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Microcytic Anaemias: Iron vs Sideroblastic

TL;DR

Microcytic anaemia questions in MRCP Part 1 frequently test your ability to distinguish iron deficiency from sideroblastic anaemia using iron studies. Heme: Microcytic Anemias (Iron vs. Sideroblastic) revolves around ferritin, serum iron, and transferrin patterns. Iron deficiency shows low ferritin, while sideroblastic anaemia presents with iron overload despite anaemia. Recognising this contrast is a dependable scoring opportunity.


Why this matters

Microcytic anaemia is a staple topic in MRCP Part 1, with predictable question patterns centred on interpretation rather than recall. Examiners often present laboratory data requiring rapid differentiation between iron deficiency anaemia (IDA) and sideroblastic anaemia.

This distinction is crucial because both conditions produce microcytosis but reflect opposite states of iron metabolism. Mastering this concept not only secures marks but also reinforces broader haematology reasoning.

For a structured pathway, review the MRCP Part 1 overview and consolidate with active recall via Free MRCP MCQs.


Core sections

1. Definition and classification

Microcytic anaemia is defined as:

  • MCV < 80 fL

The main causes can be recalled using TAILS:

  1. Thalassaemia

  2. Anaemia of chronic disease

  3. Iron deficiency

  4. Lead poisoning

  5. Sideroblastic anaemia

In MRCP Part 1, iron deficiency and sideroblastic anaemia are the most commonly contrasted entities.

2. Iron deficiency anaemia (IDA)

Aetiology

  • Chronic blood loss (e.g. gastrointestinal malignancy, menstruation)

  • Malabsorption (coeliac disease)

  • Poor dietary intake

Pathophysiology Reduced iron availability leads to impaired haemoglobin synthesis and microcytosis.

Laboratory findings

  • ↓ Ferritin (most specific marker)

  • ↓ Serum iron

  • ↑ Total iron-binding capacity (TIBC)

  • ↓ Transferrin saturation

Peripheral smear

  • Hypochromic, microcytic red cells

  • Anisopoikilocytosis

Clinical associations

  • Pica, koilonychia, glossitis

  • Occult malignancy (critical exam clue)

3. Sideroblastic anaemia

Aetiology

  • Alcohol excess

  • Drugs (notably isoniazid)

  • Myelodysplastic syndromes

  • Congenital enzyme defects (rare)

Pathophysiology Defective haem synthesis causes iron accumulation in mitochondria, forming ring sideroblasts.

Laboratory findings

  • ↑ Ferritin

  • ↑ Serum iron

  • ↓/normal TIBC

  • ↑ Transferrin saturation

Peripheral smear

  • Dimorphic red cell population

Bone marrow

  • Ring sideroblasts (diagnostic)

Clinical associations

  • Alcohol misuse

  • Vitamin B6 (pyridoxine) deficiency

4. High-yield comparison table

Feature

Iron Deficiency Anaemia

Sideroblastic Anaemia

Ferritin

↓ Low

↑ High

Serum iron

↓ Low

↑ High

TIBC

↑ High

↓/Normal

Transferrin saturation

↓ Low

↑ High

Blood film

Hypochromic microcytic

Dimorphic

Bone marrow

Absent iron stores

Ring sideroblasts

Common cause

Blood loss

Alcohol/drugs

Exam shortcut:

  • Low ferritin → iron deficiency

  • High iron + microcytosis → sideroblastic anaemia

5. The 5 most tested subtopics

  1. Iron study interpretation – pattern recognition is essential

  2. Ferritin limitations – elevated in inflammation

  3. Ring sideroblasts – pathognomonic feature

  4. Drug-induced causes – especially isoniazid

  5. Differentiation from anaemia of chronic disease

6. Rapid revision checklist

  1. Microcytosis = MCV < 80 fL

  2. Ferritin is the most specific test for iron deficiency

  3. Sideroblastic anaemia shows iron overload

  4. TIBC rises in iron deficiency, falls in sideroblastic

  5. Alcohol and isoniazid are key triggers

  6. Dimorphic film suggests sideroblastic anaemia

  7. Ferritin may be falsely normal in inflammation

  8. Always consider GI malignancy in IDA

  9. Thalassaemia has a relatively high RBC count

  10. Bone marrow findings are exam favourites


Practical examples / mini-cases

MCQ

A 58-year-old man with a history of alcohol dependence presents with fatigue. Blood tests reveal Hb 9.2 g/dL and MCV 70 fL. Iron studies show elevated ferritin and serum iron.

What is the most likely diagnosis?

A. Iron deficiency anaemiaB. Anaemia of chronic diseaseC. Sideroblastic anaemiaD. Thalassaemia trait

Answer: C. Sideroblastic anaemia

Explanation :Microcytosis with increased iron stores rules out iron deficiency. Alcohol is a well-known cause of sideroblastic anaemia. The key discriminator is paradoxically elevated iron despite anaemia.


MRCP Part 1 study setup with notes and MCQ practice for haematology revision

Common pitfalls (5 bullets)

  • Misinterpreting ferritin in inflammatory states

  • Confusing anaemia of chronic disease with iron deficiency

  • Ignoring TIBC trends

  • Missing drug-induced causes

  • Overlooking dimorphic blood films


FAQs

1. What is the most specific test for iron deficiency?

Serum ferritin is the most specific indicator. A low level confirms iron deficiency in the absence of inflammation.

2. How is sideroblastic anaemia diagnosed?

Diagnosis is confirmed by identifying ring sideroblasts in bone marrow, alongside elevated iron indices.

3. Why can ferritin be misleading?

Ferritin is an acute-phase reactant and may be elevated in inflammation, masking true iron deficiency.

4. Which drug is commonly associated with sideroblastic anaemia?

Isoniazid is the classic cause due to pyridoxine deficiency affecting haem synthesis.

5. How do you differentiate from anaemia of chronic disease?

ACD typically shows normal or increased ferritin with low TIBC, but without iron overload seen in sideroblastic anaemia.


Call to action

Refine your haematology performance with focused practice. Attempt Free MRCP MCQs or simulate exam conditions with a Start a mock test. For comprehensive coverage, revisit the MRCP Part 1 overview and pair this topic with related discussions on thalassaemia and anaemia of chronic disease.


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