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PA14.1-2 | Microcytic Anaemia Differentials: Thalassaemia, ACD, Sideroblastic — Part 2

Sideroblastic Anaemia: When Iron Cannot Enter Haemoglobin

Pathogenesis:
Haem synthesis requires protoporphyrin IX + iron (Fe²⁺) + the enzyme ferrochelatase. If any step in the protoporphyrin synthesis pathway is blocked (specifically aminolevulinic acid synthase 2, ALAS2, in the hereditary form), iron is delivered to the mitochondria of developing RBCs but cannot be incorporated into protoporphyrin. Iron accumulates in the mitochondrial ring around the nucleus → ringed sideroblast — the pathognomonic finding.

Acquired causes (more common in clinical practice):
- Alcohol — most common acquired cause; inhibits ALAS2 activity and pyridoxine (B6) metabolism
- Lead toxicity — inhibits multiple enzymes in the haem pathway, including ALAD (ALA dehydratase)
- Isoniazid (INH) — antagonises pyridoxine (B6), which is the cofactor for ALAS2
- Copper deficiency — rare; seen in malabsorption, excessive zinc intake
- Myelodysplastic syndrome (MDS) — refractory anaemia with ringed sideroblasts (RARS); SF3B1 mutation

Hereditary cause:
- X-linked sideroblastic anaemia (XLSA): ALAS2 gene mutation. Males affected (X-linked recessive). Females are carriers. Responds partially to B6 supplementation.

The paradox of sideroblastic anaemia: The body has PLENTY of iron — in fact, too much. But it can't use it to make haemoglobin. So ferritin is HIGH, transferrin saturation is HIGH (iron loads up on transferrin because it can't get into cells), and yet the patient is anaemic.

Laboratory fingerprint:

Parameter	Finding
Hb	Moderate-severe reduction (7-10 g/dL)
MCV	Low (microcytic) OR normal
RDW	HIGH — distinctive dimorphic picture
Ferritin	HIGH (iron overload)
TIBC	Low-normal (saturated)
Transferrin saturation	HIGH (>50%, often >80%)
Serum iron	HIGH

Smear — the key clue: Dimorphic RBC population — two distinct populations on the peripheral smear. One population of normal-sized, normally haemoglobinised RBCs (the ones that somehow got enough iron) and one population of microcytic, hypochromic cells. This bimodal distribution is visible and reflected in a very high RDW.

Bone marrow confirmation: Perls Prussian blue stain — demonstrates blue-staining iron granules arranged in a ring around the nucleus of erythroblasts (ringed sideroblasts, defined as ≥5 iron granules encircling ≥1/3 of the nucleus). This is the gold standard.

Treatment:
- Address the cause: stop alcohol, stop INH (and add B6 prophylactically), chelate lead if toxic levels
- B6 (pyridoxine) trial — 100-200 mg/day for 3 months; partial response in XLSA and INH-induced forms
- RBC transfusions for symptomatic severe anaemia
- Iron chelation (deferoxamine or deferasirox) if iron overload develops
- MDS-RARS: lenalidomide or luspatercept; bone marrow transplant in young eligible patients

Three-panel diagram illustrating normal haem synthesis versus sideroblastic anaemia pathogenesis and the resulting clinical paradox of iron overload with anaemia. — **Panel A:** Normal mitochondrion, ferrochelatase enzyme, protoporphyrin IX, iron (Fe²⁺), haemoglobin synthesis. **Panel B:** Ringed sideroblast, iron accumulation in mitochondrial ring, blocked ALAS2 pathway, RBC nucleus. **Panel C:** High ferritin levels, elevated transferrin saturation, anaemic patient, iron paradox concept.

A three-panel medical diagram showing a dimorphic peripheral blood smear in sideroblastic anaemia with normal normochromic RBCs, microcytic hypochromic RBCs, and lead-toxicity-associated basophilic stippling. — **Panel A:** Peripheral blood smear with normal-sized normochromic RBCs, microcytic hypochromic RBCs, enlarged central pallor, Wright-Giemsa stain at 100x. **Panel B:** Enlarged comparison of normal-sized normochromic RBC versus microcytic hypochromic RBC showing reduced cell size and increased central pallor. **Panel C:** Lead toxicity association showing coarse basophilic stippling in RBCs plus abdominal colic, Burton lead line, and wrist/foot drop.

CLINICAL PEARL

Lead poisoning: the anaemia with extra clues — don't miss it in occupational exposure:

Lead (Pb) inhibits multiple enzymes in the haem synthesis pathway, causing a microcytic (or normocytic) anaemia with basophilic stippling on the smear — coarse blue dots in the RBC cytoplasm from aggregated ribosomes that lead prevents from degrading. This is NOT the same as the fine stippling in reticulocytosis.

The clinical constellation: microcytic anaemia + basophilic stippling + abdominal colic (lead colic) + a blue-black line at the gingival margin (Burton's lead line, from lead sulphide deposits) + encephalopathy in severe cases + wrist/foot drop (peripheral neuropathy).

Who gets it: battery factory workers, car mechanics, potters using lead-based glazes, children with pica (eating paint chips in old buildings). Environmental lead from paint in buildings constructed before 1980 is the dominant paediatric exposure in India.

Order: blood lead level (>10 µg/dL is above normal; >70 µg/dL requires immediate chelation). Erythrocyte protoporphyrin (EP) is elevated — a good screening test.

A Wright-Giemsa peripheral blood smear diagram compares normal red blood cells with microcytic red cells showing coarse blue basophilic stippling from retained ribosomal RNA. — **Panel A:** Peripheral blood smear with Wright-Giemsa stain, microcytic hypochromic RBCs, coarse blue-purple basophilic stippling, and one normal RBC for scale.. **Panel B:** Magnified affected RBC beside normal RBC, labeled basophilic stippling, coarse RNA/ribosome aggregates, microcytic RBC, and normal RBC for scale.. **Panel C:** Mechanism schematic showing retained ribosomal RNA aggregates causing basophilic stippling, distinguished from Pappenheimer bodies or iron granules..

SELF-CHECK

A 19-year-old woman presents with mild fatigue. CBC: Hb 10.8 g/dL, MCV 66 fL, RBC count 5.4 million/µL. Ferritin: 72 ng/mL. Calculate the Mentzer Index and state the most likely diagnosis.

A. A. Mentzer Index 12.2 — favours IDA

B. B. Mentzer Index 12.2 — favours thalassaemia trait

C. C. Mentzer Index 18.4 — favours thalassaemia trait

D. D. Mentzer Index 18.4 — favours IDA

Reveal Answer

Answer: B. B. Mentzer Index 12.2 — favours thalassaemia trait

Mentzer Index = MCV ÷ RBC = 66 ÷ 5.4 = 12.2. A value <13 favours thalassaemia trait. The clinical picture also supports this: Hb is only mildly reduced (10.8), MCV is disproportionately low (66), RBC count is elevated (5.4 million — the marrow is pumping out many small cells), and ferritin is normal at 72 (not depleted). This is a beta-thalassaemia trait pattern. Next investigation: HPLC for HbA2. Answer A applies the correct index but wrong interpretation — index <13 means thalassaemia trait, not IDA.

SELF-CHECK

A 48-year-old woman with rheumatoid arthritis on methotrexate presents with fatigue. CBC: Hb 9.2 g/dL, MCV 76 fL. Iron studies: ferritin 210 ng/mL, TIBC 195 µg/dL (low), transferrin saturation 14%, serum iron 27 µg/dL. ESR 88 mm/hr, CRP elevated. What is the most likely diagnosis and the key discriminating lab?

A. A. Iron deficiency anaemia — low transferrin saturation is the discriminator

B. B. Anaemia of chronic disease — low TIBC is the key discriminator

C. C. Beta-thalassaemia trait — ferritin is normal in context

D. D. Sideroblastic anaemia — methotrexate causes haem synthesis blockade

Reveal Answer

Answer: B. B. Anaemia of chronic disease — low TIBC is the key discriminator

This is classic ACD in the setting of chronic RA. The key discriminator is LOW TIBC (195 µg/dL, normal 250-370). In IDA, the body upregulates transferrin synthesis → HIGH TIBC. In ACD, chronic inflammation suppresses transferrin production → LOW TIBC. Ferritin at 210 is high — confirming intact storage iron. MCV is 76 (borderline microcytic), consistent with long-standing ACD. Transferrin saturation at 14% is low due to reduced iron delivery, not depleted stores. Treatment is directed at controlling the RA. Sideroblastic anaemia from methotrexate is not recognised — methotrexate causes folate deficiency (macrocytic anaemia), not sideroblastic.

SELF-CHECK

A 23-year-old woman from a Sindhi family is investigated for anaemia. CBC: Hb 8.8 g/dL, MCV 68 fL, RBC 5.8 million/µL, RDW 13.2%. Peripheral smear: target cells and mild hypochromia, no hypersegmented neutrophils. Ferritin: 88 ng/mL. What is the best next investigation?

A. A. Serum iron and TIBC to confirm IDA

B. B. HPLC for haemoglobin electrophoresis to screen for beta-thalassaemia trait

C. C. Bone marrow examination for Perls Prussian blue stain

D. D. Direct Coombs test for autoimmune haemolysis

Reveal Answer

Answer: B. B. HPLC for haemoglobin electrophoresis to screen for beta-thalassaemia trait

This is a high-probability beta-thalassaemia trait case: Sindhi ethnicity (10-15% carrier rate), MCV 68 fL with Hb 8.8 (disproportionate microcytosis), very high RBC count 5.8 million (marrow compensating with more small cells), low RDW 13.2% (uniform small cells — isocytosis, not anisocytosis), normal-high ferritin 88 (iron stores intact), target cells on smear. Mentzer Index = 68/5.8 = 11.7 (<13 → thalassaemia). HPLC is the confirmation test: elevated HbA2 (>3.5%) diagnoses beta-thalassaemia trait. Serum iron/TIBC is not necessary here — the pattern is not IDA. Bone marrow is invasive and not the next step. Coombs is for haemolytic anaemia (not the presentation here — no jaundice, normal reticulocytes expected).

The Integrative Comparison Table: Side-by-Side Differentials

This table is the anchor of your diagnostic memory. Commit the patterns — they are the essence of PA14.1.

Parameter	IDA	Beta-Thal Trait	ACD	Sideroblastic
Hb	Low (can be severe)	Mild (10-13 g/dL)	Mild-moderate	Moderate-severe
MCV	Low	VERY low (<70, often <65)	Normal / mildly low	Low or normal
RBC count	LOW	NORMAL/HIGH	Normal/low	Low
RDW	HIGH (anisocytosis)	Low-normal (isocytosis)	Normal	VERY HIGH (dimorphic)
Ferritin	LOW (<12)	Normal/high	HIGH	HIGH
TIBC	HIGH	Normal/low	LOW	Low-normal
Transferrin sat.	LOW (<15%)	Normal	Low	HIGH (>50%)
Mentzer Index	>13	<13	N/A	N/A
Serum iron	LOW	Normal	Low	HIGH
Inflammatory markers	Normal	Normal	HIGH (ESR, CRP)	Normal
Smear hallmark	Pencil cells, hypochromia	Target cells, isocytosis	Non-specific	Dimorphic population
Marrow	Normal (↓ iron stores on Perls)	Normal	Normal	Ringed sideroblasts (Perls +)
HPLC	Normal	↑ HbA2 (>3.5%)	Normal	Normal
Treatment direction	Iron supplementation	NO iron; genetic counselling	Treat underlying disease	Remove cause; B6 trial

Memory anchor for TIBC: Think of it as the 'hook' for iron. Iron-starved body (IDA) makes more hooks (HIGH TIBC). Inflamed body (ACD) makes fewer hooks because it downregulates all proteins (LOW TIBC). Thalassaemia and sideroblastic — hooks are normal.

Memory anchor for RDW: IDA = anisocytosis (high RDW — variable cell sizes as iron depletes). Thal trait = isocytosis (low RDW — all cells uniformly small). Sideroblastic = dimorphic (very high RDW — two distinct populations).

Side-by-side comparison of four microcytic anemias showing red blood cell morphology and diagnostic parameters for IDA, beta-thalassemia trait, ACD, and sideroblastic anemia. — **Panel A:** Iron Deficiency Anemia - severe microcytosis, high RDW, anisocytosis, hypochromic cells. **Panel B:** Beta-Thalassemia Trait - very low MCV, normal/high RBC count, uniform small cells. **Panel C:** Anemia of Chronic Disease - mild microcytosis, normal/low RBC count, variable morphology. **Panel D:** Sideroblastic Anemia - ringed sideroblasts, iron deposits, moderate-severe anemia.

The Diagnostic Decision Tree: Approach to Microcytic Anaemia

Apply this algorithm every time you see MCV <80 fL:

Step 1: Check Ferritin first.
- Ferritin <12 ng/mL → IDA. Confirm with low transferrin saturation + high TIBC. No further work-up needed unless treatment fails.
- Ferritin ≥12 → proceed to Step 2.

Step 2: Is there inflammation? Check CRP, ESR, clinical context.
- Elevated inflammatory markers + clinical setting (RA, TB, malignancy, CKD) → ACD likely.
- Confirm with LOW TIBC (typically <250 µg/dL), low-normal transferrin saturation, low-normal serum iron. Ferritin is HIGH (storage intact).
- If ferritin is 'normal' (30-100) but inflammatory markers are sky-high, ferritin may be artificially elevated — sTfR/log ferritin ratio >2 suggests concurrent IDA.

Step 3: Is the microcytosis disproportionate? Apply Mentzer Index.
- MCV disproportionately low (<70) for the degree of anaemia (Hb 10-13)? RBC count high (>5.0)? RDW low-normal? Mentzer <13?
- Ethnicity: from Sindhi, Punjabi, Gujarati, Bengali community?
- Target cells on smear?
- If YES to most: screen with HPLC. HbA2 >3.5% confirms beta-thal trait. Normal HPLC with clinical suspicion → alpha-thal trait (genetic testing).

Step 4: Look at the smear carefully.
- Dimorphic population (two clearly distinct RBC populations) + high ferritin + high transferrin saturation (>50%)?
- History of alcohol, INH, MDS, lead exposure?
- → Sideroblastic anaemia. Confirm with bone marrow Perls stain (ringed sideroblasts).

Step 5: Basophilic stippling + occupational history + abdominal/neurological symptoms?
- Order blood lead level. → Lead poisoning.

The red flags for a wrong diagnosis:
- Patient fails to respond to iron after 4-6 weeks → reconsider the diagnosis.
- Ferritin rising on iron therapy (expected when IDA is corrected) but Hb not rising → may have thal trait as the primary cause.
- 'Normal' ferritin in a patient from a high-risk community — request HPLC before starting iron.

Flowchart diagram showing the stepwise diagnostic approach to microcytic anemia with decision points for ferritin levels, inflammation markers, Mentzer index, and blood smear examination. — **Main Flow:** Sequential decision steps from MCV <80 fL through ferritin check, inflammation assessment, Mentzer index calculation, and blood smear examination leading to specific anemia diagnoses (IDA, ACD, Thalassemia, Sideroblastic anemia).