same field. You also notice coarse blue stippling in multiple cells. What is the term for this smear finding, and what is the most likely diagnosis?
A. Anisocytosis — suggests iron deficiency anaemia; order serum ferritin
B. Dimorphic RBC population — suggests sideroblastic anaemia; the coarse stippling and occupational history point to lead poisoning
C. Rouleaux formation — suggests multiple myeloma; order serum protein electrophoresis
D. Polychromasia — suggests haemolytic anaemia; order reticulocyte count and Coombs test
Reveal Answer
Answer: B. Dimorphic RBC population — suggests sideroblastic anaemia; the coarse stippling and occupational history point to lead poisoning
The dimorphic RBC population — two distinct crowds of cells (microcytic hypochromic + normocytic normochromic) in the same field — is pathognomonic for sideroblastic anaemia. The extremely high RDW (22.4%) reflects this dual population. Coarse basophilic stippling is the hallmark of lead poisoning (lead inhibits ALA dehydratase and ferrochelatase, blocking multiple steps in haem synthesis). The occupational history (mill worker with abdominal colic — 'lead colic') confirms the diagnosis. Next step: whole blood lead level, urine δ-ALA, and X-ray long bones (lead lines in children). Anisocytosis is variation in size, not two distinct populations. Rouleaux are stacked coin-like formations of normal RBCs. Polychromasia is the blue-grey hue of reticulocytes.
Lead Poisoning Smear — A Recognisable Pattern
Lead poisoning produces a specific smear variant of sideroblastic anaemia that has one highly visible, near-pathognomonic feature: coarse basophilic stippling.
Lead poisoning smear:
- Coarse basophilic stippling — much more prominent than in thalassaemia or other causes. The dots are large, irregular, and numerous. Lead inhibits 5'-nucleotidase, causing ribosomal RNA to aggregate → dark blue precipitates
- Mild microcytosis — not always severe; MCV may be low-normal
- Dimorphic population — may be present (sideroblastic component)
- Anaemia — typically mild to moderate (Hb 9–11 g/dL)
Clinical context is diagnostic: Paint exposure (children), battery manufacturing, smelting, traditional kohl (surma) — these occupational/exposure histories + coarse stippling = lead poisoning until proven otherwise. Order whole blood lead level (not serum — lead is in RBCs).
Lead Poisoning Blood Smear: Diagnostic Features and Clinical Context
SELF-CHECK
A 7-year-old child from a household where the walls were recently scraped and repainted presents with irritability, learning regression, and Hb 10.2 g/dL. The smear shows prominent coarse basophilic stippling in many RBCs. Which investigation is the correct NEXT step?
A. Serum ferritin — to rule out iron deficiency
B. HPLC haemoglobin electrophoresis — to rule out thalassaemia trait
C. Whole blood lead level — to confirm lead poisoning (lead partitions into RBCs, not serum)
D. Bone marrow aspiration — to identify ringed sideroblasts
Reveal Answer
Answer: C. Whole blood lead level — to confirm lead poisoning (lead partitions into RBCs, not serum)
The clinical picture (child, paint exposure, neurological symptoms + coarse basophilic stippling) is lead poisoning. The correct specimen is WHOLE BLOOD, not serum — lead is sequestered inside red cells (bound to haemoglobin and enzyme inhibition sites), so serum lead levels are falsely low. A blood lead level ≥5 µg/dL (CDC action level for children) confirms exposure. Additional tests: X-ray long bones (lead lines at metaphyses in children), urine δ-ALA, erythrocyte protoporphyrin. Serum ferritin and HPLC are not the priority here — stippling pattern + occupational/environmental history directs you straight to blood lead.