Define anemia ,, morphological classification
Reading File
Finding Sources
Finding Sources
Finding Sources
Finding Sources
Reading File
Finding Sources
Reading File
Reading File
Reading File
Reading File
Reading File
Reading File
Loading Image
Reading File
Loading Image
Here is a complete, textbook-sourced answer on the definition and morphological classification of anemia.
Anemia: Definition and Morphological Classification
Definition
Anemia is defined as a significant reduction in the volume of erythrocytes (red blood cells) in the body, resulting in decreased oxygen-carrying capacity of the blood. The red blood cell is an anucleate biconcave disc (~8 µm in diameter) whose major component is hemoglobin - a molecule that delivers O₂ from the lungs to peripheral tissues and removes CO₂.
Because direct measurement of red cell mass is complex, surrogate measures are used clinically:
- Hemoglobin concentration (most preferred - directly measured)
- Hematocrit (packed cell volume - calculated in modern analyzers)
- RBC count per µL (rarely used alone to define anemia)
WHO Diagnostic Thresholds for Anemia
| Population | Hemoglobin (g/dL) |
|---|---|
| Adult men | < 13.0 |
| Adult women (non-pregnant) | < 12.0 |
| Pregnant women | < 11.0 |
| Children 6 months - 5 years | < 11.0 |
- Goldman-Cecil Medicine, p. 1666
- Brenner and Rector's The Kidney, p. 3199
Morphological Classification (by Red Cell Size / MCV)
This system, pioneered by hematologist Max Wintrobe, classifies anemia by the Mean Corpuscular Volume (MCV) - the average size of a red blood cell measured in femtoliters (fL). It is the most practical first step in evaluating anemia and guides the diagnostic workup.
1. Microcytic Anemia (MCV < 80 fL)
Due to any process that interferes with hemoglobin synthesis - less hemoglobin = smaller red cells. The red cells are also typically hypochromic (increased central pallor due to reduced hemoglobin content).
Causes (mnemonic: TAILS or SLIT):
| Cause | Mechanism |
|---|---|
| Iron deficiency | Cannot synthesize heme (most common cause worldwide) |
| Thalassemia | Defective globin chain synthesis (alpha or beta) |
| Anemia of chronic disease/inflammation | Impaired iron reutilization due to cytokine-mediated hepcidin upregulation (often normocytic; MCV rarely < 75 fL) |
| Sideroblastic anemia | Defective heme synthesis; ring sideroblasts in marrow |
| Lead poisoning | Suppresses ferrochelatase, interfering with porphyrin ring synthesis |
Note: The lowest MCVs are seen in hemoglobinopathies and thalassemias. A normal RDW in microcytic anemia favors thalassemia over iron deficiency.
2. Normocytic Anemia (MCV 80-100 fL)
The MCV is within the normal range. This is a broad category and includes all causes of anemia not due to a defect in hemoglobin synthesis or DNA synthesis.
Causes:
| Cause | Notes |
|---|---|
| Aplastic anemia | Bone marrow failure |
| Anemia of chronic disease | Most common presentation is normocytic |
| Renal disease | Decreased erythropoietin production |
| Hemolytic anemia | Increased RBC destruction (may show reticulocytosis) |
| Acute blood loss | Early hemorrhage before iron stores deplete |
| Endocrinopathies | Hypothyroidism (also can be macrocytic), Addison's disease |
| Marrow infiltration / myeloma | Replacement of normal marrow |
| Pure red cell aplasia | Selective suppression of erythroid precursors |
3. Macrocytic Anemia (MCV > 100 fL)
Due to defects in DNA synthesis (megaloblastic) or membrane/lipid defects (non-megaloblastic). Macrocytic anemias are further divided by the shape of macrocytes:
A. Megaloblastic (Oval macrocytes + hypersegmented neutrophils)
- Defective DNA synthesis → impaired nuclear maturation with normal cytoplasmic development
- Vitamin B12 (cobalamin) deficiency
- Folate deficiency
- Medications - chemotherapy agents (methotrexate, hydroxyurea), some antiseizure drugs
- Myelodysplasia
B. Non-megaloblastic (Round macrocytes)
- Due to changes in red cell membrane lipids; no hypersegmented neutrophils
- Liver disease
- Alcohol use (direct toxic effect + liver disease)
- Hypothyroidism
- Dysproteinemia
- Reticulocytosis (reticulocytes are larger than mature RBCs)
- Hypoxia, Smoking
Summary Table
| Type | MCV | Key Mechanism | Key Causes |
|---|---|---|---|
| Microcytic | < 80 fL | Impaired Hb synthesis | Iron deficiency, Thalassemia, Sideroblastic, Lead, ACD |
| Normocytic | 80-100 fL | Varied | Aplastic, ACD, Renal, Hemolysis, Acute blood loss |
| Macrocytic | > 100 fL | DNA synthesis defect or membrane lipid change | B12/Folate deficiency, Liver disease, Alcohol, MDS |
Additional Indices Used in Morphological Classification
- MCH (Mean Corpuscular Hemoglobin) - pg/cell - Low MCH = hypochromia (impaired Hb synthesis)
- MCHC (Mean Corpuscular Hemoglobin Concentration) - g/dL - Low MCHC = hypochromia
- RDW (Red Cell Distribution Width) - measures anisocytosis (variation in RBC size); elevated in iron deficiency, normal in thalassemia trait
- MCV is calculated as: MCV (fL) = [Hct (%) × 10] / [RBC count (10⁶/µL)]
Sources: Harrison's Principles of Internal Medicine 22E, p. 491-492; Goldman-Cecil Medicine, p. 1673-1674; Frameworks for Internal Medicine, p. 330-331
This is a shared conversation. Sign in to Orris to start your own chat.