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Now I have rich content from Harrison's and Rosen's Emergency Medicine. Let me compile the comprehensive CBC guide.

Complete Blood Count (CBC) - Complete Guide

What is a CBC?

A CBC (Complete Blood Count) is a panel of laboratory tests that measures the cellular components of blood. It is performed using automated electronic cell counters that directly measure red cell size (MCV), number of red cells, and hemoglobin levels, with additional values derived from these measurements. It is the most commonly ordered hematologic test worldwide.

Harrison's Principles of Internal Medicine 22E, p. 488

ANATOMY OF BLOOD - The Components Measured

Blood is a specialized connective tissue consisting of cellular elements suspended in plasma. The CBC evaluates three major cell lines, all derived from pluripotent hematopoietic stem cells in the bone marrow:

Cell Line	Origin	Normal Lifespan
Red Blood Cells (Erythrocytes)	Bone marrow (erythroid lineage)	~120 days
White Blood Cells (Leukocytes)	Bone marrow (myeloid/lymphoid lineage)	Hours to years
Platelets (Thrombocytes)	Bone marrow megakaryocytes	7-10 days

Bone marrow anatomy: Normal marrow has ~40-50% nucleated cellular elements and ~50-60% fat. The ratio changes in disease (erythroid hyperplasia in hemolytic anemia, myeloid hyperplasia in infection/leukemia).

PART 1: RED BLOOD CELLS (RBCs) / ERYTHROCYTES

Physiology

Biconcave disc, ~7-8 µm diameter, no nucleus, no mitochondria
Primary function: oxygen transport via hemoglobin (Hb)
Hemoglobin = 4 globin chains + 4 heme groups; each heme binds 1 O₂
Adult hemoglobin (HbA) = 2 alpha + 2 beta chains (α₂β₂)
RBCs use anaerobic glycolysis (Embden-Meyerhof pathway) for energy
2,3-DPG regulates oxygen affinity: higher 2,3-DPG = lower O₂ affinity = better tissue O₂ delivery

Compensatory mechanisms in anemia:

Increased heart rate and cardiac output (immediate)
Increased 2,3-DPG (hours to days)
Increased plasma volume (weeks)

Normal CBC Values - RBC Parameters

Parameter	Formula	Normal Values
Hemoglobin (Hgb)	Direct measurement	Men: 13.5-17.5 g/dL; Women: 12.0-16.0 g/dL
Hematocrit (Hct)	RBC count × MCV ÷ 10	Men: 41-53%; Women: 36-46%
RBC Count	Direct count	Men: 4.5-5.9 × 10⁶/µL; Women: 4.0-5.2 × 10⁶/µL
MCV	Hct ÷ RBC count × 10	85-95 fL
MCH	Hgb ÷ RBC count × 10	28.5-32.3 pg
MCHC	Hgb ÷ Hct × 100	33.8-34.2 g/dL
RDW	SD of MCV/Mean MCV × 100	11.5-14.5%

Harrison's 22E (Table 66-1), p. 488; Rosen's Emergency Medicine (Table 109.2)

Note: MCHC changes very little in most anemias and is of limited value. MCH varies linearly with MCV and also adds limited independent value. MCV is the most clinically useful index.

PART 2: THE PERIPHERAL BLOOD SMEAR

The blood smear allows direct visualization of blood cells and complements the CBC indices:

Normal peripheral blood smear showing uniform biconcave RBCs with central pallor and small platelets

Normal blood smear: uniform RBCs with central pallor, scattered platelets (small purple dots)

Key smear findings:

Finding	Description	Significance
Anisocytosis	Variation in cell size	Correlates with elevated RDW
Poikilocytosis	Variation in cell shape	Defective maturation or fragmentation
Polychromasia	Large, grayish-blue cells	Premature reticulocytes (EPO stimulation)
Hypochromia	Enlarged area of central pallor	Iron deficiency, thalassemia
Teardrop cells (dacrocytes)	Teardrop shape	Marrow disruption, severe iron deficiency
Schistocytes	Fragmented cells	Microangiopathic hemolytic anemia (TTP, HUS, DIC)
Sickle cells	Crescent shape	Sickle cell disease
Target cells	Bull's-eye appearance	Thalassemia, liver disease, HbC
Spherocytes	Small, round, no central pallor	Hereditary spherocytosis, autoimmune hemolytic anemia

PART 3: RETICULOCYTE COUNT

Reticulocytes contain mRNA for ~24h after marrow release; detected by new methylene blue stain
Measure of red cell production
Separates hyperproliferative (hemolysis/blood loss) from hypoproliferative (production defect) anemia

Corrected reticulocyte count = Measured retic count × (Patient Hct / 45%)

Absolute reticulocyte count = (Reticulocyte % / 100) × RBC count

Normal absolute count: 25,000-75,000 cells/µL
No hematocrit correction needed for absolute count
Harrison's 22E, p. 490-491

PART 4: WHITE BLOOD CELLS (WBCs) / LEUKOCYTES

Normal WBC Parameters

Parameter	Normal Range
Total WBC count	4,500-11,000 /µL
Neutrophils	50-70% (absolute: 1,800-7,700/µL)
Lymphocytes	20-40% (absolute: 1,000-4,800/µL)
Monocytes	2-8% (absolute: 200-900/µL)
Eosinophils	1-4% (absolute: 100-400/µL)
Basophils	0.5-1% (absolute: 0-200/µL)

WBC Differential - Functions

Cell	Key Function
Neutrophils	First responders to bacterial/fungal infection; phagocytosis
Lymphocytes	Adaptive immunity - B cells (antibodies), T cells (cell-mediated)
Monocytes	Phagocytosis; become macrophages in tissues
Eosinophils	Parasitic infections; allergic/hypersensitivity reactions
Basophils	Allergic responses; IgE receptors; release histamine

Pathological WBC Changes

Abnormality	Definition	Causes
Leukocytosis	WBC > 11,000/µL	Infection, inflammation, steroids, leukemia
Leukopenia	WBC < 4,500/µL	Viral infections, aplastic anemia, chemotherapy, HIV
Neutrophilia	Neutrophils > 7,700/µL	Bacterial infection, stress, steroids, MI
Neutropenia	ANC < 1,500/µL	Chemotherapy, aplasia, B12/folate deficiency
Lymphocytosis	Lymphocytes > 4,800/µL	Viral infections (EBV, CMV), CLL, pertussis
Lymphopenia	Lymphocytes < 1,000/µL	HIV/AIDS, immunosuppression, SLE
Eosinophilia	Eosinophils > 400/µL	Parasites, asthma, allergies, Hodgkin lymphoma
Basophilia	Basophils > 200/µL	CML, allergic states, hypothyroidism
Monocytosis	Monocytes > 900/µL	TB, subacute bacterial endocarditis, inflammatory disorders
Left shift	Bands/immature neutrophils in blood	Severe bacterial infection, sepsis

Note: Neonates and children have a lower total WBC and neutrophil count with a higher percentage of lymphocytes compared to adults.

PART 5: PLATELETS

Normal Values and Physiology

Normal platelet count: 150,000-400,000 /µL
Produced by megakaryocytes in the bone marrow
Lifespan: 7-10 days; destroyed in spleen
Function: primary hemostasis (platelet plug formation)
- Adhesion via GPIb-von Willebrand factor
- Activation (shape change, granule release)
- Aggregation via GPIIb/IIIa-fibrinogen

Pathological Platelet Changes

Abnormality	Definition	Causes
Thrombocytopenia	Platelets < 150,000/µL	ITP, TTP, HUS, DIC, heparin-induced (HIT), hypersplenism, aplasia
Thrombocytosis	Platelets > 400,000/µL	Reactive (infection, iron deficiency, post-splenectomy) or primary (ET, CML)

Bleeding risk by platelet count:

< 100,000: surgical bleeding risk
< 50,000: spontaneous bleeding risk
< 10,000-20,000: risk of spontaneous intracranial hemorrhage

PART 6: CLASSIFICATION OF ANEMIA (CBC-Based)

The MCV is the cornerstone of CBC-based anemia classification:

A. Microcytic Anemia (MCV < 80 fL)

Cause	Additional CBC Clues	Key Test
Iron deficiency anemia	Low MCH, high RDW, thrombocytosis	Serum ferritin (low), TIBC (high)
Thalassemia	Low MCV but high/normal RBC count, low RDW	Hb electrophoresis
Anemia of chronic disease	Normal/low RDW	Serum ferritin (normal/high)
Lead poisoning	Basophilic stippling on smear	Blood lead level
Sideroblastic anemia	Ring sideroblasts on bone marrow	Bone marrow biopsy

Thalassemia pathophysiology: Autosomal recessive disorder with decreased globin chain synthesis. Beta-thalassemia has reduced/absent beta-globin -> excess alpha-chains -> reactive oxygen species -> erythroid apoptosis -> hemolytic anemia. Alpha-thalassemia has excess beta-chains forming HbH.

B. Normocytic Anemia (MCV 80-100 fL)

Cause	Key Features
Anemia of chronic disease	Most common; normocytic or microcytic
Aplastic anemia	Pancytopenia (low RBC + WBC + platelets)
Acute blood loss	Low Hgb, normal MCV initially
Hemolytic anemia	Elevated reticulocyte count, elevated LDH, low haptoglobin
Renal failure	Low EPO, decreased production
Hypothyroidism	May be normocytic or macrocytic

C. Macrocytic Anemia (MCV > 100 fL)

Cause	Smear Finding	Key Test
B12 deficiency	Hypersegmented neutrophils, oval macrocytes	Low serum B12, elevated methylmalonic acid
Folate deficiency	Hypersegmented neutrophils, oval macrocytes	Low serum folate
Liver disease	Target cells, acanthocytes	LFTs
Alcohol	Macrocytes without hypersegmentation	History
Myelodysplastic syndrome	Dysplastic cells	Bone marrow biopsy
Hydroxyurea/Methotrexate	Drug-induced	Medication history

PART 7: WBC DISEASE STATES

Leukemia (Malignant WBC Proliferation)

Type	CBC Features
AML (Acute Myeloid)	High WBC with blasts (>20%), Auer rods on smear
ALL (Acute Lymphoblastic)	High WBC with lymphoblasts; most common childhood leukemia
CML (Chronic Myeloid)	Very high WBC (50,000-200,000), left shift, basophilia, thrombocytosis
CLL (Chronic Lymphocytic)	High WBC with mature lymphocytes (>5,000); smudge cells on smear

Reactive WBC Changes

Infectious mononucleosis (EBV): Lymphocytosis with atypical lymphocytes
Sepsis: Leukocytosis with left shift (bands/metamyelocytes); may have leukemoid reaction (WBC > 50,000)
Viral infections: Leukopenia with relative lymphocytosis
Allergic reactions/parasites: Eosinophilia

PART 8: PLATELET DISEASE STATES

Thrombocytopenia

ITP (Immune Thrombocytopenic Purpura):

Autoimmune destruction via anti-platelet IgG antibodies
Isolated thrombocytopenia; normal WBC, Hgb
Treatment: steroids, IVIG, anti-D, thrombopoietin receptor agonists, splenectomy

TTP (Thrombotic Thrombocytopenic Purpura):

ADAMTS13 deficiency -> uncleaved vWF multimers -> platelet microthrombi
Pentad: thrombocytopenia, MAHA, fever, renal failure, neurological symptoms
Treatment: plasma exchange (PEX)

HIT (Heparin-Induced Thrombocytopenia):

Platelet drop >50% after heparin exposure (days 5-10)
Paradoxically prothrombotic (not bleeding)
Stop heparin immediately; use alternative anticoagulant (argatroban, bivalirudin)

PART 9: PHARMACOLOGY - DRUGS AFFECTING THE CBC

Drugs That Cause Anemia

Drug	Mechanism	CBC Pattern
Methotrexate, hydroxyurea, 5-FU	Folate antagonism/DNA synthesis inhibition	Macrocytic anemia
Chloramphenicol	Bone marrow suppression	Aplastic anemia, pancytopenia
NSAIDs	GI blood loss	Microcytic (iron deficiency)
Dapsone, primaquine	Oxidative hemolysis in G6PD deficiency	Hemolytic anemia
Methyldopa, penicillin	Drug-induced autoimmune hemolysis	AIHA - normocytic
Ribavirin	Direct hemolysis	Hemolytic anemia

Drugs That Cause Thrombocytopenia

Drug	Mechanism
Heparin	HIT (immune-mediated platelet activation)
Quinine/quinidine	Drug-dependent antibodies
Chemotherapy agents	Bone marrow suppression
Valproic acid	Dose-dependent thrombocytopenia
Linezolid	Myelosuppression
TMP-SMX	Immune-mediated

Drugs That Cause Leukopenia/Agranulocytosis

Drug	Notes
Clozapine	Requires mandatory WBC monitoring weekly for 6 months
Methimazole/carbimazole	Monitor in hyperthyroid patients
Carbamazepine, phenytoin	Rare but serious
Chemotherapy (any)	Expected dose-dependent
Colchicine (overdose)	Bone marrow suppression

Drugs That Treat CBC Abnormalities

Drug	Use	Mechanism
Erythropoietin (EPO, darbepoetin)	Anemia of CKD	Stimulates erythropoiesis
Ferrous sulfate 325mg TID	Iron deficiency anemia	Iron replacement
B12 (cyanocobalamin)	B12 deficiency anemia	Cofactor for DNA synthesis
Folic acid 1-5mg/day	Folate deficiency	DNA synthesis
G-CSF (filgrastim)	Chemotherapy-induced neutropenia	Stimulates neutrophil production
Eltrombopag, romiplostim	ITP, low platelets	Thrombopoietin receptor agonists
IVIG	ITP	Blocks Fc receptor-mediated platelet destruction
Prednisone	ITP, AIHA	Immunosuppression
Hydroxyurea	Sickle cell disease	Increases HbF production

PART 10: COMPLICATIONS OF CBC ABNORMALITIES

Complications of Anemia

Severity	Complication
Mild (Hgb 10-12)	Fatigue, dyspnea on exertion, tachycardia
Moderate (Hgb 8-10)	High-output cardiac failure, pallor, impaired cognition
Severe (Hgb < 8)	Cardiac failure, myocardial ischemia, shock
Chronic severe	Tissue hypoxia, organ dysfunction, impaired wound healing
Pregnancy	Preterm birth, low birth weight, maternal mortality

Compensatory failure: When plasma volume expansion overwhelms cardiac capacity, edema and heart failure develop.

Complications of Leukopenia/Neutropenia

ANC Level	Risk
ANC < 1,000/µL	Increased infection risk
ANC < 500/µL	Severe risk (fever = oncologic emergency)
ANC < 100/µL	Life-threatening infections (pseudomonas, fungal, gram-negative sepsis)

Febrile neutropenia: Temperature > 38.3°C with ANC < 500 -> empirical broad-spectrum antibiotics (piperacillin-tazobactam or cefepime) immediately.

Complications of Thrombocytopenia

Platelet 50,000-100,000: Post-surgical/traumatic bleeding
Platelet 10,000-50,000: Spontaneous mucosal bleeding (petechiae, purpura, epistaxis, GI bleeding)
Platelet < 10,000: Spontaneous intracranial hemorrhage risk

Complications of Thrombocytosis

Platelet > 1,000,000 (extreme): Paradoxical bleeding (acquired vWF deficiency)
Primary thrombocytosis (ET, CML): Arterial/venous thrombosis, stroke, MI, Budd-Chiari syndrome

SUMMARY TABLE - CBC Normal Reference Ranges

Parameter	Normal Range
Hemoglobin (Men)	13.5-17.5 g/dL
Hemoglobin (Women)	12.0-16.0 g/dL
Hematocrit (Men)	41-53%
Hematocrit (Women)	36-46%
RBC (Men)	4.5-5.9 × 10⁶/µL
RBC (Women)	4.0-5.2 × 10⁶/µL
MCV	85-95 fL
MCH	28.5-32.3 pg
MCHC	33.8-34.2 g/dL
RDW	11.5-14.5%
WBC	4,500-11,000/µL
Neutrophils	50-70% (ANC 1,800-7,700)
Lymphocytes	20-40% (1,000-4,800/µL)
Monocytes	2-8%
Eosinophils	1-4%
Basophils	0.5-1%
Platelets	150,000-400,000/µL
Reticulocytes	0.5-1.5% (absolute: 25,000-75,000/µL)

Sources:

Harrison's Principles of Internal Medicine 22E (2025), Chapter 66 - Anemia, pp. 488-491
Rosen's Emergency Medicine - Chapter 109, Anemia
Washington Manual of Medical Therapeutics - Hematology

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