Anemia is one of the major reasons behind patients presenting with shortness of breath, fatigue, pallor and weakness in clinics. Anemia is reduction of the oxygen carrying capacity of the blood that results in decreased oxygen supply to tissue cells. It is of utmost importance for doctors to rule out anemia as a cause of this range of symptoms as not giving prompt medical treatment can result in severer symptoms and even fatality.
Human blood from a case of iron deficiency anemia

Image: “Human blood from a case of iron deficiency anemia (Giemsa stain). License: CC BY-SA 3.0

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Definitions of Blood Components

Red blood cells


Image: “Red blood cells (eritrociti)” License: Public Domain

Red blood cells, also known as Erythrocytes, make up the cellular part of the blood, give it its red color and also bind and carry oxygen to all parts of the body. Under the microscope, they appear to be circular and biconcave in shape.


Hemoglobin is the biochemical component of the red blood cells (RBCs) that is responsible for binding oxygen. It is a metalloprotein, with iron being the metal attached to globular protein.


Hematocrit is the measure of the total volume % of red blood cells in the blood. For men, it measured to be 45 % and in women 40 %. It is an important component of a patient’s complete blood profile.

Mean corpuscular volume

Mean corpuscular volume is the average volume of red blood cells and is reflective of RBC size. RBC size increasing or decreasing changes the mean corpuscular volume and helps physicians define the type of anemia and its causes. Normal MCV is 80–96µm³.

Epidemiology of Anemia

According to WHO anemia in non-pregnant and otherwise healthy people is characterized by hemoglobin value lower than 12.5 g/dLAcute anemias affect young adults and people in their 50’s.  Females in their reproductive age (due to menses and pregnancy) are usually more prone to be anemic. Overall, females have twice as much susceptibility to developing anemia than males.

Certain races, due to specific genetic factors, have the prevalence of different types of anemias. For example, thalassemia is found majorly in South East Asian region, whereas sickle cell anemia is more prevalent in Africa.

Types of Anemia

Categorized by mean corpuscular volume, anemia can be differentiated into microcytic, macrocytic and normocytic anemias.  Normocytic anemias can be further divided into intrinsic and extrinsic RBC defect and blood loss.

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Microcytic Anemia

In this type of anemia, the MCV is lower than 80 µm³. They are caused by problems in hemoglobin synthesis. There are three types of microcytic anemia.

Iron deficiency anemia

Iron is the main component of hemoglobin, lack of iron results in decreased synthesis of hemoglobin, resulting in reduced red blood cells size. It can be due to four reasons:

  1. Decrease intake of iron: Infants younger than 6 months old can develop iron deficiency due to their being no iron in breast milk.
  2. Increase iron demand of the body: Children and adolescent during their rapid growth phase and pregnant women develop this type of iron deficiency due to increasing in blood volume and need to make more hemoglobin.
  3. Decrease iron absorption: Stomach acids and small intestine are involved in release and absorption of iron from food. Gastrectomy, duodenal diseases like celiac disease lead to reduced absorption of iron leading to deficiency.
  4. Blood loss: In young women, heavy menstruation is the leading cause of iron deficiency. Blood loss in males is commonly due to peptic ulcer diseases. In older patients, colon cancer can be the underlying cause of blood loss leading to iron deficiency. In developing countries, hookworm infection is also necessary to rule out as a cause of iron deficiency anemia.


Along with usual anemia symptoms of fatigue, shortness of breath, weakness, pallor, patients with iron deficiency anemia also show signs of PICA (the urge to eat inedible objects), koilonychia (spooning of nails), Plummer-Vinson syndrome (esophageal web and glossitis) and restless leg syndrome.

Lab reports show a decrease in serum iron, iron saturation, ferritin and an increase in total iron binding capacity.

Diagnostics – iron studies

Iron studies play a vital diagnostic role in determining the types of anemia and may other diseases involving iron metabolism. It is composed of four main values:

  • Ferritin: It is the iron-binding storage protein. It functions to store iron in a non-toxic form. Bone marrow macrophages are one of the storage sites of ferritin. Low ferritin levels are a diagnostic feature of iron deficiency anemia. Ferritin synthesis increases in anemia of chronic disease and iron overload disease.
  • Serum iron: Iron that is bound to transferrin; transferrin is the binding protein for iron in blood and synthesized in the liver
  • Total iron binding capacity: This is the concentration of transferrin. Total iron binding capacity is always inversely proportional to ferritin levels.
  • Iron saturation: Percentage of available binding sites on transferrin

Anemia of chronic disease

Strongly associated with inflammatory diseases. In a chronic inflammatory state the liver produces a protein called hepcidin. It works to stop iron absorption from the duodenum and prevents iron recycling by inhibiting the breakdown of old RBCs. Reduction in serum iron results in anemia of chronic disease.


Treatment of the underlying cause (inflammation due to infection, malignancy autoimmune etc.).


  • Ferritin is increased
  • Total binding capacity
  • % saturation
  • Serum iron decreased

Sideroblastic anemia

Sideroblast, as seen in sideroblastic anemia

Image: “Sideroblast, as seen in sideroblastic anemia.” Original by Paulo Henrique Orlandi Mourao, zooming in and minor modification by Mikael Häggström. License: CC BY-SA 3.0

This anemia occurs due to abnormalities in the synthesis of protoporphyrin, resulting in decreased synthesis of hemoglobin because protoporphyrin along with iron makes heme (which joins with globular protein to make hemoglobin).

Iron accumulates in the cells because it is not being utilized to make heme due to the absence of protoporphyrin. This results in the characteristic ring sideroblast – iron-laden mitochondria making a ring around the nucleus of the RBC precursor cells.


Ferritin, along with serum iron and % saturation is increased. Total iron binding capacity is decreased.


This disease occurs due to abnormalities in synthesis of the globular chains that attach to heme to make hemoglobin. There are two types of thalassemias:

  • Alpha thalassemia: Occurs because of gene deletion
    • 3 gene deletion: HbH, this is because beta chains start making tetramers in the absence of alpha chains; symptoms apparent after birth
    • 4 gene deletion: HbBart, incompatible with life; fetus dies inutero due to hydropsfetalis. This is due to tetramers formed by gamma chains because there are no alpha chains
  • Beta thalassemia: Occurs due to gene mutation; there are 2 types:
    • Minor: Mild, asymptomatic
    • Major: Normal in fetus; severe anemia after birth


Severe anemia, extramedullary hematopoiesis leading to hepatosplenomegaly and chipmunk appearance of face are symptoms of thalassemia.

Macrocytic Anemia

In this type of anemia, the mean corpuscular volume is more than 96 µm³.

Megaloblastic anemia

Hypersegmented neutrophils seen in megaloblastic anemia

Image: “Hypersegmented neutrophils seen in megaloblastic anemia” by United States Department of Veterans Affairs. License: Public Domain

This is the type of anemia caused specifically by vitamin B12 and/or folate deficiency. It is characterized by MCV > 100 and hyper segmented neutrophils and megaloblastic changes in all rapidly dividing cells of the body, for example, the tongue.


Alcoholism and liver disease are causes of the megaloblastic anemia, as well as drugs like 5-flourouracil. There are no hypersegmented neutrophils or megaloblastic changes in rapidly dividing cells found here.

Vitamin B12 deficiency

Vitamin B12 is a water-soluble vitamin and found in animal-derived proteins. Most common cause of vitamin B12 deficiency is pernicious anemia, which is an autoimmune-mediated destruction of the parietal cells of the stomach that decreases the production of hydrochloric acid and intrinsic factor both of which are needed for absorption of vitamin B12.

Other causes include pancreatic insufficiency (as pancreatic enzymes are needed to free vitamin B12) and damage to the part of the small intestine (terminal Ileum) where vitamin B12 is absorbed.

Vitamin B12 has two biochemical functions: it takes methyl group from methylated tetrahydrofolate and gives it to homocysteine, which in turn converts into methionine. Tetrahydrofolate can now be utilized for DNA synthesis. The second biochemical function is a conversion of methylmalonic acid into succinyl CoA.

Due to these functions, vitamin B12 deficiency results in increased homocysteine and methylmalonic acid in the blood. The latter can also contribute to subacute combined degeneration of the spinal cord as methylmalonic acid accumulates and degenerates the myelin in the spinal cord.

Other symptoms include glossitis, megaloblastic anemia, and hypersegmented neutrophils.

Folate deficiency

Folate is derived from green leafy vegetables like Asparagus and broccoli. The body doesn’t have large stores of folate so an increased demand or poor diet can result in folate deficiency within a monthThe symptoms and lab values of folate deficiency megaloblastic anemia are the same except the methyl malonic acid levels are normal in folate deficiency.

Normocytic Anemia

The mean corpuscular volume in normocytic anemia is within the range of 80–96 µm³.

Reticulocyte count

Reticulocytes seen in peripheral blood smear of a patient with hemolytic anemia

Image: “Reticulocytes seen in peripheral blood smear of a patient with hemolytic anemia” by Ed Uthman, MD. License: CC-BY 3.0

Reticulocytes are young red blood cells which are large and stain blue due to the presence of RNA. The reticulocyte count is of great clinical significance as it reflects the nature of anemia. Normal reticulocyte count without any underlying pathology is 1–2 %.

In case of normocytic anemia, if the Reticulocyte count is…

  • …more than 3 %: Indicative of bone marrow functioning normally and anemia not being due to underproduction of red blood cells
  • …less than 3 %: Indicative of abnormality in the bone marrow hence a poor response to anemia

Reticulocytes saw in peripheral blood smear of a patient with hemolytic anemiaNormocytic anemia occurs in three conditions:

  • Acute blood loss: Reticulocyte count > 3 %
  • Intrinsic defect of RBC: Reticulocyte count < 3 %
  • Extrinsic defect of RBC: Reticulocyte count < 3 %

Extrinsic defect in red blood cells

Peripheral blood smear – patient with thrombotic thrombocytopenic purpura

Image: “Peripheral blood smear: patient with thrombotic thrombocytopenic purpura” by Erhabor Osaro. License: CC BY-SA 3.0

This includes the following types of blood disorders:

  1. Immune hemolytic anemias: IgG or IgM-mediated destruction of RBCs; direct and indirect Coombs tests are used for diagnostic purposes.
  2. Micro/macroangiopathic hemolytic anemias: RBCs getting damaged due to micro or macro thrombi, resulting in Schistocytes (fragmented RBCs). Diseases include Thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, disseminated intravascular coagulation (DIC).
  3. Malaria: Destruction of RBCs by plasmodium species transmitted by the female anopheles mosquito.

Clinical findings of extrinsic defect in RBCs

  • Anemia
  • Splenomegaly – due to increased load on spleen to destroy so many abnormal RBCs
  • Jaundice – due to increased destruction of defective RBCs by the spleen and raised total bilirubin levels; this may also increase risk of developing gallstones

Intrinsic defect in red blood dells

This includes the membrane defects, abnormal hemoglobin, and enzymatic defects.

Membrane defects

  • Hereditary spherocytosis: Inherited defect in the proteins that maintain the integrity of the RBC membrane, leading to a round, small cells with no central pallor; diagnosed with osmotic fragility test.
  • Paroxysmal nocturnal hemoglobinuria: Inherited abnormalities/absence of surface proteins that protect the RBCs from the complement; increased oxidative stress (e.g acidosis during sleeping) results in the destruction of RBCs overnight and dark colored urine (containing hemoglobin) in the morning. Diagnosed with sucrose and acidified serum test.

Abnormal hemoglobin

sickle cell anemia

Image: “Sickle Cell Anemia” by BruceBlaus. License: CC BY-SA 4.0 The picture is cropped.

Sickle cell anemia is a genetic defect in the synthesis of hemoglobin chain. Glutamic acid gets substituted by Valine, resulting in HbS which sickles (polymerizes) when deoxygenates. Sickling can lead to a range of severe symptoms:

  • Vaso-occlusion leading to tissue infarction (for example, dactylitis – swollen hands due to vaso-occlusion of hand bones)
  • Auto-splenectomy occurs if spleen gets infarcted; this increases chances of acquiring infections and is main cause of death in children
  • Extreme pain
  • Acute chest syndrome: Infarction of lung vessels; causes death in adults with Sickle cell anemia
  • Renal papillary necrosis – occlusion of renal blood vessels  – results in visible hematuria and leaking of proteins into the urine
sickle cell disease

Image: “Figure A shows normal red blood cells flowing freely in a blood vessel. The inset image shows a cross-section of a normal red blood cell with normal hemoglobin. Figure B shows abnormal, sickled red blood cells blocking blood flow in a blood vessel. The inset image shows a cross-section of a sickle cell with abnormal (sickle) hemoglobin forming abnormal strands.” by The National Heart, Lung, and Blood Institute. License: Public Domain

Enzymatic defects

Glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency) leads to reduced production of NADPH which is needed to reduce oxidized Glutathione. Glutathione is oxidized while protecting the RBCs from hydrogen peroxide. Therefore, G6PD deficiency results in destruction of RBCs by oxidative stress.

Heinz bodies are characteristic of this disorder. When they’re removed by Splenic macrophages we see bite cellsSymptoms include hemoglobinuria and back pain (due to nephrotoxicity of the Heinz bodies).

Popular Exam Questions

The correct answers can be found below the references.

1. A 34-year-old went on a business trip to India. On his return, he presented in the clinic with jaundice, painful splenomegaly and fever spikes. A direct Coomb’s test is negative. Patient’s MCV is within the range of 80–100 µm³. The reticulocyte count is discovered to be > 3 %. The case complicates further when gross hematuria is observed in the urine bag. A peripheral blood smear shows a sausage shapes body in the red blood cell. Which of the following is the most likely diagnosis?

  1. Plasmodium falciparum malaria
  2. Sickle cell anemia
  3. G6PD deficiency
  4. Autoimmune hemolytic anemia
  5. Disseminated Intravascular Coagulation

2. A 4-year-old boy presents in the clinic with shortness of breath, pallor, and fatigue. His mean corpuscular volume is 67 µm³. On taking a history the patient’s mother tells the doctor that she often finds him eating scraped off paint from the wall. The patient also suffers colicky abdominal pain and vomiting. Lead poisoning is diagnosed by the doctor. The following image shows the peripheral blood smear. Which of the following best describes the type of anemia the patient has?

  1. Microangiopathic anemia
  2. Megaloblastic Anemia
  3. Sideroblastic Anemia
  4. Thalassemia
  5. Sickle Cell Anemia
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