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PA14.1-2 | Microcytic Anaemia Differentials: Thalassaemia, ACD, Sideroblastic — Part 1
CLINICAL SCENARIO
Two patients. Same CBC. Haemoglobin 9.5 g/dL. MCV 70 fL. Hypochromic, microcytic picture.
Patient A: a 22-year-old woman, Sindhi community, second-year MBBS student, no symptoms except mild fatigue during exams. Ferritin comes back: 68 ng/mL.
Patient B: a 25-year-old man, vegetarian, poor dietary history, fatigue, koilonychia. Ferritin: 4 ng/mL.
Patient B gets iron supplements. He recovers. Haemoglobin rises to 12 in three months.
Patient A also gets iron supplements — misdiagnosed as IDA. Six months later, her ferritin is 180. A year later, 340. Her liver enzymes are creeping up. She never needed the iron.
The CBC was identical. The diagnosis was completely different.
This SDL teaches you how to read the iron studies, ethnicity, and smear together — so you never make that mistake.
WHY THIS MATTERS
Microcytic anaemia is the single most common haematological finding in outpatient practice in India. IDA accounts for the majority — but in a country with a beta-thalassaemia carrier rate of 3-4% nationally and up to 10-15% in certain communities (Sindhis, Punjabis, Gujaratis, Bengalis), thalassaemia trait is routinely missed or misdiagnosed as IDA.
For your clinical career: the patient who keeps coming back despite iron therapy, the anaemic woman whose ferritin mysteriously refuses to fall after months of supplementation, the mildly anaemic person from a high-prevalence community — these are your HPLC patients.
Anaemia of chronic disease affects every ward: rheumatology (RA, SLE), respiratory (TB), oncology, nephrology (CKD). You will order ESR, CRP, and ferritin in that context and be expected to interpret the pattern.
Sideroblastic anaemia is rare — but when the smear shows a dimorphic picture and you think of it, you look like the sharpest person in the ward round.
Knowing the differentials also matters for NMC CBUC PA14.1, which explicitly tests differential diagnosis of microcytic hypochromic anaemia.
RECALL
From SDL1 (Iron Metabolism):
- Iron is absorbed in the duodenum as Fe²⁺; hepcidin is the master regulator — it degrades ferroportin to reduce iron export from enterocytes and macrophages.
- Inflammation (IL-6) drives hepcidin synthesis → this is the mechanism of anaemia of chronic disease.
- Ferritin is a storage protein (normal range 12-200 ng/mL women, 30-300 ng/mL men); it is also an acute-phase reactant — it rises with inflammation regardless of iron stores.
- TIBC (total iron-binding capacity) reflects transferrin availability: high in IDA (body tries to scavenge more iron), low in ACD and thalassaemia.
From SDL2 (IDA):
- Classic IDA labs: low Hb, low MCV, low ferritin (<12 ng/mL), HIGH TIBC, low transferrin saturation (<15%), HIGH RDW (anisocytosis), low reticulocyte count. Smear: pencil cells, hypochromia, anisocytosis.
- RDW is high in IDA because RBC size becomes progressively more variable as iron runs out.
- A trial of iron therapy is reasonable in mild suspected IDA; if Hb rises ≥1 g/dL in 4 weeks, IDA is confirmed. If it doesn't rise — rethink the diagnosis.
This SDL is the next step: the patient has microcytosis, but the picture doesn't fit classic IDA. What else could it be?
The TAILS Mnemonic: Four Causes of Microcytosis
When you see microcytosis (MCV <80 fL), the differential has five entries. A useful aide-mémoire is TAILS:
- T — Thalassaemia (alpha or beta)
- A — Anaemia of Chronic Disease (ACD) — microcytic in advanced/long-standing cases
- I — Iron Deficiency Anaemia (IDA) — the most common
- L — Lead poisoning
- S — Sideroblastic anaemia
In Indian clinical practice, the working differential is almost always IDA vs thalassaemia trait vs ACD, with sideroblastic anaemia occasionally entering when the smear is dimorphic. Lead poisoning is screened for in occupational exposure or paediatric pica.
IMPORTANT: ACD is usually normocytic (MCV 80-95). It becomes microcytic only in prolonged, severe cases. If you see MCV 75-78 in a patient with chronic RA or TB — advanced ACD is contributing.
The key insight: IDA is a diagnosis of iron store depletion. The others are not — and treating them with iron is at best useless, at worst harmful.
TAILS Mnemonic for Microcytic Anemia: Differential Diagnosis and RBC Morphology
Thalassaemia Trait: Pathogenesis, Lab Pattern, Indian Context
One-line pathogenesis (defer molecular genetics to H5-SDL2):
In thalassaemia trait (carrier state), one alpha or beta globin gene is defective → reduced globin chain output → fewer haemoglobin molecules per RBC → smaller, paler cells. BUT the total RBC count is maintained or increased because the marrow compensates by making more (smaller) RBCs.
The critical distinction: trait vs major
- Thalassaemia trait (carrier): one functional gene pair intact. Mild anaemia (Hb 10-13), asymptomatic. Detected incidentally. No transfusion needed.
- Thalassaemia major: both gene pairs defective. Severe anaemia from infancy, transfusion-dependent, splenomegaly, bony deformities. This SDL focuses on trait — major is a paediatric haematology discussion.
Laboratory fingerprint of beta-thalassaemia trait:
| Parameter | Finding | Why |
|---|---|---|
| Hb | 10-13 g/dL (mild) | Compensated |
| MCV | Often <70 fL, sometimes <65 fL | Disproportionately low |
| RBC count | NORMAL or ELEVATED (>5.0 million/µL) | Compensatory erythroid output |
| RDW | LOW-NORMAL (<14%) | Uniform small cells — isocytosis |
| Ferritin | Normal or HIGH | Iron stores intact |
| TIBC | Normal or low | Not iron-depleted |
| Transferrin saturation | Normal | |
| Reticulocyte count | Normal | Steady-state |
Mentzer Index = MCV ÷ RBC count
- MCV <13 favours thalassaemia trait (small cells, many of them)
- MCV >13 favours IDA (small cells, fewer of them)
- Example: MCV 68, RBC 5.6 → 68/5.6 = 12.1 → thalassaemia trait
- Example: MCV 72, RBC 3.8 → 72/3.8 = 18.9 → IDA
- Sensitivity ~80%, not diagnostic alone. Use as the first screening step.
Smear: Target cells (codocytes), mild hypochromia. Basophilic stippling may be present.
Confirmation — HPLC:
- Beta-thalassaemia trait: HbA2 elevated (>3.5%), often 4-7%. HbF may be mildly elevated.
- Normal HPLC: HbA ~97%, HbA2 1.5-3.5%, HbF <1%.
- Alpha-thalassaemia trait (2-gene deletion): HPLC is NORMAL — diagnosis is by genetic testing or exclusion after ruling out IDA and beta-trait. Clinically common (HbH disease requires 3-gene deletion).
Indian epidemiology:
Beta-thalassaemia carrier rate is 3-4% nationally. In Sindhi, Punjabi, Gujarati, and Bengali communities, rates reach 10-15%. India has the highest number of thalassaemia major births globally (~10,000/year). ICMR recommends premarital HPLC screening in high-prevalence communities and mandates it where both partners show microcytosis.
Clinical rule: Never give iron to a confirmed thalassaemia trait patient unless concurrent IDA is documented (ferritin low + transferrin saturation <15%). Iron overload from unnecessary supplementation accumulates in the liver, heart, and endocrine glands over years.
Thalassaemia Trait: Pathophysiology and Laboratory Pattern
CLINICAL PEARL
Iron + thalassaemia trait = diagnostic trap (the 'normal ferritin' deception):
A patient with beta-thal trait develops concurrent IDA — a real and common combination, especially in women with heavy menstrual blood loss. Her ferritin, which should be near zero from iron depletion, is 28 ng/mL — 'normal' by lab reference range. But 28 is low-normal for a thal-trait patient who should have stores of 80-120. The thalassaemia trait masks the IDA.
Clue: her Hb has dropped from a stable 11 to 8.5. Her MCV has fallen from 68 to 61. RDW, usually low in thal trait, is now 17% — the new IDA is causing size variability. Transferrin saturation: 8%. TIBC: elevated.
Treat the iron — but use the absolute ferritin cut-off of <30 ng/mL in the thal-trait context, not the standard <12 ng/mL.
Anaemia of Chronic Disease (ACD): The Hepcidin-Driven Differential
Pathogenesis (connecting to SDL1):
In any chronic inflammatory state, IL-6 and other cytokines drive hepatic hepcidin synthesis. Hepcidin degrades ferroportin on macrophages and duodenal enterocytes → iron is trapped inside macrophages (the body's storage cells) and cannot be exported to the bone marrow for erythropoiesis. The result: the body has PLENTY of stored iron but the bone marrow cannot access it → functional iron deficiency.
Simultaneously, inflammatory cytokines (TNF-α, IL-1) suppress EPO production and impair erythroid progenitor responsiveness. This is a three-hit mechanism: (1) reduced iron availability, (2) reduced EPO drive, (3) reduced marrow responsiveness.
Morphology: Usually normocytic (MCV 80-95 fL). Microcytic in advanced, long-standing ACD only (months to years of severe inflammation). If you see ACD patient with MCV <78, consider additional IDA or mixed deficiency.
Laboratory fingerprint of ACD:
| Parameter | Finding | Why |
|---|---|---|
| Hb | Mild-moderate reduction (8-11 g/dL) | Multifactorial suppression |
| MCV | Usually normal; occasionally low | Normocytic first |
| RDW | Normal to mildly elevated | Uniform suppression |
| Ferritin | HIGH or NORMAL | Storage iron intact + acute-phase reactant |
| TIBC | LOW | Less transferrin synthesised in inflammation |
| Transferrin saturation | Low (but not as dramatically as IDA) | Less iron reaching transferrin |
| ESR/CRP | ELEVATED | Inflammation markers — the diagnostic clue |
| Serum hepcidin | Elevated (not routinely tested) | Research use |
The TIBC discriminator: This is the single most useful split between IDA and ACD.
- IDA: ferritin LOW + TIBC HIGH → body desperately makes more transferrin to catch any available iron.
- ACD: ferritin HIGH/NORMAL + TIBC LOW → chronic disease suppresses albumin and transferrin synthesis.
If IDA and ACD co-exist: Soluble transferrin receptor (sTfR) is elevated even in ACD+IDA — sTfR/log ferritin ratio >2 suggests concurrent IDA in an inflamed patient.
Common disease settings in India:
- Chronic infections: pulmonary TB (very common), HIV, chronic osteomyelitis
- Autoimmune: RA, SLE, inflammatory bowel disease
- Malignancy: lymphoma, carcinoma (any solid tumour)
- CKD: reduced EPO + chronic inflammation
Treatment: Treat the underlying disease. EPO/darbepoetin in CKD and chemotherapy-induced anaemia (if Hb <10 and symptomatic). IV iron is considered only if concurrent IDA is documented — not for pure ACD.
Anaemia of Chronic Disease: Hepcidin-Driven Pathogenesis and Laboratory Profile