Blood indicis in ANEMIA
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Blood Indices in Anemia
Anemia is defined as blood counts (primarily hemoglobin) below normal for a given population. The blood indices - numbers generated by automated cell counters and confirmed on peripheral smear - are the backbone of anemia classification and diagnosis.
1. Core Blood Indices: Definitions and Normal Ranges
| Index | What it Measures | Formula | Normal Range |
|---|---|---|---|
| Hemoglobin (Hb) | Oxygen-carrying capacity | Direct photometric | Men: 13.2-16.6 g/dL; Women: 11.6-15.0 g/dL |
| Hematocrit (Hct) | % of blood that is RBCs | MCV × RBC count | Men: 38-49%; Women: 35-45% |
| RBC Count | Number of red cells | Direct count | Men: 4.4-5.6 ×10⁶/µL; Women: 3.9-5.0 ×10⁶/µL |
| MCV | Average RBC volume | Hct ÷ RBC count × 10 | 78-98 fL |
| MCH | Avg. Hb mass per cell | Hb ÷ RBC | 26-34 pg/cell |
| MCHC | Avg. Hb concentration per packed cell | Hb ÷ Hct | 31-37 g/dL (or 31-37%) |
| RDW | Variation in RBC size (anisocytosis) | Coefficient of variation of MCV | 11.5-14.5% |
| Reticulocyte count | Immature RBCs in circulation | Direct count | 0.5-1.5% (or 25,000-75,000/µL absolute) |
Note: In early anemia, MCV may change before Hb and Hct become abnormal. - Textbook of Family Medicine 9e
2. Morphologic Classification by MCV
The first and most practical step in evaluating anemia is classifying by MCV (pioneered by hematologist Max Wintrobe):
Microcytic Anemia (MCV < 80 fL)
Any process that interferes with hemoglobin synthesis leads to smaller red cells.
| Cause | Key Feature | Ferritin | Serum Iron | TIBC | RDW |
|---|---|---|---|---|---|
| Iron Deficiency | Most common; hypochromic, microcytic cells; target cells, pencil cells | Low (<30 ng/mL) | Low | High (elevated) | High (anisocytosis) |
| Anemia of Chronic Disease | Slightly hypochromic/microcytic; often normocytic | Normal/High | Low | Low/Normal | Normal |
| Thalassemia | Target cells prominent; very low MCV relative to mild anemia | Normal/High | Normal/High | Normal | Normal (uniform microcytosis) |
| Sideroblastic Anemia | Ring sideroblasts in marrow | High | High | Low | High (dimorphic population) |
- Robbins & Kumar Basic Pathology, p. 394
- Harrison's Principles of Internal Medicine 22E
Normocytic Anemia (MCV 80-100 fL)
Broad differential; reticulocyte count is the key to subdivide:
| Cause | Reticulocyte Count | Key Indices / Findings |
|---|---|---|
| Acute blood loss | High (later) | Normal indices initially; Hb/Hct drop after fluid equilibration (12-24 hrs) |
| Hemolytic anemia | High | Elevated indirect bilirubin, elevated LDH, low haptoglobin; schistocytes (microangiopathic) or spherocytes |
| Anemia of chronic disease | Low/Normal | Normal MCV; low serum iron; normal/high ferritin |
| Renal failure | Low | Burr cells (echinocytes); low erythropoietin; elevated creatinine/BUN |
| Aplastic anemia | Low | Pancytopenia; hypocellular bone marrow; no immature cells on smear |
| Myelodysplastic syndrome | Low | Refractory; dysplastic cells; may evolve with macro- or microcytosis |
| Endocrinopathies | Low | Hypothyroidism, Addison's disease |
Macrocytic Anemia (MCV > 100 fL)
Subdivided into oval macrocytes vs. round macrocytes:
Oval macrocytes = DNA synthesis defect:
- Vitamin B12 deficiency - pernicious anemia (anti-intrinsic factor antibodies), gastrectomy, ileal resection, Crohn's; also causes subacute combined degeneration of the cord
- Folate deficiency - poor diet, alcohol, pregnancy, methotrexate
- Chemotherapy / antiseizure medications
- Myelodysplasia
Round macrocytes = membrane defect / other:
- Alcohol use, liver disease, hypothyroidism, reticulocytosis, smoking, dysproteinemia
Megaloblastic anemia hallmark on smear: hypersegmented neutrophils (≥5% with ≥5 lobes, or any neutrophil with ≥6 lobes). MCV often >110-115 fL in severe cases. - Henry's Clinical Diagnosis and Management by Laboratory Methods
3. Mechanism-Based Classification (Reticulocyte-Driven)
The absolute reticulocyte count distinguishes the two broad mechanisms:
| Reticulocyte Count | Mechanism | Interpretation |
|---|---|---|
| Elevated (>100,000/µL) | Hyperproliferative | Bone marrow responding - blood loss or hemolysis |
| Low/Normal | Hypoproliferative | Bone marrow not responding adequately - nutritional, aplastic, infiltrative, or renal causes |
4. Specific Diagnostic Indices by Anemia Type
Iron Deficiency Anemia
- MCV low, MCH low, MCHC low (hypochromic)
- Serum ferritin < 30 ng/mL (most sensitive early marker)
- Serum iron low, TIBC high, transferrin saturation low
- RDW elevated (anisocytosis)
- Peripheral smear: microcytic, hypochromic cells, pencil/elliptocytes, target cells
- Platelet count often elevated
- Erythropoietin elevated but response blunted
- Robbins & Kumar Basic Pathology, p. 393-394
Anemia of Chronic Inflammation
- MCV normal or slightly low
- Serum iron low, TIBC low (distinguishes from iron deficiency where TIBC is high)
- Ferritin normal or high (acute-phase reactant)
- Hepcidin elevated (blocks ferroportin, traps iron in macrophages)
- Storage iron in bone marrow increased (opposite of IDA)
Megaloblastic Anemia (B12 / Folate Deficiency)
- MCV markedly elevated (often >110 fL)
- Hypersegmented neutrophils on smear
- Pancytopenia in severe cases
- Elevated homocysteine (both B12 and folate deficiency)
- Elevated methylmalonic acid (B12 deficiency only - useful distinguisher)
- Serum B12 < 200 pg/mL (though 200-400 may still be symptomatic)
- RBC folate more reliable than serum folate for tissue stores
Hemolytic Anemia
- Reticulocyte count high
- Indirect (unconjugated) bilirubin elevated
- LDH elevated
- Serum haptoglobin low (consumed by free Hb)
- Peripheral smear: schistocytes (microangiopathic), spherocytes (autoimmune/hereditary spherocytosis), sickle cells, etc.
- Coombs test positive in autoimmune hemolytic anemia
Aplastic Anemia
- Normocytic, normochromic
- Pancytopenia (all three cell lines low)
- Reticulocyte count very low
- No immature cells on peripheral film
- Serum iron may be elevated (lack of erythropoiesis)
- Bone marrow biopsy: severely hypoplastic/aplastic with fat replacement
Renal Failure Anemia
- Normocytic, normochromic
- Low reticulocyte count
- Low erythropoietin
- WBC and platelet counts usually normal (unlike aplastic)
- Peripheral smear: burr cells (echinocytes)
5. Quick Reference Summary Table
| Anemia Type | MCV | MCHC | Reticulocytes | Ferritin | Other Key Test |
|---|---|---|---|---|---|
| Iron deficiency | Low | Low | Low | Low | TIBC high |
| Anemia of chronic disease | Normal/Low | Normal/Low | Low | Normal/High | TIBC low |
| Thalassemia | Low | Normal/Low | Normal/High | Normal/High | Hb electrophoresis |
| B12/Folate deficiency | High | Normal | Low | Normal | B12, folate, MMA, homocysteine |
| Hemolytic anemia | Normal/High | Normal | High | Normal | Bilirubin, LDH, haptoglobin, Coombs |
| Aplastic anemia | Normal | Normal | Very low | Normal/High | Bone marrow biopsy (hypocellular) |
| Acute blood loss | Normal | Normal | Low → High | Normal | Clinical history; Hct falls after 12-24h |
| Renal failure | Normal | Normal | Low | Normal | Creatinine, EPO level |
| Sideroblastic | Low/Normal | Low | Low/Normal | High | Ring sideroblasts on marrow |
Sources:
- Robbins & Kumar Basic Pathology (Robbins Pathology), pp. 383-396
- Harrison's Principles of Internal Medicine 22E (2025), pp. 491-492
- Textbook of Family Medicine 9e, pp. 227-228
- Henry's Clinical Diagnosis and Management by Laboratory Methods, pp. 130-131
- Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 661+
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