anemia and it’s classification

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anemia classification morphology microcytic normocytic macrocytic diagram

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Anemia and Its Classification

Definition

Anemia is defined as a reduction in red blood cell (RBC) mass or hemoglobin concentration below the normal range for a given population. Practically, it means a blood count below the established population normal. The WHO criteria for anemia are:

Hemoglobin < 13 g/dL in adult males
Hemoglobin < 12 g/dL in adult females
Hemoglobin < 11 g/dL in pregnant women

Two Major Classification Systems

Anemias are classified by two complementary approaches: morphological (by red cell size/MCV) and pathophysiological (by mechanism). Both are used together in clinical practice.

1. Morphological Classification (by MCV)

Pioneered by hematologist Max Wintrobe, this approach groups anemia by Mean Corpuscular Volume (MCV). It is a practical first step in workup because it is derived directly from the CBC.

A. Microcytic Anemia (MCV < 80 fL)

Microcytosis results from any process that interferes with hemoglobin synthesis - less hemoglobin means smaller red cells.

Cause	Mechanism
Iron deficiency anemia	Insufficient iron to synthesize heme
Thalassemia	Defective globin chain synthesis
Anemia of chronic disease/inflammation	Cytokine-mediated iron sequestration; often normocytic but can be microcytic
Sideroblastic anemia	Defective heme synthesis; iron accumulates in mitochondria
Lead poisoning	Inhibits heme synthesis enzymes

Mnemonic: TAILS - Thalassemia, Anemia of chronic disease, Iron deficiency, Lead, Sideroblastic

B. Normocytic Anemia (MCV 80-100 fL)

A broad category; the reticulocyte count is the most useful next step.

Cause	Examples
Aplastic anemia	Bone marrow failure
Anemia of chronic disease (most cases)	Chronic kidney disease, endocrine deficiency
Hemolytic anemias (early/compensated)	Sickle cell disease (homozygous)
Bone marrow infiltration	Leukemia, metastatic malignancy, granulomas
Renal failure	EPO deficiency
Endocrinopathies	Hypothyroidism, adrenal insufficiency, androgen deficiency
Acute blood loss
Mixed deficiency (B12 + iron)	May cancel out to give normal MCV

C. Macrocytic Anemia (MCV > 100 fL)

Divided into megaloblastic (oval macrocytes) and non-megaloblastic (round macrocytes):

Megaloblastic (oval macrocytes) - defects in DNA synthesis:

Vitamin B12 deficiency
Folate deficiency
Drugs (hydroxyurea, methotrexate, some anticonvulsants, chemotherapy)
Myelodysplastic syndrome (MDS)

Non-megaloblastic (round macrocytes) - membrane defects or other:

Alcohol use disorder
Liver disease
Hypothyroidism
Reticulocytosis (young RBCs are larger)
Dysproteinemia
Hypoxia, smoking

2. Pathophysiological Classification (by Mechanism)

This is the most clinically meaningful classification, from Robbins Pathology (Pathologic Basis of Disease):

A. Blood Loss

Type	Examples
Acute blood loss	Trauma, surgical hemorrhage
Chronic blood loss	GI tract lesions, gynecologic disorders (leading to iron deficiency)

B. Increased RBC Destruction (Hemolysis)

Inherited / Intrinsic Defects:

Category	Specific Examples
RBC membrane disorders	Hereditary spherocytosis, hereditary elliptocytosis
Enzyme deficiencies	G6PD deficiency (HMP shunt), Pyruvate kinase deficiency (glycolysis)
Hemoglobin abnormalities	Sickle cell disease, thalassemia, unstable hemoglobins
Acquired genetic defects	Paroxysmal nocturnal hemoglobinuria (PNH) - loss of GPI-linked proteins

Acquired / Extrinsic Defects:

Category	Specific Examples
Immune-mediated	Autoimmune hemolytic anemia (warm/cold), Hemolytic disease of the newborn, transfusion reactions, drug-induced
Microangiopathic hemolytic anemia (MAHA)	HUS, TTP, DIC - mechanical RBC fragmentation (schistocytes)
Cardiac traumatic hemolysis	Defective cardiac valves
Infections	Malaria, babesiosis
Physical/chemical injury	Burns, oxidant drugs
Hypersplenism/Sequestration

C. Decreased RBC Production (Hypoproliferative)

Subcategory	Examples
Nutritional deficiencies	Iron, B12, folate deficiency
EPO deficiency	Chronic kidney disease
Stem cell depletion	Aplastic anemia (Fanconi anemia, acquired)
Bone marrow infiltration	Leukemia, metastatic cancer, granulomatous disease (myelophthisic anemia)
Inflammation-mediated iron sequestration	Anemia of chronic disease - hepcidin blocks iron release
Immune-mediated progenitor injury	Pure red cell aplasia
Primary hematopoietic neoplasms	MDS, leukemia

3. Kinetic (Reticulocyte-Based) Classification

The reticulocyte count rapidly differentiates the mechanism:

Reticulocyte Count	Interpretation
Elevated (> 2%)	Hyperproductive response - bone marrow is responding to RBC loss (hemorrhage or hemolysis)
Low/Normal	Hypoproliferative - bone marrow is failing (nutritional deficiency, aplasia, infiltration, EPO deficiency)

A corrected reticulocyte count or reticulocyte production index (RPI) < 2 suggests inadequate marrow response.

Quick Summary Diagram

ANEMIA
├── MORPHOLOGICAL (MCV)
│   ├── Microcytic (<80)  → Iron def, Thal, ACD, Sideroblastic, Lead
│   ├── Normocytic (80-100) → Aplastic, ACD, Hemolytic, Renal, Bone marrow infiltration
│   └── Macrocytic (>100)
│       ├── Megaloblastic (oval macro) → B12/Folate def, MDS, Drugs
│       └── Non-megaloblastic (round macro) → Alcohol, Liver dz, Hypothyroid
│
└── PATHOPHYSIOLOGICAL
    ├── Blood Loss (Acute / Chronic)
    ├── Increased Destruction (Hemolysis)
    │   ├── Intrinsic (membrane, enzyme, Hb defects)
    │   └── Extrinsic (immune, MAHA, infection)
    └── Decreased Production (Hypoproliferative)
        ├── Nutritional deficiency
        ├── Bone marrow failure/infiltration
        └── EPO deficiency

Sources:

[Harrison's Principles of Internal Medicine 22E](Harrison's, p. 492) - Anemia Definition and Classification
Robbins, Cotran & Kumar - Pathologic Basis of Disease, Table 14.1: Classification of Anemia According to Underlying Mechanism
Tintinalli's Emergency Medicine - Table 144-2: Classification of Anemia
Goldman-Cecil Medicine - Table 144-9: Classification of Anemia by MCV

etiological classification

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Excellent! I have all the material needed. Here is the comprehensive etiological classification of anemia:

Etiological Classification of Anemia

The etiological classification organizes anemias by their root cause - what is actually happening biologically to reduce the red cell mass. There are three master etiological categories:

Overview Diagram

Anemia classification flowchart showing microcytic, normocytic, and macrocytic branches with causes

Frameworks for Internal Medicine - Anemia classification by morphology and etiology

CATEGORY I - Nutritional / Deficiency Anemias

Caused by lack of substrates needed for RBC synthesis or maturation.

1. Iron Deficiency Anemia (IDA)

The most common cause of anemia worldwide.

Causes:
- Insufficient intake - malnutrition, poor diet
- Decreased absorption - celiac disease, post-gastrectomy, achlorhydria
- Chronic blood loss - GI lesions (peptic ulcer, CRC, hookworm), menorrhagia, hemorrhoids
- Increased demand - pregnancy, infancy, adolescence
Result: Microcytic, hypochromic anemia
Diagnosis confirmed by serum ferritin < 15 ng/mL

2. Megaloblastic Anemias (DNA Synthesis Defects)

Impaired DNA synthesis causes ineffective erythropoiesis with large, immature cells.

Type	Cause
Vitamin B12 deficiency	Pernicious anemia (anti-IF antibodies), strict veganism, gastrectomy, Crohn's disease, fish tapeworm (D. latum)
Folate deficiency	Poor diet (green leafy vegetables), alcohol use, pregnancy (increased demand), malabsorption, drugs (methotrexate, trimethoprim)
Copper deficiency	Excess zinc supplementation, penicillamine use, bariatric surgery
Drug/toxin-induced	Nitrous oxide (inactivates B12), hydroxyurea, chemotherapy, anticonvulsants

3. Protein-Calorie Malnutrition

Global undernutrition leads to normocytic or macrocytic anemia
Reduces EPO synthesis and erythropoietic marrow activity

CATEGORY II - Hemolytic Anemias (Increased RBC Destruction)

Red cell lifespan is shortened (normal = 120 days). Characterized by:

Raised reticulocyte count (hyperproliferative)
Raised LDH and unconjugated bilirubin
Decreased serum haptoglobin

Divided into Intracorpuscular (Intrinsic) and Extracorpuscular (Extrinsic) defects.

A. Intracorpuscular (Intrinsic/Inherited) Defects

i. Red Cell Membrane Disorders

Condition	Defect
Hereditary spherocytosis	Spectrin/ankyrin defect - loss of membrane surface area, rigid spherical cells trapped in spleen
Hereditary elliptocytosis	Spectrin dimer self-association defect - elliptical fragile cells
Hereditary stomatocytosis	Abnormal cation permeability

ii. Red Cell Enzyme Deficiencies

Enzyme	Pathway	Consequence
G6PD deficiency	Hexose monophosphate shunt	Cannot neutralize oxidants; precipitated by infections, drugs (primaquine, dapsone), fava beans
Pyruvate kinase deficiency	Glycolysis	ATP depletion; RBCs cannot maintain shape and integrity
Glutathione synthetase deficiency	HMP shunt	Oxidative hemolysis

iii. Hemoglobin Disorders (Hemoglobinopathies)

Qualitative defects (abnormal structure):

Sickle cell disease (HbS) - point mutation Glu→Val in β-globin; polymerization under hypoxia causes sickling, vaso-occlusion, and hemolysis

Quantitative defects (deficient synthesis):

Thalassemia syndromes - reduced/absent α or β globin chain production; unbalanced chains precipitate and destroy RBCs
- α-thalassemia: deletion of α-globin genes (1-4 genes)
- β-thalassemia: mutations in β-globin gene

iv. Acquired Intrinsic Defect

Paroxysmal Nocturnal Hemoglobinuria (PNH) - acquired somatic mutation in PIGA gene; loss of GPI-anchored complement regulatory proteins (CD55, CD59); complement-mediated lysis, especially at night

B. Extracorpuscular (Extrinsic/Acquired) Defects

i. Immune-Mediated Hemolysis

Type	Details
Autoimmune hemolytic anemia (AIHA) - Warm type	IgG antibodies active at 37°C; extravascular hemolysis; associated with SLE, CLL, drugs
AIHA - Cold type	IgM antibodies active at < 37°C; complement fixation; associated with Mycoplasma, EBV, lymphoma
Drug-induced hemolysis	Hapten mechanism (penicillin), immune complex (quinidine), autoantibody (methyldopa)
Hemolytic disease of the newborn (HDN)	Maternal IgG crosses placenta (Rh or ABO incompatibility)
Transfusion reactions	ABO mismatch - acute intravascular hemolysis

ii. Microangiopathic Hemolytic Anemia (MAHA)

Mechanical fragmentation (schistocytes on smear):

Thrombotic thrombocytopenic purpura (TTP) - ADAMTS13 deficiency
Hemolytic uremic syndrome (HUS) - Shiga toxin-producing E. coli (O157:H7)
DIC - Disseminated intravascular coagulation
Malignant hypertension, HELLP syndrome

iii. Infectious Causes

Malaria (Plasmodium falciparum - most severe; direct RBC invasion and lysis)
Babesiosis (tick-borne; similar to malaria)
Clostridial sepsis - phospholipase toxins destroy RBC membrane

iv. Physical/Chemical Causes

Thermal injury (burns) - direct heat damage to membrane
March hemoglobinuria - mechanical trauma from repetitive physical activity (marathon runners, bongo drumming)
Cardiac hemolysis - turbulence from defective cardiac valves

v. Hypersplenism / Sequestration

Enlarged spleen traps and destroys normal RBCs (liver cirrhosis, portal hypertension, storage diseases)

CATEGORY III - Hypoproliferative Anemias (Decreased RBC Production)

Bone marrow is failing to produce adequate RBCs. Reticulocyte count is low.

1. Erythropoietin (EPO) Deficiency

Chronic kidney disease (CKD) - most common cause; reduced EPO synthesis by peritubular fibroblasts
Endocrine deficiencies - hypothyroidism, adrenal insufficiency, androgen deficiency

2. Bone Marrow Failure (Aplastic Anemia)

Acquired aplastic anemia - autoimmune T-cell destruction of stem cells; triggered by drugs (chloramphenicol, NSAIDs), viruses (hepatitis, EBV), radiation
Congenital aplastic anemia - Fanconi anemia (DNA repair defect, telomerase mutations)
Pure red cell aplasia (PRCA) - isolated failure of erythroid precursors; associated with Parvovirus B19 (transient, especially in sickle cell - "aplastic crisis"), thymoma

3. Bone Marrow Infiltration (Myelophthisic Anemia)

Normal marrow replaced by abnormal cells:

Leukemia and lymphoma
Metastatic carcinoma (breast, prostate, lung, thyroid)
Granulomatous disease (TB, sarcoidosis)
Myelofibrosis
Storage diseases (Gaucher's, Niemann-Pick)

4. Anemia of Chronic Disease / Inflammation (ACD)

Chronic infections, autoimmune diseases, malignancy
Mechanism: hepcidin (acute phase reactant) is upregulated by IL-6 → blocks ferroportin → iron sequestered in macrophages → unavailable for erythropoiesis
Usually normocytic; may become microcytic
Ferritin is normal or HIGH (unlike IDA where it is low)

5. Sideroblastic Anemia

Defective heme synthesis; iron accumulates in mitochondria of erythroblasts → ring sideroblasts on Prussian blue stain
Inherited - X-linked (ALAS2 gene mutation)
Acquired - alcohol, lead, isoniazid (B6 antagonist), copper deficiency, MDS

CATEGORY IV - Blood Loss Anemia

Type	Features
Acute hemorrhage	Normocytic initially; reticulocyte count rises after 3-5 days; volume loss with hypotension
Chronic hemorrhage	Iron stores gradually depleted → converts to iron deficiency (microcytic) anemia

Summary Table

Etiology	MCV	Reticulocytes	Key Lab Finding
Iron deficiency	Low	Low	↓ Ferritin, ↓ Fe, ↑ TIBC
Thalassemia	Low	Low/Normal	Normal ferritin, HbA2 ↑ (β-thal)
Sideroblastic	Low/Normal	Low	Ring sideroblasts, ↑ Fe
ACD	Normal/Low	Low	↑ Ferritin, ↓ TIBC, ↑ CRP
B12/Folate deficiency	High	Low	↑ MMA (B12), ↑ homocysteine
Hemolytic anemia	Normal	HIGH	↑ LDH, ↓ haptoglobin, ↑ bilirubin
Aplastic anemia	Normal	Very Low	Pancytopenia, hypocellular marrow
CKD	Normal	Low	↓ EPO
Acute blood loss	Normal	↑ (after 3-5 days)	Clinical bleeding

Sources:

Robbins & Kumar Basic Pathology - Hemolytic Anemia (p. 384)
Frameworks for Internal Medicine - Microcytic, normocytic, macrocytic causes
Harriet Lane Handbook 23e - Causes of Anemia and evaluation indices
Harrison's Principles of Internal Medicine 22E - Anemia Definition and Classification

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