Anemias

Anemia is one of the most common problems in all of medicine — and one of the most misunderstood. It is not a disease in itself but a signpost: a reduction in the oxygen-carrying capacity of the blood that always points to an underlying cause. A tired 30-year-old woman, a pale elderly man passing black stool, a jaundiced child with a family history of "yellow eyes" — all may have anemia, but the diagnoses and treatments could not be more different. Learning anemia well means learning to think mechanistically: is the marrow not making enough red cells, or is the body losing or destroying them too fast? Master that single question and the rest of hematology becomes far less intimidating.

This page teaches you to classify anemia the way clinicians actually do — by red cell size and by mechanism — then walks through the major types you must know for exams and practice, and closes with the extraordinary story of how pernicious anemia went from a uniformly fatal disease to a curable vitamin deficiency.

Learning Objectives

• Define anemia correctly and explain why it is a sign, not a diagnosis
• Classify anemias by MCV (microcytic, normocytic, macrocytic) and by mechanism (production, destruction, loss)
• Describe the pathophysiology, laboratory findings, and treatment of iron-deficiency, megaloblastic, and hemolytic anemias
• Interpret core investigations: CBC, MCV, reticulocyte count, peripheral smear, iron studies, and B12/folate levels
• Recognize red-flag causes (occult GI malignancy, hemolytic crises, aplastic marrow) that demand urgent action
• Recount how the discovery of vitamin B12 transformed pernicious anemia from fatal to treatable

Quick Answer

Anemia is a fall in hemoglobin below the age- and sex-adjusted reference (roughly below 13 g/dL in men and 12 g/dL in non-pregnant women). The fastest clinical shortcut is to classify by MCV (mean corpuscular volume): microcytic (small cells, less than 80 fL) suggests iron deficiency or thalassemia; macrocytic (large cells, over 100 fL) suggests B12/folate deficiency or liver/alcohol effects; normocytic points to acute blood loss, hemolysis, chronic disease, or marrow failure. The reticulocyte count then separates under-production (low retics) from destruction or loss (high retics). Iron-deficiency anemia — the world's commonest — is treated by finding the source of bleeding and replacing iron. B12 and folate deficiencies are corrected with replacement, but B12 must be excluded before giving folate. Hemolytic anemias require identifying whether the cause is inside the red cell (hereditary) or outside it (immune, mechanical).

Where It Came From

For most of history, severe anemia was a death sentence with a poetic name: chlorosis, the "green sickness" of pale young women (iron deficiency), and pernicious anemia — pernicious meaning "deadly," because it always killed. Physicians in the 19th century could describe the lemon-yellow pallor, the beefy sore tongue, and the numb feet, but had nothing to offer.

The breakthrough came from an unglamorous experiment. In the 1920s, George Whipple bled dogs to make them anemic and tested which foods restored their blood fastest; liver was the winner. George Minot and William Murphy reasoned that if liver rebuilt blood in dogs, it might help humans, and in 1926 they fed patients with pernicious anemia enormous quantities of raw liver — up to half a pound a day. Patients who had been dying recovered. It was one of the most dramatic therapeutic successes in medical history, and the three shared the 1934 Nobel Prize.

But why did liver work? William Castle showed in the 1920s–30s that patients needed two things: an "extrinsic factor" in food and an "intrinsic factor" made by the stomach. Only when both were present could the vital substance be absorbed. The mystery substance — the extrinsic factor — was finally isolated in 1948 as vitamin B12 (cobalamin), a cobalt-containing molecule, by teams led by Karl Folkers (Merck) and E. Lester Smith (in the UK). Later, Dorothy Hodgkin used X-ray crystallography to solve its complex structure, winning her own Nobel Prize in 1964. Castle's "intrinsic factor" turned out to be a specific stomach protein needed to absorb B12 in the terminal ileum — and its autoimmune destruction is the true cause of pernicious anemia. A disease that once meant certain death now needs only a simple injection.

Classifying Anemia: The Two Questions That Organize Everything

Every anemia workup answers two questions. First, what size are the red cells? The MCV, reported automatically on every CBC, splits anemias into three buckets:

MCV	Category	Typical causes
Less than 80 fL	Microcytic	Iron deficiency, thalassemia, anemia of chronic disease (late), sideroblastic
80–100 fL	Normocytic	Acute blood loss, hemolysis, anemia of chronic disease, renal failure, early marrow failure
Over 100 fL	Macrocytic	B12 deficiency, folate deficiency, alcohol, liver disease, hypothyroidism, myelodysplasia

Second, is the marrow responding? The reticulocyte count measures young red cells released from marrow. A high reticulocyte count means the marrow is working hard — appropriate for blood loss or hemolysis. A low count in the face of anemia means production is failing — deficiency, marrow disease, or suppression. This single test often redirects the whole workup.

A useful mental model: think of the red cell factory. Microcytic usually means a raw-material problem (not enough iron/hemoglobin, so cells stay small). Macrocytic often means a DNA-synthesis problem (nuclei can't keep up with the growing cell, producing big immature cells). Normocytic with high retics means cells are being lost or destroyed faster than made.

Iron-Deficiency Anemia

This is the single most common anemia worldwide. Iron is the core of the heme molecule; without it, hemoglobin production stalls and red cells emerge small and pale (microcytic, hypochromic). Causes cluster by age and sex: in menstruating women, heavy periods and pregnancy dominate; in men and postmenopausal women, gastrointestinal blood loss until proven otherwise — and that includes colorectal cancer. In children and in low-resource settings, poor dietary intake and hookworm are major contributors.

Symptoms include fatigue, exertional breathlessness, and pallor, plus iron-specific signs: koilonychia (spoon nails), angular cheilitis, a smooth sore tongue, and pica (craving ice or dirt). Laboratory hallmarks are low MCV, low MCH, raised red cell distribution width (RDW), and the decisive iron studies: low ferritin (the most useful single test — low ferritin confirms iron deficiency), low serum iron, high total iron-binding capacity, and low transferrin saturation. Ferritin is an acute-phase reactant, so inflammation can falsely raise it.

Worked example. A 62-year-old man reports 3 months of tiredness. Hb 9.1 g/dL, MCV 71 fL, ferritin 6 ng/mL. This is iron deficiency — but the crucial next step is not simply iron tablets. At his age, occult GI bleeding must be excluded; he needs upper and lower endoscopy to look for a source, classically a right-sided colon cancer or peptic ulcer. Treating the anemia without finding the cause could let a curable cancer advance.

Treatment: oral ferrous sulfate (or ferrous fumarate/gluconate) is first-line; take on an empty stomach with vitamin C to aid absorption; every-other-day dosing improves tolerance and absorption. Expect a reticulocyte rise within a week and Hb rising by about 2 g/dL over 3–4 weeks; continue for 3 months after Hb normalizes to refill stores. IV iron is used when oral iron fails, is not tolerated, or in malabsorption/chronic kidney disease.

Megaloblastic and Other Macrocytic Anemias

Megaloblastic anemia arises when DNA synthesis is impaired but cytoplasm and RNA keep growing, producing large cells (megaloblasts in marrow, macro-ovalocytes and hypersegmented neutrophils on smear). The two classic culprits are vitamin B12 and folate deficiency — both are cofactors in DNA nucleotide production.

Folate deficiency develops within weeks to months because stores are small: causes include poor diet, alcoholism, pregnancy, and drugs (methotrexate, phenytoin). B12 deficiency takes years because stores are large; causes include pernicious anemia (autoimmune loss of intrinsic factor), gastrectomy, terminal ileal disease (Crohn's), and strict veganism. B12 deficiency is distinguished by neurological features: peripheral neuropathy and subacute combined degeneration of the cord (dorsal columns and corticospinal tracts), causing loss of vibration/proprioception, ataxia, and weakness.

Critical rule: never give folate alone to a macrocytic patient without checking B12. Folate can correct the anemia while the B12-driven neurological damage silently progresses and may become irreversible. Always replace B12 first (or both together). B12 is given as intramuscular hydroxocobalamin; pernicious anemia requires lifelong replacement because absorption cannot be restored.

Not all macrocytosis is megaloblastic. Alcohol, liver disease, hypothyroidism, and reticulocytosis cause a non-megaloblastic macrocytosis without hypersegmented neutrophils. Myelodysplastic syndromes in older adults are an important cause of unexplained macrocytic anemia that does not respond to vitamins.

Hemolytic Anemias

Here red cells are destroyed faster than they can be replaced. The marrow ramps up, so the reticulocyte count is high. Breakdown products rise: unconjugated bilirubin (jaundice), LDH, and free hemoglobin, while haptoglobin falls as it mops up released hemoglobin. A direct antiglobulin (Coombs) test is the pivotal branch point.

Classify hemolysis by where the defect lies:

• Intrinsic (usually hereditary): membrane defects (hereditary spherocytosis — spherocytes, positive osmotic fragility, splenomegaly), enzyme defects (G6PD deficiency — episodic hemolysis triggered by oxidant drugs, fava beans, or infection, with bite cells and Heinz bodies), and hemoglobinopathies (sickle cell disease, thalassemias).
• Extrinsic (acquired): autoimmune hemolytic anemia (Coombs-positive; warm IgG or cold IgM types), microangiopathic hemolysis (schistocytes in TTP, HUS, DIC — a hematologic emergency), mechanical (prosthetic valves), infections (malaria), and drugs.

Case vignette. A 22-year-old man of Mediterranean descent develops dark urine and jaundice two days after starting an antibiotic for a UTI. Hb has dropped, retics are high, LDH elevated, haptoglobin undetectable, and the smear shows bite cells. This is a G6PD-deficiency hemolytic episode triggered by an oxidant drug. Management is supportive — stop the offending drug, hydrate, transfuse if severe — and counsel him to avoid future oxidant triggers.

Treatment of hemolysis depends on cause: corticosteroids for warm autoimmune hemolysis, avoidance of triggers in G6PD, folate supplementation in chronic hemolysis, splenectomy in selected hereditary spherocytosis, and disease-specific therapy for sickle cell and thalassemia.

Real-World Applications

• Primary care: anemia is often the first clue to occult disease. Iron deficiency in an older adult mandates a hunt for GI cancer; macrocytosis may reveal alcohol misuse or hypothyroidism.
• Obstetrics: anemia in pregnancy raises risks of preterm birth and low birth weight; routine iron and folate supplementation is standard antenatal care.
• Surgery and critical care: preoperative anemia worsens outcomes; correcting iron deficiency before elective surgery reduces transfusion needs.
• Global health: iron deficiency and hookworm-related anemia impair childhood cognition and adult productivity, making them major public-health targets.
• Everyday relevance: understanding that fatigue can stem from a treatable vitamin or iron deficiency — rather than being dismissed — changes lives.

Common Mistakes

1. Treating iron deficiency without finding the cause. Iron tablets fix the number but hide the reason. Missing a colon cancer or peptic ulcer is a serious error, especially in men and postmenopausal women. Always ask where the iron went.
2. Giving folate to a B12-deficient patient. Folate corrects the blood picture but lets neurological damage from B12 deficiency progress, sometimes irreversibly. Check B12 before or alongside folate replacement.
3. Assuming all microcytic anemia is iron deficiency. Thalassemia trait also causes microcytosis but with a normal or high red cell count and normal ferritin. Giving iron to a thalassemia carrier is useless and risks overload. The Mentzer index (MCV divided by RBC count) helps: below 13 favors thalassemia.
4. Ignoring the reticulocyte count. Without it, you cannot tell a marrow that is failing from one working overtime — the two demand opposite investigations.

Comparison and Connections

Feature	Iron deficiency	B12/folate deficiency	Hemolysis
MCV	Low (microcytic)	High (macrocytic)	Normal or high
Reticulocytes	Low	Low	High
Key labs	Low ferritin, high TIBC	Low B12/folate, hypersegmented neutrophils	High LDH/bilirubin, low haptoglobin
Smear clue	Hypochromic microcytes	Macro-ovalocytes	Spherocytes/schistocytes/bite cells
First treatment	Iron + find bleeding source	B12 or folate (B12 first)	Treat cause (steroids, avoid triggers)

Anemia connects widely: iron studies overlap with the anemia of chronic disease (where inflammation traps iron, giving low serum iron but normal or high ferritin); renal failure causes anemia through low erythropoietin; and marrow failure syndromes like aplastic anemia produce pancytopenia. See also related material in Pathology and Physiology.

Practice Questions

Recall

Q: What single laboratory test best confirms iron-deficiency anemia, and what happens to it in inflammation? A: Serum ferritin — low ferritin confirms iron deficiency. Because ferritin is an acute-phase reactant, inflammation can raise it and mask true iron deficiency.

Understanding

Q: Why does vitamin B12 deficiency cause neurological problems while folate deficiency generally does not? A: B12 is a cofactor not only for DNA synthesis but also for methylmalonyl-CoA conversion and myelin maintenance. Its deficiency damages the dorsal columns and corticospinal tracts (subacute combined degeneration), producing sensory loss, ataxia, and weakness — features folate deficiency lacks.

Application

Q: A 68-year-old woman has Hb 8.5, MCV 70, ferritin 4. What is the diagnosis and the essential next step? A: Iron-deficiency anemia. Because she is postmenopausal, the essential next step is investigation for GI blood loss (upper and lower endoscopy) to exclude malignancy — not just iron replacement.

Analysis

Q: A patient has anemia with high reticulocytes, raised LDH and bilirubin, low haptoglobin, and schistocytes on smear. Coombs is negative. What category is this, and why is it urgent? A: Microangiopathic hemolytic anemia (mechanical red cell fragmentation). A negative Coombs with schistocytes points to TTP, HUS, or DIC — potentially life-threatening conditions where TTP in particular requires urgent plasma exchange.

FAQ

Is anemia a diagnosis? No. It is a sign of an underlying process. The clinical task is always to find why the hemoglobin is low.

How fast should iron tablets work? Reticulocytes rise within about a week, and hemoglobin climbs roughly 2 g/dL over 3–4 weeks. If there is no response, reconsider the diagnosis (wrong cause, poor adherence, ongoing bleeding, or malabsorption).

Can I just eat more spinach to fix iron deficiency? Diet alone rarely corrects an established deficiency. Plant (non-heme) iron is poorly absorbed; medicinal iron supplements are usually needed, plus treating the cause.

Why do B12 injections rather than tablets for pernicious anemia? Pernicious anemia destroys intrinsic factor, so B12 cannot be absorbed from the gut normally. Injections bypass the problem. (High-dose oral B12 can work in some cases via passive absorption, but injections are standard for confirmed pernicious anemia.)

What is the difference between anemia of chronic disease and iron deficiency? Both can be microcytic, but in anemia of chronic disease inflammation locks iron away — serum iron is low but ferritin is normal or high, unlike the low ferritin of true iron deficiency.

Is a slightly low hemoglobin in pregnancy normal? Some fall is physiological due to plasma volume expansion (dilutional), but true iron and folate deficiency are common and treatable, which is why routine screening and supplementation are standard.

Quick Revision

• Anemia is a sign, not a disease — always find the cause.
• Classify by MCV (micro/normo/macro) and by reticulocyte count (production vs destruction/loss).
• Iron deficiency: microcytic, low ferritin, high TIBC; in older adults, hunt for GI bleeding.
• Megaloblastic: macrocytic, hypersegmented neutrophils; B12 has neuro signs; never give folate alone without checking B12.
• Hemolysis: high retics, high LDH/bilirubin, low haptoglobin; Coombs separates immune from non-immune.
• Pernicious anemia: autoimmune loss of intrinsic factor; lifelong B12 injections; once fatal, now curable.

Related Topics

Prerequisites

• Physiology — oxygen transport and erythropoiesis
• Biochemistry — heme synthesis, iron and vitamin metabolism

Related Topics

• Pathology — mechanisms of red cell destruction and marrow failure
• Hematology branch overview: Hematology

Next Topics

• Coagulation disorders and bleeding (within Hematology)
• Hematologic malignancies (leukemias and lymphomas) — see Oncology