Anemia

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"anemia"[MeSH Terms] AND "diagnosis"[MeSH Terms]

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I now have comprehensive content from Harrison's, Robbins, Goldman-Cecil and other authoritative sources. Let me compile the full overview.

Anemia — Comprehensive Overview

Definition

Anemia is defined as a reduction in red blood cell (RBC) mass below normal for a given population. Standard WHO thresholds are:

Men: Hemoglobin (Hb) < 13 g/dL
Women: Hb < 12 g/dL
Pregnant women: Hb < 11 g/dL

Classification

Anemias are classified in two complementary ways:

1. By Red Cell Size (MCV-Based)

Type	MCV	Causes
Microcytic	< 80 fL	Iron deficiency, thalassemia, anemia of chronic disease (ACD), sideroblastic anemia, lead poisoning
Normocytic	80–100 fL	Acute blood loss, hemolysis, aplastic anemia, renal disease, hypothyroidism, multiple myeloma
Macrocytic	> 100 fL	B12/folate deficiency (oval macrocytes), liver disease, alcohol use, hypothyroidism, reticulocytosis, myelodysplasia (round macrocytes)

Note: The RDW (red cell distribution width) is a useful clue — a normal RDW in microcytosis favors thalassemia over iron deficiency (which typically raises RDW).

2. By Mechanism (Reticulocyte-Based)

Category	Reticulocyte Count	Causes
Hyperproduction (loss/destruction)	↑ Elevated	Bleeding, hemolytic anemia (autoimmune, mechanical, congenital)
Underproduction	↓ Low or inappropriately normal	Nutritional deficiency, marrow replacement, aplastic anemia, EPO deficiency

Major Types in Detail

🔴 Iron Deficiency Anemia (IDA)

Most common anemia worldwide — affects ~25–50% in lower-resource countries.

Pathophysiology:

Iron is essential for heme synthesis (4 Fe atoms per hemoglobin molecule)
Iron absorption is regulated by hepcidin (liver peptide) via ferroportin inhibition
Inflammation raises hepcidin → blocks iron absorption and release from stores
Causes: Chronic blood loss (GI, menstrual), poor dietary intake, malabsorption (celiac, post-gastrectomy), pregnancy

Lab findings:

Test	Iron Deficiency
Serum ferritin	↓ < 30 μg/L (depleted stores)
Serum iron	↓
TIBC	↑
Transferrin saturation	↓ < 20%
MCV	↓ (microcytic)
RDW	↑
Peripheral smear	Hypochromic, microcytic cells

Treatment: Oral ferrous sulfate (150–200 mg elemental iron/day); IV iron (e.g., ferric carboxymaltose) for malabsorption/intolerance. Treat underlying cause.

🟠 Megaloblastic Anemia (B12 / Folate Deficiency)

Mechanism: Defective DNA synthesis in erythroid precursors → nuclear–cytoplasmic asynchrony → large, immature megaloblasts.

Key features:

Oval macrocytes on smear
Hypersegmented neutrophils (≥5 lobes — pathognomonic)
Bone marrow: hypercellular with giant megaloblasts
B12 deficiency also causes subacute combined degeneration of the spinal cord (demyelination of posterior and lateral columns)

Causes of B12 deficiency: Pernicious anemia (anti-intrinsic factor antibodies), strict veganism, terminal ileal disease (Crohn's), gastrectomy Causes of folate deficiency: Poor diet, pregnancy, malabsorption, methotrexate, phenytoin, alcohol

Lab: ↑ MCV, ↑ homocysteine (both), ↓ methylmalonic acid (MMA only in B12 deficiency — key differentiator)

Treatment: B12 (IM cyanocobalamin for pernicious anemia; oral B12 if dietary); folic acid 1–5 mg/day orally for folate deficiency.

Megaloblastic anemia — peripheral blood (oval macrocytes, hypersegmented neutrophil) and bone marrow (megaloblasts)

🟡 Anemia of Chronic Disease / Anemia of Inflammation (ACD/AI)

Second most common anemia globally (up to 40% of hospital patients).

Pathophysiology:

Chronic inflammation ↑ IL-6 → ↑ hepcidin → ferroportin degradation → impaired iron release from macrophages and reduced intestinal absorption
Inflammatory cytokines (TNF) also suppress EPO production and erythropoiesis directly

Lab:

Typically normocytic (sometimes mildly microcytic, MCV rarely < 75 fL)
Low serum iron AND low TIBC (distinguishes from IDA where TIBC is high)
Ferritin normal or ↑ (acute phase reactant)
Low reticulocyte count

Treatment: Treat underlying disease; IV iron ± ESAs (erythropoiesis-stimulating agents) in renal/cancer contexts.

🔵 Hemolytic Anemias

Accelerated RBC destruction → shortened lifespan (< 120 days normal). Hallmarks: ↑ reticulocytes, ↑ LDH, ↓ haptoglobin, indirect hyperbilirubinemia, splenomegaly.

Extravascular vs. Intravascular Hemolysis

Feature	Extravascular	Intravascular
Mechanism	Splenic macrophage phagocytosis	RBC lysis within vessels
Hemoglobinuria	Absent	Present
Hemoglobinemia	Absent	Present
Jaundice	Yes	Less prominent
Haptoglobin	↓	↓↓
Iron deficiency	No	Yes (iron lost in urine)
Splenomegaly	Common	Less common

Classification by Cause

Intrinsic (Intracorpuscular) Defects:

Category	Disease
Membrane defects	Hereditary spherocytosis (AD; spectrin/ankyrin mutations), hereditary elliptocytosis
Enzyme defects	G6PD deficiency (X-linked; oxidative stress triggers hemolysis), pyruvate kinase deficiency
Hemoglobinopathies	Sickle cell disease (HbS polymerization → vaso-occlusion), unstable hemoglobins
Deficient globin synthesis	Thalassemia syndromes (α and β)
Acquired (PNH)	Paroxysmal nocturnal hemoglobinuria (GPI-anchor deficiency → complement-mediated lysis)

Extrinsic (Extracorpuscular) Defects:

Category	Examples
Autoimmune	Warm AIHA (IgG, Coombs+), Cold agglutinin disease (IgM)
Microangiopathic (MAHA)	TTP, HUS, DIC, malignant hypertension — schistocytes on smear
Infections	Malaria, babesiosis, clostridial sepsis
Mechanical	Prosthetic valve hemolysis, march hemoglobinuria

Diagnostic workup: Direct antiglobulin test (DAT/Coombs), peripheral smear, osmotic fragility, G6PD assay, Hb electrophoresis, flow cytometry (PNH).

⚪ Aplastic Anemia

Bone marrow failure with pancytopenia (anemia + thrombocytopenia + neutropenia). Marrow is markedly hypocellular.

Pathogenesis:

Immune-mediated (70–80%): Autoreactive T-cells destroy hematopoietic stem cells; responds to immunosuppression
Intrinsic stem cell defect: Telomerase mutations (5–10%), short telomeres (50% of cases)
Precipitants: drugs (chloramphenicol, benzene, carbamazepine), viral infections, radiation; often idiopathic

Clinical: Insidious onset of weakness, pallor, petechiae (thrombocytopenia), recurrent infections (neutropenia). No splenomegaly (distinguishes from other causes).

Treatment:

< 40 years, suitable donor → hematopoietic stem cell transplantation (HSCT) — often curative
Not transplant candidates → immunosuppression (anti-thymocyte globulin + cyclosporine); restores hematopoiesis in 60–70%

🟤 Myelophthisic Anemia

Space-occupying marrow lesions (metastatic breast/lung/prostate cancer, TB, granulomas) displace normal hematopoiesis.

Peripheral smear: Leukoerythroblastosis — teardrop cells, nucleated RBCs, immature granulocytes. Requires bone marrow biopsy for diagnosis.

Diagnostic Approach

                         ↓ Hb / Hct
                              │
              ┌───────────────┼───────────────┐
           Microcytic       Normocytic      Macrocytic
           (MCV <80)       (MCV 80–100)    (MCV >100)
              │                │                │
      Iron studies     Reticulocyte count   B12, folate,
      Ferritin, TIBC        │                TFTs, LFTs
              │         ┌────┴────┐
              │      Elevated    Low/Normal
              │         │            │
              │      Hemolysis    Aplastic/ACD/
              │      workup       Renal/Marrow Rx
        ┌─────┴──────┐
     Low ferritin   Normal/↑ ferritin
     = IDA          ↑ TIBC → IDA
                    Normal TIBC → ACD
                    ↑ TIBC → Thalassemia trait

Key labs to order:

CBC with differential and reticulocyte count
Peripheral blood smear (mandatory)
Iron studies: serum Fe, ferritin, TIBC, transferrin saturation
B12, folate, LDH, haptoglobin
Direct Coombs (if hemolysis suspected)
Bone marrow biopsy (if cause unclear or pancytopenia)

Clinical Features (General)

Severity	Hb	Symptoms
Mild	10–12 g/dL	Often asymptomatic; fatigue on exertion
Moderate	8–10 g/dL	Fatigue, palpitations, dyspnea on exertion, pallor
Severe	< 8 g/dL	Rest dyspnea, tachycardia, angina, heart failure
Very severe	< 6 g/dL	High-output cardiac failure, altered consciousness

Specific signs: conjunctival pallor, koilonychia (spoon nails — IDA), glossitis, angular cheilitis, jaundice (hemolysis), splenomegaly (hemolysis, thalassemia), neurological signs (B12 deficiency).

Treatment Summary

Type	First-line Treatment
Iron deficiency	Oral/IV iron; treat cause
B12 deficiency	IM cyanocobalamin (if pernicious anemia); oral B12
Folate deficiency	Oral folic acid 1–5 mg/day
ACD/AI	Treat underlying disease; IV iron ± ESAs
Hemolytic (AIHA warm)	Corticosteroids → rituximab → splenectomy
Aplastic anemia	HSCT (young) or immunosuppression (ATG + CsA)
Sickle cell	Hydroxyurea, transfusion support, HSCT
Thalassemia	Transfusions, iron chelation, HSCT

Sources:

Harrison's Principles of Internal Medicine 22E, Ch. 66 (Anemia Definition & Classification)
Robbins & Kumar Basic Pathology, Ch. 10 (Red Cell Disorders — Iron Deficiency, Hemolytic Anemias, Aplastic Anemia)
Goldman-Cecil Medicine, Ch. 144–151 (Anemias)
Recent meta-analyses (PMID 40159279 — IV iron in heart failure + iron deficiency; PMID 40392014 — mixed AIHA)

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