W1L2 - Iron Deficiency Anaemia

Question 1

Iron Metabolism Overview

Answer 1

1. Ferrous iron (Fe2+) in heme within RBC is the ultimate objective
2. Delivered from ferritin stores in macrophages
3. Delivered to macrophages via transferrin
4. Absorbed from dietary sources of iron via the duodenum

Question 2

Iron Absorption from the Gut

Answer 2

To move from the lumen of the intestine into the bloodstream, iron must cross both the apical brush-border membrane and the basolateral membrane of enterocytes

1. Nonheme iron traverses the brush-border membrane via divalent metal transporter 1 (DMT1)
2. DMT1 requires ferrous iron (Fe2+) as a substrate, but most dietary iron is in the ferric (Fe3+) form => iron needs to be reduced before it can be absorbed
3. Duodenal cytochrome B is one brush-border reductase
4. When not immediately required, iron becomes sequestered in the cell within ferritin
5. If the iron is required, it can be exported rapidly across the enterocyte basolateral membrane via ferroportin-1 (FPN1)

Question 3

How is the efficiency of iron absorption from the gut enhanced?

Answer 3

It’s enhanced by copper dependent iron oxidase hephaestin

- converts newly transported Fe2+ to the Fe3+ form

Question 4

Iron Transport

Answer 4

1. Diferric transferrin delivers iron to cells by binding to transferrin receptor 1 (TfR1)
2. The transferrin-TfR1 complex is internalised via clathrin-mediated endocytosis
3. Reduction of transferrin bound Fe3+ releases iron from transferrin
4. The iron moves into the cytoplasm across the endosomal membrane via DMT1
5. The iron may be used metabolic functions, sequestered within the iron-storage protein ferritin for later use, or exported through FPN1

Question 5

Transferrin

Answer 5

Major transport protein for iron
Each transferrin molecule can bind 2 atoms of iron (Fe3+).
Under normal circumstances, ∼30% of the iron-binding sites on the plasma transferrin pool are occupied (i.e. 30% total saturation)
Total saturation of transferrin less than ∼16% is correlated with a reduced erythropoiesis

Question 6

Iron Storage

Answer 6

1. It is critical that iron is stored in a nontoxic state so that it can be used for future metabolic needs
2. Ferritin is the major intracellular iron-storage protein
3. When high concentrations of iron-laden ferritin accumulate within the cell, the ferritin molecules aggregate, and ultimately fuse with lysosomes
4. This process leads to the degradation of ferritin, and the resulting mixture of Fe3+ cores and peptides is known as hemosiderin
5. Small amounts of ferritin are secreted from the cell, and the amount that is secreted strongly correlates with the concentration of intracellular iron

Question 7

Iron Metabolism in Erythropoiesis

Answer 7

Erythroblastic island
Nurse cell transfers iron to erythroid cells
Nurse cell = iron containing macrophage

Question 8

Iron Deficiency Anaemia (IDA) - Clinical Features

Answer 8

Slow onset
- months to years
Early - no apparent clinical signs
As IDA develops:
- weakness/lethargy
- koilonychia (concavity of nails)
- glossitis
- pica (abnormal appetite) eating ice, dirt/clay
- muscle dysfunction
- inability to regulate body temp
- irreversible mental & motor development, learning difficulties

Question 9

Underlying Causes of Iron Deficiency

Answer 9

Most common nutritional deficiency in the world
Insufficient dietary intake
- made worse by rapid growth
- predominantly affects children
Poor uptake of dietary iron
- achlorhydria
- gastrectomy
Blood loss
- parasitic infections
- elderly: GIT bleeds, malignancy, peptic ulcers
- females: menstrual loss, pregnancy
- frequent blood donation

Question 10

Stages of Iron Deficiency

Answer 10

1. Iron Depletion (ID)
2. Iron Deficient Erythropoiesis (IDE)
3. Iron Deficiency Anaemia (IDA)

Question 11

Laboratory Assessment of IDA - Blood Film

Answer 11

Microcytes
Hypochromasia
Poikilocytosis
- elliptocytes
- target cells
Decreased reticulocyte count

Question 12

Laboratory Assessment of IDA - Bone Marrow

Answer 12

Cellularity is normal or increased
Mild to moderate erythroid hyperplasia
Decreased M:E ratio
Fe stain Perl’s Prussian Blue shows absence of hemosiderin in macrophages

Question 13

Laboratory Assessment of IDA - FBC

Answer 13

Decreased haemoglobin
Decreased MCV (< 75 fL) (microcytosis)
Decreased MCH (14 - 26 pg)
Decreased MCHC (220 310 g/L) (hypochromic)
Increased RDW (due to microcytosis)
Increased sub-populations of microcytic, hypochromic RBC

Question 14

Cellular Haemoglobin in Reticulocytes (CHr)

Answer 14

RBC parameter
Used to detect hypochromic reticulocytes
Reticulocytes persist 1-2 days after release into PB => Hb content in reticulocytes is more reflective of Fe status in erythropoiesis
CHr can be accurately and inexpensively measured by automated hematology analysers
CHr has been shown to be more effective than other indices of iron metabolism for the diagnosis of iron deficiency

Question 15

Further Assessment - Iron Studies

Answer 15

Serum iron (SI)
Transferrin (Tf)/TIBC
% saturation
Ferritin (SF)
Soluble transferrin receptor (sTfR)
(iron content of bone marrow iron)

Question 16

Soluble Transferrin Receptor (sTfR)

Answer 16

Transferrin receptor is cleaved by membrane protease in erythroid cells when it is not stabilised by diferric transferrin
Therefore, sTfR levels are increased in ID, and also during enhanced erythropoietin activity such as hemolytic anemia or other conditions that increase red cell mass/production
However, sTfR is not affected by the acute-phase response
Accordingly, its level is useful for differential diagnosis of ID and anemia of chronic disease (ACD)

Question 17

Anaemia of Chronic Disease (ACD)

Answer 17

Mild-moderate anaemia
Often normocytic, normochromic
Typically no increase in reticulocytes
Occurs with a wide range of chronic diseases

Question 18

Pathophysiology of Anaemia of Chronic Disease - Dysregulation of Iron Homeostasis

Answer 18

1. Upregulation of ferritin mRNA, caused by cytokines IL-1 & INF
2. Increased translation of ferritin mRNA
3. Sequesters iron (so not available for erythropoiesis)
4. IL-6 => increased hepcidin production => decreased iron availability

Question 19

Pathophysiology of Anaemia of Chronic Disease - Erythropoiesis is Inhibited

Answer 19

1. IL-1, TNF-α inhibit renal EPO production
2. EPO receptor expression down regulated => inhibition response to EPO & apoptosis of the erythroid progenitors
3. Decreased numbers of reticulocytes

Question 20

IDA vs ACD

Answer 20

IDA
- reticulocyte hemoglobin content (CHr) decreased
- sTfR increased prior to anaemia
- decreased MCV
ACD
- both CHr and sTfR are not influenced by inflammation

Question 21

Sideroblastic Anaemias

Answer 21

Typically, normal erythropoiesis consumes 20mg Fe/day
Disturbed incorporation of Fe into haeme => build up of Fe in mitochondria (site of haeme formation)
‘Ringed sideroblasts’ – non ferritin iron in mitochondria & Pappenheimer bodies
Detected with Prussian blue stain

Question 22

Pappenheimer Bodies

Answer 22

Appear similar to basophilic stippling due to RNA with Romanowsky stains
Require Prussian blue stain to confirm iron content
Can be recognised in a wide range of disorders resulting in altered iron metabolism

Question 23

Hepcidin Regulation of Iron

Answer 23

Hepcidin is synthesised by hepatocytes & cleared by kidneys
Anaemia & hypoxia (Epo) decrease hepcidin production allowing normal or increased Fe uptake & metabolism
Inflammation (IL-6) induces high levels of hepcidin resulting in abnormal Fe metabolism & anaemia of chronic disease

Question 24

Hepcidin Peptide

Answer 24

Inhibits iron absorption in duodenal enterocytes
Inhibits macrophage Fe release
Inhibits erythroid progenitors