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Harrison's Hematology - Section 9 Master Guide

Chapters 65, 66, 67, 68, 69 & Chapter 70 (Lymph Node/Spleen)

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CHAPTER 65 - Interpreting Peripheral Blood Smears

The Skill Every Internist Must Master

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PLATELETS (examine first)

• Count 5-6 fields under oil immersion (×100), average per field, multiply by 20,000 = rough platelet count
• Normal: 1-2 μm diameter, blue granulated appearance; ~1 platelet per 20 red cells
• Large platelets = rapid turnover (young platelets are bigger); OR rare inherited syndromes
• Low count with no large platelets = marrow production problem
• Platelet clumping = EDTA-related artifact causing falsely low automated count
• Neutrophil fragmentation = falsely elevated automated platelet count
• Absent platelet granules = gray platelet syndrome (rare congenital) or marrow disease
• Elevated count = myeloproliferative disorder OR reactive (systemic inflammation)

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RED CELLS (examine second)

• Iron deficiency: RDW elevated (mixed-size cells, early iron-deficient cells are small)
• Thalassemia: RDW usually normal (uniformly small cells)

Specific RBC Morphology

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WHITE CELLS (examine third)

• Normal differential: Neutrophils 50-70%, Lymphocytes 20-40%, Monocytes 2-8%
• Left shift = increased band forms (immature neutrophils) - sign of acute infection/inflammation
• Hypersegmented neutrophils (>5 lobes) = B12/folate deficiency
• Toxic granulation + Döhle bodies = severe bacterial infection, sepsis
• Pelger-Hüet anomaly = bilobed neutrophils; genetic (LBR mutation) or acquired (MDS)
• Auer rods = pathognomonic for AML (myeloid blasts)
• Atypical lymphocytes = EBV (infectious mononucleosis), CMV, viral infections

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CHAPTER 66 - Anemia and Polycythemia

Epidemiology

Normal Red Cell Production

• Hematopoietic stem cell → regulated by transcription factors GATA-1 and FOG-1 for erythroid/megakaryocytic lineage
• EPO (erythropoietin) = primary regulatory hormone from kidney; absence → apoptosis of erythroid progenitors
• First morphologically recognizable erythroid precursor = pronormoblast → 4-5 divisions → 16-32 mature RBCs
• Normal RBC: 8 μm, anucleate, biconcave disc; survives 100-120 days
• Daily replacement: 0.8-1% of all circulating RBCs

Classification of Anemia (3 mechanisms)

1. Hypoproliferative Anemia (most common - 75% of all anemias)

• Inadequate EPO production OR marrow unable to respond
• Causes: Iron deficiency, inflammation (anemia of chronic disease), renal disease (low EPO), marrow suppression

2. Maturation Defect Anemia

• Nuclear maturation defect (DNA synthesis impaired): B12/folate deficiency → megaloblastic anemia, macrocytosis, hypersegmented neutrophils
• Cytoplasmic maturation defect (hemoglobin synthesis impaired): Iron deficiency, thalassemia → microcytic, hypochromic

3. Blood Loss / Hemolysis

• Blood loss or accelerated destruction → reticulocytosis (marrow responding)
• Intravascular hemolysis: hemoglobinuria, low haptoglobin, elevated LDH
• Extravascular hemolysis: spleen destroys RBCs, elevated indirect bilirubin

Key Laboratory Tests

Specific Causes

• Most common nutritional deficiency worldwide
• Most common in premenopausal women (menstrual losses)
• Classic finding: microcytic, hypochromic anemia
• Early iron deficiency may not show microcytosis
• Low ferritin + low transferrin saturation

• Required for RBC DNA synthesis → deficiency = impaired DNA synthesis
• Classic finding: macrocytosis + hypersegmented neutrophils + remarkably hypercellular marrow with large abnormal precursors
• Neurologic manifestations (subacute combined degeneration)

• Same hematologic picture as B12 (grouped as megaloblastic anemia)
• No neurological manifestations
• Rare in countries supplementing flour with folate
• See in poor diet, severe malabsorption

• Associated with neurologic disease AND neutropenia

• Severe anemia + clinical findings of scurvy

• Infections (histoplasmosis, TB) or neoplasms (prostate cancer metastases, multiple myeloma) crowd out hematopoietic elements
• Blood smear: nucleated RBCs + tear-drop cells + immature WBCs = leukoerythroblastic picture
• Mandates bone marrow biopsy

• Kidney = primary source of EPO → renal disease → reduced EPO → hypoproliferative anemia
• Normocytic, normochromic; low reticulocyte count

Polycythemia

• Relative polycythemia = decreased plasma volume (dehydration, "stress polycythemia")
• Absolute polycythemia = increased RBC mass
  • Primary: Polycythemia vera (JAK2 V617F mutation; low EPO)
  • Secondary: High EPO - appropriate (altitude, COPD, sleep apnea) or inappropriate (renal cell carcinoma, hepatocellular carcinoma producing EPO)

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CHAPTER 67 - Disorders of Granulocytes and Monocytes

Granulopoiesis

• Myeloid stem cell → committed progenitor → myeloblast → promyelocyte → myelocyte → metamyelocyte → band → neutrophil
• G-CSF (granulocyte colony-stimulating factor) is the key cytokine
• Bone marrow storage pool = 10-15 times the circulating neutrophil mass; released in infection/stress

Neutrophil Function

1. Margination - neutrophils loosely adhere to vascular endothelium (selectins)
2. Rolling - along endothelium via selectins
3. Adhesion - firm adhesion via integrins (LFA-1/Mac-1 binding ICAM-1)
4. Diapedesis - migration through vessel wall
5. Chemotaxis - movement toward bacteria (fMLF, C5a, IL-8, leukotriene B4)
6. Phagocytosis - engulf opsonized bacteria (IgG + complement)
7. Killing - oxidative burst (NADPH oxidase/NOX2 → superoxide → H2O2 → myeloperoxidase + Cl⁻ → hypochlorous acid/bleach); non-oxidative (defensins, elastase, lysozyme, lactoferrin)

Neutropenia

• Mild: 1000-1800/μL
• Moderate: 500-1000/μL
• Severe: <500/μL (risk of serious infection rises sharply)
• Profound: <100/μL (Kostmann's syndrome level)

Hereditary Neutropenias

Neutrophilia

• Infection (most important acute cause) - increased production + marrow release
• Glucocorticoids - mobilize marginated pool from blood vessel walls
• Epinephrine/exercise/stress - demargination (doubles count in minutes)
• Cigarette smoking - mild chronic elevation
• Chronic inflammation
• Myeloproliferative diseases

Neutrophil Function Defects

Chronic Granulomatous Disease (CGD)

• Defect in NADPH oxidase (NOX2) → cannot generate oxidative burst
• X-linked (most common) or autosomal recessive
• Recurrent infections with catalase-positive organisms: Staph aureus, Aspergillus, Serratia, Nocardia, Burkholderia
• Granuloma formation (try to wall off what they can't kill)
• Diagnosis: DHR (dihydrorhodamine) flow cytometry or nitroblue tetrazolium (NBT) test

Leukocyte Adhesion Deficiency (LAD)

• Type I: CD18 (beta-2 integrin) deficiency → neutrophils can't adhere/migrate
• Hallmark: Extremely high WBC count with no pus despite severe infection; delayed umbilical cord separation
• Diagnosis: flow cytometry for CD11b/CD18

Chediak-Higashi Syndrome

• Defect in LYST (lysosomal trafficking regulator) gene
• Giant lysosomes in neutrophils, monocytes, lymphocytes on smear
• Partial albinism + recurrent infections + peripheral neuropathy
• Accelerated phase = HLH-like picture

Myeloperoxidase (MPO) Deficiency

• Most common inherited neutrophil disorder
• Usually asymptomatic; mild susceptibility to Candida especially with diabetes

Eosinophilia and Basophilia

• Allergic diseases (asthma, hay fever, eczema)
• Drug reactions
• Parasitic infections (especially tissue-invasive: Toxocara, Strongyloides, Ascaris)
• Hypereosinophilic syndrome (>1500/μL for >6 months with organ damage)
• Malignancy (Hodgkin lymphoma, T-cell lymphoma)
• Adrenal insufficiency (Addison's - classic boards point)

• Myeloproliferative disorders (especially CML - basophilia is a hallmark)
• Allergic reactions, hypothyroidism

Monocytes and Macrophages

• Monocytes circulate 1-3 days then enter tissues → become macrophages
• Monocytosis: TB, fungal infections, monocytic leukemia, inflammatory bowel disease, recovery from neutropenia
• Macrophages are central to granuloma formation (TB, sarcoidosis, Crohn's)

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CHAPTER 68 - Primary and Secondary Hemophagocytic Lymphohistiocytosis (HLH)

What is HLH?

• Primary HLH: Mostly children; Mendelian (genetic)
• Secondary HLH: Far more common in adults; triggered by infections, malignancies, autoimmune diseases

Primary HLH - Genetic Forms

Secondary HLH Triggers

• Infection-associated: EBV (most common viral trigger), CMV, HIV, bacteria, parasites, fungi
• Malignancy-associated: T-cell lymphoma, NK-cell lymphoma (most common), B-cell lymphoma
• Autoimmune-associated (MAS): SoJIA, adult-onset Still's disease, SLE, vasculitis
• Transplant-associated: Post-HSCT, post-CAR-T cell therapy

HScore - Diagnostic Scoring

HLH-2004 Diagnostic Criteria (≥5/8 required)

1. Fever ≥38.5°C
2. Splenomegaly
3. Cytopenias in ≥2 cell lines (Hb <90 g/L; platelets <100×10⁹/L; neutrophils <1×10⁹/L)
4. Hypertriglyceridemia (fasting triglycerides ≥3 mmol/L) AND/OR hypofibrinogenemia (fibrinogen ≤1.5 g/L)
5. Hemophagocytosis in bone marrow, spleen, or lymph nodes
6. Low/absent NK-cell activity
7. Ferritin ≥500 μg/L (very high ferritin, often >10,000 is highly suggestive)
8. Elevated soluble CD25 (sIL-2 receptor) ≥2400 U/mL

Treatment

• Primary HLH: HLH-94/HLH-2004 protocol (dexamethasone + etoposide); followed by allogeneic HSCT (only cure)
• MAS-HLH: Treat underlying autoimmune disease + cyclosporine + anakinra (IL-1 blockade)
• Secondary HLH (infection-associated): Treat underlying trigger + immunosuppression if needed

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CHAPTER 69 - Bleeding and Thrombosis

Clinical Approach to Bleeding Disorders

1. Spontaneous vs. trauma/surgery-induced bleeding?
2. Mucosal bleeding (epistaxis, heavy periods, gum bleeding) = platelet/VWD disorder (primary hemostasis)
3. Hemarthroses (joint bleeds) = clotting factor deficiency (factor VIII, IX) = secondary hemostasis
4. Family history (inherited vs acquired)
5. Medications (aspirin, NSAIDs, anticoagulants, supplements)

Primary Hemostasis (Platelet Plug Formation)

Disorders of Primary Hemostasis (Platelet Plug)

• Von Willebrand Disease (VWD) - most common inherited bleeding disorder; VWF deficiency/dysfunction
• Bernard-Soulier syndrome - absent/dysfunctional GPIb-IX-V; giant platelets + thrombocytopenia

• Glanzmann's thrombasthenia - absent/dysfunctional GPIIb/IIIa; normal platelet count but no aggregation
• Afibrinogenemia - no fibrinogen to bridge platelets

• Decreased cyclooxygenase activity (aspirin, NSAIDs)
• Storage pool diseases (dense granule deficiency)

Secondary Hemostasis (Coagulation Cascade)

Coagulation Tests

Important Clotting Factor Disorders

Thrombosis

• Arterial thrombosis = platelet-rich ("white clot") - often from atherosclerosis, atrial fibrillation
• Venous thrombosis = fibrin-rich ("red clot") - stasis, hypercoagulability

Virchow's Triad (DVT/PE pathophysiology)

1. Stasis (immobility, heart failure)
2. Endothelial injury (surgery, trauma, indwelling catheters)
3. Hypercoagulability (inherited or acquired)

Inherited Thrombophilias

Acquired Thrombophilias

• Antiphospholipid syndrome (APS): Lupus anticoagulant + anticardiolipin antibodies + anti-β2GP1; paradoxically prolonged aPTT but causes THROMBOSIS; recurrent pregnancy loss
• Myeloproliferative neoplasms: Polycythemia vera, essential thrombocythemia
• Malignancy: Trousseau syndrome (migratory thrombophlebitis)
• Oral contraceptives, HRT
• Prolonged immobility, pregnancy, obesity

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CHAPTER 70 - Enlargement of Lymph Nodes and Spleen

Lymphadenopathy

Epidemiology

• 2/3 of patients with lymphadenopathy in primary care have nonspecific/viral upper respiratory causes

• <1% have malignancy in primary care settings
• In referral settings: 84% benign; 16% malignant (lymphoma or metastatic adenocarcinoma)
• Of benign: 63% nonspecific/reactive; rest = infectious mono, toxoplasmosis, TB most commonly

Normal Nodes (do NOT investigate further)

• Submandibular nodes <1 cm in children/young adults - normal
• Inguinal nodes up to 2 cm in healthy adults - normal
• Soft, flat, non-tender

Clinical Assessment Framework

• Age (children/young adults = benign viral; >50 years = malignancy rises)
• Symptoms: sore throat, cough, fever, night sweats, fatigue, weight loss
• Exposures: pets (cat-scratch disease, toxoplasmosis), sexual history (HIV, syphilis), TB exposure
• Occupation
• Drugs: phenytoin (diphenylhydantoin) causes lymphadenopathy
• Tobacco history + cervical nodes → malignancy must be excluded

• Localized or generalized (≥3 non-contiguous regions)
• Size, texture (soft/hard/rubbery), tenderness, fluctuance
• Overlying skin changes
• Splenomegaly
• Other organomegaly

Node Characteristics Guide

Differential Diagnosis of Lymphadenopathy

• Viral: EBV (mono), CMV, HIV, hepatitis, rubella, adenovirus
• Bacterial: Streptococcal, TB, cat-scratch disease (Bartonella), brucellosis, syphilis
• Fungal: Histoplasmosis, coccidioidomycosis
• Parasitic: Toxoplasmosis

• SLE, rheumatoid arthritis (Felty), serum sickness, drug reaction (phenytoin, allopurinol, INH)

• Hodgkin lymphoma (Reed-Sternberg cells)
• Non-Hodgkin lymphoma
• Leukemia (CLL, ALL)
• Metastatic carcinoma (breast, lung, melanoma, head/neck)

• Sarcoidosis
• Castleman disease
• Kikuchi disease (necrotizing lymphadenitis - young Asian women, self-limiting)
• Amyloidosis

When to Biopsy?

• Nodes >2 cm persisting >4-6 weeks without explanation
• Hard, fixed nodes
• Supraclavicular adenopathy (always biopsy - very high malignancy risk)
• Generalized adenopathy with constitutional symptoms ("B symptoms": fever, night sweats, weight loss >10%)
• Progressive growth
• Abnormal CBC or LDH

Splenomegaly

Functions of the Spleen

1. Filtration - removes senescent/abnormal RBCs; removes encapsulated bacteria (asplenic patients at risk for overwhelming sepsis from Pneumococcus, Meningococcus, H. influenzae)
2. Immune surveillance - T and B cell areas, immunoglobulin production
3. Storage - platelets, monocytes
4. Hematopoiesis - extramedullary hematopoiesis when marrow fails

Causes of Splenomegaly

• Chronic myeloid leukemia (CML)
• Myelofibrosis
• Visceral leishmaniasis (kala-azar)
• Malaria (hyperreactive malarial splenomegaly)
• Thalassemia major/intermedia

• Portal hypertension (cirrhosis, schistosomiasis)
• Lymphoma, leukemia
• Storage diseases (Gaucher's, Niemann-Pick)
• Autoimmune hemolytic anemia

• Infectious mononucleosis (EBV)
• Bacterial endocarditis
• Viral hepatitis
• Rheumatoid arthritis, SLE
• Systemic infections

Hypersplenism

• Splenomegaly → sequestration and destruction of blood cells
• Pancytopenia (low WBC, RBC, platelets) with hypercellular bone marrow
• Treat underlying cause; splenectomy as last resort

Assessment

• Physical exam: Normally the spleen must enlarge 2-3x before palpable
• Best imaging: ultrasound or CT (spleen >12 cm = splenomegaly)
• Nuclear scan: most sensitive for function

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MASTER CLINICAL PEARLS - High-Yield Exam Points

1. RDW elevated + microcytosis = iron deficiency; RDW normal + microcytosis = thalassemia
2. Auer rods = AML (pathognomonic)
3. Hypersegmented neutrophils = B12/folate deficiency
4. Pelger-Hüet anomaly = bilobed neutrophils (inherited LBR mutation or acquired in MDS)
5. Hemarthroses = factor deficiency (hemophilia); Mucosal bleeding = platelet/VWD problem
6. Prolonged aPTT, normal PT = intrinsic pathway defect (hemophilia A/B, VWD, lupus anticoagulant)
7. Prolonged PT, normal aPTT = extrinsic pathway (factor VII deficiency, early liver disease, warfarin)
8. Both PT + aPTT prolonged = common pathway defect, DIC, severe liver disease, supratherapeutic anticoagulation
9. APS = prolonged aPTT but causes CLOTS not bleeding; recurrent pregnancy loss
10. Factor V Leiden = most common inherited thrombophilia; activated protein C resistance
11. Ferritin >10,000 = think HLH immediately (along with fever, splenomegaly, cytopenias)
12. Felty syndrome = RA + splenomegaly + neutropenia
13. CGD = recurrent catalase-positive organism infections; NBT test negative (no oxidative burst)
14. LAD type I = very high WBC + no pus + delayed cord separation
15. Kostmann syndrome = neutrophils <100/μL; HAX-1 mutation; often fatal without G-CSF/HSCT
16. Supraclavicular lymphadenopathy = always workup aggressively; highest malignancy risk of any location
17. Massive splenomegaly = think CML, myelofibrosis, kala-azar, thalassemia, malaria
18. Asplenic patients = at risk for overwhelming sepsis from encapsulated organisms; vaccinate against Pneumococcus, Meningococcus, H. influenzae
19. Pronormoblast = first morphologically recognizable erythroid precursor
20. EPO from kidney = absence causes apoptosis of erythroid progenitors; renal failure → normocytic anemia

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What You Just Learned (Harrison's 22E, Section 9)