Microcytic & Normocytic Anemias

General Information 

RBCs & Erythropoietin

• The average RBC has a life span of 120 days; production of new RBCs (erythropoiesis) replaces approximately 1% of circulating RBCs on a daily basis. 
• EPO is the regulatory hormone, For RBC production. Normal EPO level in plasma is 10 - 25 U/L. In circulation
• The regulation of EPO production is linked to O2 availability. So, Anemia, hypoxemia, or renal artery stenosis can raise EPO production.
• Other sources of EPO: Ectopically produced in renal cell carcinoma and hepatocellular carcinoma

Spectrin is the chief protein responsible for biconcave shape of normal red blood cell.

• Variations in red cell size is called anisocytosis.
• Variations in red cell shape is called poikilocytosis. 
• Poikilocytosis suggests a defect in the maturation of red cell precursors in bone marrow or fragmentation of circulating red cells.

Reticulocytes and the reticulocyte count 

• Reticulocytes are immature RBCs that are produced in bone marrow, released into the bloodstream, and mature into RBCs within 24 to 36 hours. 
• Reticulocytes are nonnucleated spherical cells bigger than normal RBCs and are polychromatic (having a blue color) due to the presence of free ribosomes and RNA.

Reticulocyte count: Percentage of reticulocytes among the red cells present in the peripheral blood.

• The reticulocyte count is an indicator of erythropoetic activity of the bone marrow.
• Normal value is around 1.5% in adults and 1.7% in the cord blood cells.
• The reticulocyte count determines whether normocytic anemia is hypoproliferative, or hyperproliferative.

Absolute reticulocyte count (ARC): Number of reticulocytes present in 1 mm³ of blood. 

Reticulocyte Index (RI): It adjusts reticulocyte count for hematocrit.

• It reflects bone marrow activity and is also known as “Poor man’s Bone Marrow Aspirate”.
• Normal reticulocyte index is 1-3%.

Complete blood cell count (CBC)

Hb, Hct, and RBC counts 

• Hemoglobin is the molecule found inside RBCs that allows for the transport of oxygen and carbon dioxide.
• Hematocrit is the percentage of packed blood containing intact RBCs after it has been spun in a centrifuge. 
• The hematocrit is approximately 3 times the value of the hemoglobin concentration.
• Mean hematocrit value for adult males is 47% (± SD 7) and for adult females is 42% (± 5).

Red blood cell indices

Mean corpuscular volume (MCV)—average volume of RBCs 

1. Microcytic anemia: MCV <80 µm3
2. Normocytic anemia: MCV 80–100 µm3
3. Macrocytic anemia: MCV >100 µm3

MCH (Mean corpuscular hemoglobin): Average mass of hemoglobin (in picograms) per red blood cell. Normal value is 27-33 pg.

Mean corpuscular hemoglobin concentration (MCHC)—average Hb concentration in RBCs 

• Normal value is 33-37g/dl.
• Decrease correlates with decreased synthesis of Hb
• Increased mean corpuscular hemoglobin concentration: Correlates with the presence of spherical RBCs, which occurs in hereditary spherocytosis

RDW (Red cell distribution width): It is the coefficient of variation of size of RBCs. Normal value is 11.5-14.5. 

Introduction 

Anemia is defined as any reduction below normal limits of the total circulating red cell mass, which is characterized by the clinical features of pallor of skin and nails, dizziness, palpitations, lethargy and fatigue.

The World Health Organization (WHO) defines anemia as a hemoglobin level < 13 g/dL in men and < 12 g/dL in women.

Anemia affects up to one-third of the world’s population and is often associated with significant morbidity and mortality. Compared with men, prevalence is higher in women, particularly during pregnancy.

The clinical features of anemia depend on the severity, chronicity, and rate of onset. 

• When gradual in onset, even severe anemia can remain subtle as a result of physiologic adaptation. 
• Mild anemia is frequently asymptomatic, but may be incidentally diagnosed on routine laboratory measurement.

Severity of anemia:

1. Hemoglobin ≥ 9.5 g/dL is mild; 
2. Hemoglobin 8 to 9.4 is moderate
3. Hemoglobin <8 g/dL is severe.

Anemia can be microcytic, normocytic, or macrocytic.

• Note that all causes of anemia are initially normocytic because it takes some time for the abnormal-sized RBCs to outnumber the normal-sized ones.

Anemia is most often recognized by (incidental) abnormal screening laboratory tests.

• If Hb and Hct reveal anemia, the next tests to obtain to determine cause of anemia are reticulocyte count and MCV.

The first step in evaluation of the anemic patient is to assess volume status and hemodynamic stability, in order to identify any active bleeding. 

• The RBC count, hemoglobin concentration, and hematocrit can be influenced by plasma volume. These laboratory values may be falsely elevated by hypovolemia and falsely lowered by hypervolemia.
• If hemodynamically unstable, transfuse PRBCs before attempting to find the source.

Pseudoanemia or dilutional anemia refers to a decrease in Hb and Hct secondary to dilution (i.e., secondary to acute volume infusion or overload).

Iron-deficiency anemia

Iron-deficiency anemia is the most common cause of anemia worldwide, and the most common nutritional deficiency worldwide

It's a microcytic, and hypochromic anemia, that occurs due decreased synthesis of heme (iron + protoporphyrin), which leads to a decreased synthesis of Hb 

Poikilocytosis is seen in form of small and elongated red cells called pencil cells. It is also characteristic feature of this disease. 

Iron absorption & Normal value 

• Iron is absorbed primarily from the duodenum in the ferrous form.
• Absorbed iron is transferred to a plasma protein called transferrin.
• Each molecule of transferrin can transport two molecules of iron to the desired areas. Normal transferrin saturation ranges from 25 - 50%.
• Serum iron: iron bound to transferrin. Normal serum iron ranges from 50 - 150 µg/dL
• Serum total iron-binding capacity (TIBC)—correlates with the concentration of transferrin. Normal TIBC is 300 - 360 µg/dL.
• Iron is primarily stored as ferritin and hemosiderin in bone marrow macrophages. 
• Ferritin is soluble iron-binding protein; keeps iron in non-toxic form. Ferritin synthesized in bone marrow macrophages/ hepatocytes
• Hemosiderin is an insoluble product of ferritin degradation in lysosomes

Most of the body’s iron is contained in hemoglobin and not ferritin.

• Amount of stored iron in men is ~1000 mg and in women, 400 mg (decreased due to menses).
• Total iron stores in men is ~3500 mg and in women ~2400 mg.
• Functional iron is present in Hb, enzymes, and myoglobin. - Amount of functional iron in men is ~2500 mg and in women, ~2000 mg.
• The normal requirement of iron in the diet is nearly 1 mg/d.

What is Causes of IDA?

Chronic blood loss: Most common cause of iron deficiency anemia in adults 

1. Menstrual blood loss is the most common source
2. PUD (most common cause in adult men)
3. Polyps/colorectal cancer (most common cause in adults >50 years of age)

In elderly patients with iron deficiency anemia, you must rule out colon cancer. 

Dietary deficiency/increased iron requirements

1. Infants and toddlers—occurs especially if the diet is predominantly human milk, which is low in iron
2. Decreased intake of iron is the most common cause of iron deficiency in young children.
3. Adolescents—increased requirement for iron due to rapid growth
4. Pregnant women—Daily iron requirement is 3.4 mg in pregnancy and 2.5–3 mg in lactation

Other causes: 

1. Celiac sprue: Absence of the villous surface in the duodenum decreases absorption of iron.
2. Ankylostoma duodenale is associated with iron deficiency anemia.

Symptom & Signs of IDA

1. Fatigue, Impaired growth and development
2. Pica (the craving for nonfood items) and pagophagia (the craving for ice)
3. koilonychia (angular or spoon shaped nails), 
4. Angular stomatitis (ulceration at the angle of mouth), 
5. Dysphagia (as in Plummer Vinson syndrome) 
6. Palpitations (because of hyperdynamic circulation which can even precipitate congestive heart failure).

Iron deficiency anemia, especially in women and children, may be associated with pagophagia, a peculiar pica for ice.

Diagnosis and Treatment 

Diagnosis

• Serum ferritin and serum iron are decreased whereas serum transferrin and TIBC are increased.
• Serum ferritin is the single most powerful noninvasive test to evaluate for iron-deficiency anemia. 
• A serum ferritin value <15 ng/mL confirms the diagnosis of iron deficiency, while a value of >100 ng/mL rules it out. 
• RBC free protoporphyrin is normally 30-50 μg/dl whereas its value reaches > 200 μg/dl in iron deficiency anemia.

Serum ferritin is an indicator of body iron stores.

Treatment: The treatment of anemia is with the help of either oral or parenteral iron therapy the response of which is clinically assessed with the reticulocyte count on about 8th - 9th which demonstrates reticulocytosis. 

• If iron deficiency anemia is suspected, but the anemia does not respond to iron therapy, obtain a hemoglobin electrophoresis to rule out α- and β-thalassemia.

Sideroblastic Anemia 

Sideroblastic anemia refers to a group of inherited and acquired conditions characterized by impaired heme biosynthesis and the distinctive presence of ring sideroblasts.

• It's caused by abnormality in RBC iron metabolism leading to pathologic iron deposits in RBC mitochondria 
• Sideroblasts is a normoblasts having pin point iron granules in the cytoplasm or perinuclear region. 
• Sideroblastic anemias tend to produce hemoglobin in the 4 to 10 g/dL range.
• Peripheral smear is characterized by presence of microcytic hypochromic cells which also demonstrate the presence of anisopoikilocytosis. 
• There is increase in serum iron, serum ferritin, transferrin saturation and free erythrocyte porphyrin, whereas TIBC is decreased.
• Treatment involves removing offending agents and transfusing as necessary—may also consider pyridoxine

Note: Abnormal sideroblasts are also seen in thalassemia, megaloblastic anemia and hemolytic anemias.

Pathogenesis

1. Defect in heme synthesis within the mitochondria of developing RBCs in the bone marrow 
2. Iron accumulates in mitochondria, which produces ringed sideroblasts 
3. Iron stores increase markedly in the bone marrow macrophages. 
4. Sideroblasts die in the marrow (ineffective erythropoiesis) 
5. Phagocytosed by macrophages, which leads to excess iron stores

Causes of Sideroblastic anemias

1. Chronic alcoholism (most common cause): Sideroblastic anemia is present in ~30% of hospitalized chronic alcoholics.
2. Pyridoxine (vitamin B6) deficieny: Most common cause of deficiency is isoniazid (INH) therapy for TB.
3. Lead (Pb) poisoning: Most common in children ages 1 to 5 years old
4. X-linked recessive (XR) disease

Anemia of Chronic Disease (ACD)

• Anemia can occur as a result of acute or chronic inflammation through a variety of mechanisms
• It is MC anemia in hospitalized patients, and MC anemia in malignancy, alcoholism
• Anemia of CKD mainly occurs as a result of EPO deficiency.
• Pathogenesis: Changes in iron homeostasis (limited iron availability for erythropoiesis), impaired proliferation and differentiation of erythroid progenitor cells, Decreased production of and response to EPO, and shortened life span of RBCs
• It is characterized by the decreased utilization of iron from the storage from of iron.
• This result in the high serum ferritin levels, reduced TIBC, reduced transferrin saturation and decreased serum iron levels.
• Peripheral blood smear usually reveals normocytic and normochromic anemia, but may be microcytic and hypochromic as well 
• It is most likely to present as a microcytic anemia in the setting of rheumatoid arthritis and Crohn disease.
• May be difficult to differentiate from iron deficiency anemia, Serum ferritin is most useful in distinguishing the two anemias  
• No specific treatment is necessary other than treatment of the underlying process— do not give iron

Causes of Anemia of Chronic Disease

1. Chronic inflammation—rheumatoid arthritis, tuberculosis (TB), and Crohn disease
2. In chronic inflammatory conditions, there is increased secretion of cytokines like IL-1, TNF, IFN-g, etc. that cause release of the protein hepcidin because of which release of iron from the storage pool is inhibited.
3. Alcoholism: ACD is the most common anemia in alcoholism.
4. Malignancy: ACD is the most common anemia in malignancy.
5. Anemia of hypothyroidism mainly occurs as a result marrow hypoproliferation from thyroid hormone deficiency, and Decreased EPO production as a result of reduced oxygen requirements associated with the hypothyroid state.

Anemia of chronic inflammatory conditions is caused by hepcidin.

Aplastic Anemia

• Definition: Bone marrow failure leading to pancytopenia (i.e., anemia, neutropenia, thrombocytopenia)
• Two peaks of presentation—between 15 and 25 years or >60 years
• Full blood count—Pancytopenia (anemia, leukopenia and thrombocytopenia). 
• The anemia is typically moderate to severe, normochromic, and normocytic, with a profound reticulocytopenia. 
• Bone marrow biopsy (for definitive diagnosis)— either dry tap or markedly acellular or hypocellular marrow.
• Lymphocytes are present in the marrow and peripheral blood, because the pluripotential stem cell is proximal to the myeloid progenitor stem cell

Causes of Aplastic Anemia

1. Idiopathic (MCC): Approximately 50%–70% of cases are idiopathic
2. Drugs: Most common known cause of aplastic anemia
3. Whole-body ionizing radiation (therapeutic or nuclear accident)
4. Thymoma (may be associated with pure RBC aplasia)

Symptoms of Aplastic Anemia

1. Fever, due to infection associated with neutropenia 
2. Bleeding, due to thrombocytopenia 
3. Fatigue, due to anemia

Pure red cell aplasia is a type of inherited or acquired bone marrow failure that presents with isolated anemia.