Defects of Blood: Definitions, Classification, and CBC Correlation

What the CBC Measures

1. RBC count (erythrocytes)
2. WBC count (total and differential)
3. Platelet count
4. Hemoglobin (Hb) - direct measurement
5. Red cell indices: MCV, MCH, MCHC, RDW - derived parameters
6. Hematocrit (HCT)
7. WBC differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils)

• Tietz Textbook of Laboratory Medicine, 7th Edition

PART I: DEFECTS OF RED BLOOD CELLS (Erythrocyte Disorders)

A. ANEMIA (Decreased RBCs / Hemoglobin)

1. Microcytic Hypochromic Anemia

• Hb: Low
• MCV: <80 fL (microcytic)
• MCH: <27 pg
• MCHC: Low
• RDW: Elevated (especially in iron deficiency)

• Tietz Textbook of Laboratory Medicine, 7th Ed

• In iron deficiency anemia, the RDW rises early (anisocytosis) before MCV falls - making RDW elevation the earliest CBC clue.
• In thalassemia trait, RBC count is often elevated or normal despite low MCV, which helps distinguish it from IDA where RBC is reduced.
• The MCV swells artifactually after 2-3 days of EDTA storage - samples should not be excessively delayed before analysis.

2. Macrocytic / Megaloblastic Anemia

• Hb: Low
• MCV: >100 fL (macrocytic)
• Hypersegmented neutrophils on smear (>5 lobes in >5% of neutrophils) - classic finding
• WBC and platelets may also be reduced (pancytopenia in severe cases)
• Reticulocytes: Low

3. Normocytic Normochromic Anemia

• Hb: Low
• MCV: 80-100 fL (normal)
• MCH, MCHC: Normal
• RDW: May be elevated in mixed deficiency

4. Key RBC Indices Explained

PART II: DEFECTS OF WHITE BLOOD CELLS (Leukocyte Disorders)

CBC WBC Component: Differential Count

A. Neutrophilia (High Neutrophils on CBC)

• Bacterial infections, tissue damage, corticosteroids, pregnancy
• Toxic granulation (heavy staining of azurophilic granules) and vacuolation on smear suggest active infection or inflammation

B. Neutropenia (Low Neutrophils)

• Absolute neutrophil count (ANC) <1.5 × 10⁹/L
• Causes: Viral infections, chemotherapy, autoimmune, aplastic anemia
• Risk of bacterial infection rises sharply when ANC <0.5 × 10⁹/L

C. Pelger-Huet Anomaly (Morphological Defect)

• Hypolobulation of neutrophil nuclei with increased chromatin clumping
• Inherited form: bilobed "pince-nez" shaped nuclei
• Acquired (pseudo-Pelger): seen in myelodysplastic syndromes (MDS), drugs (tacrolimus, mycophenolate)

D. Dohle Bodies

• Pale blue-gray amorphous inclusions near the neutrophil cell membrane
• Seen in: infection, inflammation
• Similar but larger/more angular inclusions occur in May-Hegglin anomaly (MYH9-related disorder)

E. Leukocytosis (Total WBC >11 × 10⁹/L)

• Bacterial infection (neutrophilic shift), viral infection (lymphocytosis), allergy/parasites (eosinophilia)
• Leukemoid reaction: WBC >50 × 10⁹/L with left shift (bands, metamyelocytes) - mimics leukemia
• Leukemia: Blast cells on differential; confirmed by bone marrow biopsy

F. Leukopenia (Total WBC <4 × 10⁹/L)

• Viral infections (HIV, EBV), chemotherapy, aplastic anemia, B12/folate deficiency
• CBC in aplastic anemia: Pancytopenia (low RBC + low WBC + low platelets) with very low reticulocyte count

PART III: DEFECTS OF PLATELETS (Thrombocyte Disorders)

A. Thrombocytopenia (Low Platelets)

• 50,000-150,000/μL: Mild - usually no spontaneous bleeding but aggravated post-trauma
• 20,000-50,000/μL: Can aggravate post-traumatic bleeding
• <20,000/μL: Risk of spontaneous, non-traumatic bleeding; intracranial hemorrhage is the most feared complication
• PT and PTT remain normal in isolated thrombocytopenia

• Robbins, Cotran & Kumar Pathologic Basis of Disease

• Always confirm an automated count <60 × 10⁹/L with a peripheral blood smear before altering clinical management (never transfuse platelets for unexpected thrombocytopenia without microscopic confirmation)
• EDTA-induced pseudothrombocytopenia: In vitro platelet clumping in EDTA-anticoagulated tubes causes falsely low automated counts - confirmed by redrawing in citrate or heparin, or viewing smear for platelet aggregates
• Platelet satellitism: Platelets surrounding neutrophils - another EDTA artifact
• Fibrin strands on smear suggest coagulation activation and erroneous counts

B. Thrombocytosis (High Platelets >400,000/μL)

• Reactive: Iron deficiency, infection, post-splenectomy, inflammation
• Primary (essential thrombocythemia): Myeloproliferative neoplasm; paradoxically can cause both thrombosis and bleeding

C. Qualitative Platelet Defects

PART IV: COMBINED / MULTI-LINEAGE DEFECTS

Pancytopenia (Low RBC + WBC + Platelets on CBC)

Summary: CBC as a Diagnostic Tool

CBC Result                    → Think First
─────────────────────────────────────────────────────
Low Hb + Low MCV + High RDW  → Iron Deficiency Anemia
Low Hb + Low MCV + Normal RDW → Thalassemia Trait
Low Hb + High MCV             → B12/Folate deficiency
Low Hb + Normal MCV           → Acute loss, hemolysis, renal failure
Pancytopenia + Low Retics     → Aplastic anemia
Pancytopenia + High MCV       → Megaloblastic anemia
High WBC + Blasts             → Acute leukemia (urgent smear/BM biopsy)
Low Platelets + Normal PT/PTT → Primary thrombocytopenia (ITP)
Low Platelets + High PT/PTT   → DIC or TTP

Key Takeaway

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Robbins, Cotran & Kumar Pathologic Basis of Disease
• Goldman-Cecil Medicine, International Edition