It is estimated that there are more than 2 billion cases of iron deficiency anemia in the world making iron deficiency the commonest cause of microcytic anemia worldwide.⁶ Iron deficiency is caused by blood loss, inadequate dietary iron, rapid growth without iron supplementation, or poor iron absorption. Iron is normally the most abundant trace metal in the human body, totaling 3 to 4 g, mainly contained in hemoglobin, myoglobin, and enzymes; only a minor fraction, about 1 g, is stored as a reserve for production of new hemoglobin. Normal iron balance is tightly maintained, with daily needs of about 1 mg of absorbed iron, reflecting a dietary intake of about 20 mg, to balance obligatory excretion of about 1 mg of iron in sweat, shed skin or shed mucosal cells. Little absorbable iron is present in many foods, including most fruits and vegetables and grains, where iron is tightly complexed to phytates. Good iron sources are meat, poultry, fish, eggs, and legumes. Because daily iron loss is slight in adult males, men need little dietary iron, and deficiency from inadequate dietary intake is uncommon in men. When iron utilization is increased as in infancy, during growth spurts, and in pregnancy, or when concurrent blood loss occurs, as in menstruation, dietary iron intake may be insufficient, especially in women and children who tend to consume less than the recommended minimal daily requirement.⁷,⁸ The problem increases during pregnancy, when maternal iron is diverted to the fetus for hematopoiesis and with breastfeeding, as iron is lost in the milk.

Plasma iron levels are closely regulated by the hepatic peptide hormone hepcidin, which limits the release of newly absorbed iron from both enterocytes in the GI tract and from macrophages in the bone marrow and reticuloendothelial organs by binding to the membrane iron transporter, ferroportin, which promotes degradation of both proteins and thus limits the amount of iron bound to transferrin available to maturing erythroblasts for hemoglobin synthesis.⁹ Inflammation, chronic disease, and cancer, as well as high plasma iron levels, all favor increases in hepcidin, resulting in decreased iron availability, and thus, decreased erythropoiesis associated with these disease states. Conversely, in iron deficiency, hepcidin falls, promoting iron trafficking. Erythroferrone present on developing erythroblasts is a natural inhibitor of hepcidin. Numerous other proteins including hemojuvelin, bone morphogenic protein-2, SMADs, furin, and TMPRSS6 (transmembrane protease serine member 6) affect hepcidin transcription. During infection, stimulation of toll receptors can directly depress ferroportin levels independently of hepcidin and thus lower plasma iron levels; in addition, activated neutrophils release the iron-binding protein lactoferrin, which further limits the availability of iron required for bacterial growth.¹⁰

The reduction of body iron in developing iron deficiency anemia follows a standard pattern. Well prior to any anemia, iron stores (i.e., bone marrow ferritin) disappear, followed by a decrease in plasma iron levels and increase in unbound transferrin. Next, a normocytic anemia develops, which only with increasing iron deficiency becomes microcytic and then hypochromic. The reticulocyte count decreases. Thus, with mild and recent iron deficiency, the red cell indices (MCV, mean corpuscular hemoglobin [MCH], mean corpuscular hemoglobin concentration [MCHC]) and the blood smear remain normal, but over time, increasingly severe anemia develops and the MCV and MCHC fall. Anisocytosis, resulting in an increase in RDW, occurs. Later morphologic changes in iron deficiency anemia include poikilocytosis, microcytosis, and hypochromia. Tiny microcytes, elongated pale elliptical RBCs (pencil cells), and target cells may be visible on the blood smear, but some of the erythrocytes may appear normal. Often, the platelet count is elevated, with characteristically very small, single platelets. In overt iron deficiency anemia, the serum iron is markedly decreased, whereas the total
iron-binding capacity (i.e., transferrin level) is elevated, and the transferrin saturation is below 10%. The serum ferritin is markedly decreased (<20 ng/mL). Soluble transferrin receptors increase in iron deficiency but remain normal in ACD. Usually, the diagnosis of iron deficiency anemia is established by these tests, but occasionally, obtaining a bone marrow sample to verify the absence of iron stores or a trial of iron therapy may be necessary to confirm the presence of iron deficiency.

The clinical features of iron deficiency are generally similar to other anemias, e.g., fatigue, weakness, pallor, poor exercise tolerance, but three distinctive findings are only seen in iron deficiency anemia: pica, koilonychia, and blue sclera. Pica is the craving for, and ingestion of, certain unusual substances, such as starch, dirt, clay, cardboard, and ice (pagophagia). Koilonychia, marked by fingernails that become thin, brittle, and concave (spoon-shaped), is virtually pathognomonic of iron deficiency. Thinning of the sclera from impaired epithelial growth causes a blue tint because of the more visible choroid beneath.

Iron deficiency anemia in adults frequently results from chronic blood loss. The major cause of blood loss in women of childbearing age is heavy menstruation or repeated pregnancies. In non-menstruating women and in men, the most common reason for iron deficiency is GI hemorrhage. However, in resource-poor countries, lack of dietary iron or parasitic infestation with GI or genitourinary bleeding are common causes of iron deficiency. Infants, children, and pregnant women develop iron deficiency because of poor dietary intake of iron in the face of increased demands such as growth spurts or later stages of pregnancy (a developing fetus takes about 400 mg of iron from the mother). Other less common reasons for iron deficiency include chronic hematuria, epistaxes, hemoptysis, or intrapulmonary hemorrhage in such disorders as idiopathic pulmonary hemosiderosis, microscopic polyangiitis, or Goodpasture syndrome. In hereditary hemorrhagic telangiectasia, repeated mucosal bleeding from the vascular malformations may result in severe chronic iron deficiency anemia only remediable by parenteral iron repletion. Rare causes of iron deficiency include intravascular hemolysis from such diseases as paroxysmal nocturnal hemoglobinuria (PNH) or mechanical heart valve-related fragmentation of RBC, conditions which result in chronic intravascular hemolysis that leads to chronic urinary excretion of iron as hemoglobinuria (for PNH) or hemosiderinuria (for both).

Iron deficiency occasionally develops from iron malabsorption related to chronic malabsorptive GI disease such as inflammatory bowel disorders or sprue, or following gastric bypass surgery. Iron is absorbed throughout the GI tract, but best in the upper duodenum. In the presence of small intestinal disease, such as celiac sprue, or after gastric resection or bariatric surgery, which may accelerate the movement of intestinal materials through the duodenum and thereby diminish absorption time, iron deficiency may develop. Indeed, iron deficiency anemia, often refractory to oral iron, frequently develops within a year in the majority of patients who have undergone gastric bypass procedures and may require parenteral iron for correction.

A rare variant of iron deficiency anemia, iron-refractory iron deficiency anemia (IRIDA), representing a mutation in the TMPRSS6 gene, has improved our understanding of susceptibility to iron deficiency.¹¹ In IRIDA, the TMPRSS6 mutation interrupts the normal pathway that inhibits hepcidin transcription, so that hepcidin levels remain high, leading to blocked GI absorption of iron, and a microcytic, hypochromic anemia develops that is refractory to oral iron. Anemia severe enough to require parenteral iron may be required in some menstruating women. However, as normal release of macrophage iron is also blocked, IRIDA patients will show increased iron stores when treated with parenteral iron. Studying other variants in the TMPRSS6 gene has revealed that some variants are associated with low hepcidin levels which have been described in blood donors with a relative resistance to development of iron deficiency, whereas carriers of variants with high hepcidin levels are more susceptible to iron deficiency anemia after blood donations. Similarly, male blood donors who carry a common polymorphism for the HIF-1 α gene that affects protein stabilization and hypoxia have higher hemoglobin and ferritin levels after blood donation than persons who have the wild-type allele. In the future, therapies that alter hepcidin concentration may become important in managing both iron deficiency and iron overload.¹²

Sideroblastic anemia results from impaired synthesis of heme. This impairment results in the presence of ring sideroblasts in the bone marrow. Ring sideroblasts are erythroblasts which contain a perinuclear ring of at least five blue granules representing iron-loaded mitochondria covering at least one-third of the nuclear circumference upon staining with Prussian blue. Sideroblastic anemia can be a genetic or acquired condition.¹³ The most common inherited form is X-linked sideroblastic anemia, which is caused by a germline mutation in the erythroid-specific aminolevulinic acid (ALA) synthase gene (ALAS2). ALAS2 catalyzes an early step in the synthesis of heme and mutation of this gene leads to impaired production. Because ALAS2 is an early step in the synthesis of heme, there is no buildup of toxic porphyrin metabolites or the associated symptoms seen with the porphyrias. Recent studies have identified mutations in a range of additional genes which can cause sideroblastic anemia. These include mutations in the glutaredoxin 5 (GLRX5) and the SLC25A38 genes, which encode mitochondrial proteins. Although X-linked sideroblastic anemia generally only affects males, there are documented cases of affected females as a result of autosomal dominance and lyonization. Acquired sideroblastic anemia is most often caused by myelodysplastic syndromes (MDS) or lead poisoning. MDS are a group of hematopoietic stem cell diseases with dysplasia in one or all of the major myeloid cell lineages; patients with MDS have a 15% chance
of developing acute leukemia. The WHO defines three distinct entities for MDS with sideroblastic anemia: refractory anemia with ringed sideroblasts (RARS), refractory anemia with ring sideroblasts associated with marked thrombocytosis (RARS-T), and refractory cytopenia with multilineage dysplasia with at least 15% ring sideroblasts [RCMD(+RS)].

Beyond MDS, lead poisoning is a major cause of acquired sideroblastic anemia. Lead can inhibit porphobilinogen synthase, which leads to impaired heme synthesis. In addition, lead inhibits ferrochelatase, which catalyzes the insertion of iron into the completed porphyrin ring to produce heme. This inhibition results in the formation of zinc protoporphyrin (containing zinc instead of iron). Measurement of the zinc protoporphyrin:heme ratio can be used to detect lead poisoning. Beyond this, routine measurements of blood lead levels are possible using either atomic absorption spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS).

Megaloblastic anemia is most often caused by nutritional deficiencies (especially deficiencies of folic acid or vitamin B₁₂, vitamins essential for normal DNA synthesis) or medications that impair DNA synthesis or block folic acid metabolism, such as cytotoxic agents used in cancer chemotherapy or immunosuppression (e.g., cyclophosphamide, azathioprine, hydroxyurea, methotrexate, trimethoprim).¹⁵,¹⁶ In either instance, DNA synthesis is slowed whereas protein synthesis continues normally, resulting in abnormally large erythrocyte precursors (megaloblasts) in the bone marrow and large well-hemoglobinized RBCs (macrocytes) in the blood. In addition to the macrocytic anemia, macrocytic leukopenia and giant platelets are often present, and other nonhematopoietic cells such as skin cells and cervical mucosal cells are also macrocytic.

Nuclear maturation of hematopoietic cells in the bone marrow involves rapid cell division, which requires high levels of DNA synthesis. In situations where these high levels of DNA synthesis cannot be supported, as in deficiencies of essential nutrients, there is defective nuclear maturation of hematopoietic cells in the bone marrow.¹⁷ The defective nuclear maturation is primarily due to inadequate production of thymidine, an essential nucleic acid specific for DNA. Folic acid and vitamin B₁₂ are essential in the production of thymidine from deoxyuridine, participating in methylation of the latter nucleotide. Tetramethyl folate is a methyl carrier, whereas vitamin B₁₂ is the cofactor for the methyl transfer reaction. Whereas thymidine deficiency slows nuclear maturation and cell division, cytoplasmic maturation continues normally, as it is regulated by RNA (which does not require thymidine). As a consequence, hematopoietic precursor cells become megaloblastic, i.e., unusually large with relatively immature nuclei at each growth stage. This nuclear-cytoplasmic asynchrony is seen not only in erythroblasts but also in granulocyte precursors and megakaryocytes/platelets. The bone marrow is markedly hypercellular, but many of these megaloblastic precursors are destroyed in the marrow before release into the circulation, termed ineffective erythropoiesis. This intramedullary cell wastage is reflected by peripheral leukopenia and thrombocytopenia as well as anemia, and by elevated levels of serum iron, unconjugated bilirubin, and LDH.¹⁸ Early in megaloblastic anemias, mild macrocytosis may be the only abnormality in the peripheral blood, where only a small percentage of RBCs are actually macroovalocytes. As anemia worsens, the peripheral blood smear typically shows many RBC anomalies including anisocytosis, poikilocytosis, teardrop cells, schistocytes, and basophilic stippling. Polychromasia is uncommon, and the reticulocyte count is inappropriately low. Leukopenia and thrombocytopenia may occur. The presence of oval-shaped macrocytes, giant platelets, and hypersegmented neutrophils (>5% of cells with five lobes or any with six or more lobes) strongly suggests a megaloblastic process.

The clinical features of folic acid and vitamin B₁₂ deficiency include signs of a hemolytic anemia such as pallor, mild jaundice, and exercise intolerance. The anemia may develop very slowly so that exercise intolerance is minimal even at very low hemoglobin levels. In addition, vitamin B₁₂ deficiency can result in GI complaints, such as diarrhea and weight loss, and episodes of glossitis, leading to erythema, soreness, and, eventually, to loss of papillae, causing a smooth, beefy red tongue. Most importantly, vitamin B₁₂ deficiency impairs nerve myelination, leading to degeneration of white matter in the brain and in both the dorsal and lateral columns of the spinal cord (subacute combined degeneration). Dorsal column involvement causes diminished vibratory sensation, creating numbness and tingling in the feet and hands (stocking-glove neuropathy), and decreased proprioception, producing gait difficulties and a positive Romberg sign. Lateral column damage causes limb weakness, spasticity, hyperactive reflexes, and a positive Babinski sign. Evidence of cerebral involvement includes depression, dementia, confusion, delusions, and hallucinations. With regard to folic acid deficiency, the symptoms of anemia are similar to those of B₁₂ deficiency, but the neurologic changes do not occur, as folic acid is not involved in myelination in adults.

Folic acid deficiency is usually caused by an inadequate diet. Rich sources are fruits, vegetables, and animal protein, but cooking easily destroys folate. Furthermore, the body stores of folate are small, and only a few months of poor intake, caused by food fads, ignorance, poverty, or alcoholism, are necessary before anemia develops. A major concern for folate deficiency is in pregnancy where the folate requirement is increased and inadequate folic acid intake results in low birth rates and neural tube defects.

Alcohol intake compounds the problem by increasing urinary folate excretion, impeding liver storage of folate, and decreasing folate absorption, which occurs primarily in the duodenum and jejunum. Disorders affecting these portions of the intestine, such as sprue, lymphoma, amyloidosis, and Crohn’s disease, can cause folate malabsorption. Folic acid deficiency also can occur when the body’s demand for it increases, not only in pregnancy but also in any condition associated with increased cell turnover, such as chronic hemolytic anemias, acute exacerbations of hemolytic anemia, leukemia, and exfoliative dermatitis. Some medications, such as methotrexate and trimethoprim, cause folate deficiency by altering its metabolism. The measurement of serum folate level is very sensitive to recent folate intake and thus testing should be performed on fasting samples. Although some have advocated for the use of red cell folate as a more accurate indicator of long-term folate storage, this test has a number of analytical issues (including high coefficient of variation) that make it of limited value; published studies have shown that red cell folate performs no better than fasting serum folate measurements.¹⁹ Although folic acid deficiency is possible, recent studies have shown that the introduction of mandatory folic acid supplementation in cereal grain in the United States has drastically reduced the prevalence of folic acid deficiency in the population and has been associated with a documented reduction in neural tubular defects and as well as macrocytic anemia due to folate deficiency.

Vitamin B₁₂ is the common name for either cyanocobalamin or hydroxycobalamin; these compounds are cobalamins, which include a group of molecules with a central cobalt atom bound by tetrapyrrole rings. In serum, methylcobalamin is the major form, whereas deoxyadenosylcobalamin is the primary form found in the cytosol. Cobalamins are not synthesized by humans but must be acquired in the diet, mainly meat, poultry, seafood, and dairy. The recommended daily allowance of cobalamins is 5 µg, and the total body content is about 2 to 5 mg, about 1 mg being present in the liver. Because the daily losses are minute, cobalamin deficiency from diet alone takes years and occurs almost exclusively in strict vegetarians, e.g., vegans. Therefore, in adults, the main cause of vitamin B₁₂ deficiency is impaired absorption. Food cobalamin binds to a substance in gastric juice called R protein (haptocorrin) and is released by pancreatic enzymes when it reaches the second portion of the duodenum. Vitamin B₁₂ then binds to intrinsic factor, a glycoprotein produced by the parietal cells in the fundus and cardia of the stomach, and the complex then travels to the distal ileum. Intrinsic factor receptors are present on the ileal mucosa, especially in the terminal ileum, where cobalamin is specifically absorbed from the cobalamin-intrinsic factor complex in a receptor-mediated process.

A major cause of cobalamin deficiency is from reduced intrinsic factor activity. This can occur from destruction of parietal cells by gastric resection or from chronic gastric inflammation due to autoimmune mechanisms that lead to mucosal atrophy in the stomach’s fundus and body. The latter disorder, pernicious anemia, occurs primarily in older adults, often with a family history of the disease; northern European descent; or concurrent autoimmune disorders, such as Graves disease, vitiligo, or Hashimoto thyroiditis. About 90% of patients with pernicious anemia have antibodies to parietal cells, compared with 5% in the general population, and approximately 60% have antibodies to intrinsic factor, which are rare in healthy people.

Disorders of the ileum, such as Crohn’s disease or lymphoma, can also cause cobalamin deficiency because of vitamin B₁₂ malabsorption. Malabsorption of cobalamin also can occur with pancreatic insufficiency, when inadequate pancreatic enzymes fail to release cobalamin from the R proteins. Another cause of cobalamin deficiency is its consumption in the small intestine by a fish tapeworm, Diphyllobothrium latum, found mostly in fish from Canada, Alaska, and the Baltic Sea, and acquired by humans by eating undercooked fish or fish roe. Excessive intestinal bacteria in diseases associated with impaired motility, or intestinal stasis, such as systemic sclerosis, extensive diverticula, or surgical blind loops, also can consume enough cobalamin to cause vitamin B₁₂-related macrocytic anemia.

Assessment of vitamin B₁₂ levels is more complicated than folic acid levels. The Schilling test had previously been used to help distinguish among the causes of cobalamin deficiency. Unfortunately, because of technical difficulties producing and measuring the cobalt radioisotopes required, this test is no longer available in the United States. The most common tests offered clinically are quantitative immunoassays. These assays typically first convert serum cobalamins to cyanocobalamin and then quantify this species. Recently, approximation of vitamin B₁₂ levels has been accomplished by measuring serum homocysteine and methylmalonic acid (MMA). Homocysteine is increased in both vitamin B₁₂ and folate deficiency because methionine synthesis is impaired by deficiency of either. This testing has higher sensitivity because the increase occurs earlier in nutritional deficiency. A cobalamin-dependent, but folate-independent, enzymatic reaction leads to increased serum levels of MMA in cases of vitamin B₁₂ deficiency. This finding also tends to precede changes in serum cobalamin. Accordingly, measurement of both homocysteine and MMA can reliably detect, and distinguish between, folate and cobalamin deficiencies. When both are elevated, cobalamin deficiency is confirmed, although concurrent folate deficiency is possible. If homocysteine is elevated and MMA is normal, folate deficiency is likely. If both are normal, deficiency of either is highly
improbable. If cobalamin deficiency is present, the presence of antibody against intrinsic factor confirms the diagnosis of pernicious anemia.

Macrocytic anemia also occurs frequently in MDS because of abnormal erythrocyte maturation and ineffective erythropoiesis caused by a clonal expansion of abnormal hematopoietic stem cells. Another source of mild macrocytosis is reticulocytosis, an increase in young erythrocytes released early from the marrow, in hemolysis. Reticulocytes may be presumptively identified on peripheral blood smears because they still contain residual blue-staining nuclear RNA, as well as red-staining hemoglobin, producing a purplish color with the Romanowsky stains ordinarily used for peripheral blood films. The presence of a few reticulocytes is common on normal smears, but numerous polychromatophilic cells, representing a stress erythropoiesis, can result in mild macrocytosis. Even more immature normoblasts that have not yet shed their nuclei (nucleated RBC) can be detected in severe anemias, especially in chronic severe hemolytic anemias. Macrocytosis, usually with an MCV of 100 to 110, but typically without anemia, is present in about 60% of alcoholics. The cause may be deficiency of folic acid or vitamin B₁₂ due to poor nutrition, but this anemia can also be a direct effect of ethanol byproducts (acetaldehyde) to depress hematopoiesis. In addition, there are a number of inherited dyserythropoietic anemias or marrow failure states, which are characterized by macrocytosis, such as Fanconi syndrome, dyskeratosis congenita, or congenital dyserythropoietic anemia.

Anemia typically occurs with chronic renal disease only after the creatinine clearance decreases below 40 mL/min, which corresponds to a serum creatinine of about 2.5 mg/mL. Studies have shown that anemia of chronic renal disease can be more accurately assessed with cystatin C compared to creatinine. The anemia tends to worsen as the renal function decreases, but it usually stabilizes at a hematocrit of 15% to 30%. The cause of the kidney disease is not usually important in determining the severity of anemia, but anemia is typically less severe with polycystic kidney disease possibly due to erythropoietin production by the renal cyst epithelial cells. Several factors contribute to the anemia of chronic renal disease, the most important, however, being inadequate renal production of erythropoietin, a glycoprotein hormone synthesized in the kidney and responsible for the proliferation, maturation, and differentiation of erythrocytes in the bone marrow.²¹ Erythropoietin is also an erythroblast survival factor. In addition, red cell survival is shortened in uremia, and various toxins ordinarily excreted by the kidney accumulate in the serum and appear to depress erythropoiesis. Replacement by recombinant erythropoietin or erythropoietin-stimulating agents has become central to the management of anemia in chronic renal disease.²²,²³

The anemia is normochromic, normocytic, and most RBCs are unremarkable on the peripheral smear. Burr cells (echinocytes), however, may form via unknown mechanisms, and sometimes schistocytes appear. Both burr cells and schistocytes have shortened survival.

Anemia, usually normocytic, occurs in several endocrine disorders. About 30% of patients with hypothyroidism have anemia, and about one-third of these are macrocytic. The anemia, usually mild, seems to relate to the hormone deficiency itself, and its severity is related to the duration and degree of hypothyroidism. Approximately 10% to 25% of patients with hyperthyroidism, usually with severe, prolonged disease, are anemic. The mechanism is uncertain.

Most patients with adrenal insufficiency have anemia, usually normocytic, normochromic. In those with autoimmune causes, pernicious anemia, producing a macrocytic anemia, is present in about 10%. Androgen deficiency also is a cause of normochromic, normocytic anemia. Hypopituitarism causes anemia through deficiencies of the previously mentioned thyroid, adrenal, and androgenic hormones.

A small number of patients with hyperparathyroidism have a normocytic, normochromic anemia, with bone marrow examinations typically demonstrating fibrosis. The increased parathyroid hormone may also decrease erythropoiesis.

ACD is quite common with infections, noninfectious inflammatory states that last more than 1 or 2 months, or
malignancy.²⁰ In about 25% of cases of ACD, anemia may be associated with congestive heart failure, hypertension, or diabetes mellitus. The anemia is mild to moderate, usually with a hemoglobin of about 10 mg/dL, but, in about 20% of patients, it can be more severe. The RBCs are typically normocytic, normochromic, but in about 30% they are mildly microcytic (usually 70-79 fL), and in about 50% they are hypochromic (MCHC 26-32). On peripheral smear, the RBC may display mild poikilocytosis and anisocytosis, but markedly small and thin cells, often seen in iron deficiency, are absent.

In ACD, both serum iron and iron-binding capacity are low, unlike iron deficiency in which low serum iron and elevated iron-binding capacity coexist. As in iron deficiency, the iron saturation may be below 10%. The serum ferritin, which is characteristically less than 15 µg/L in iron deficiency, is at least 100 µg/L in ACD and usually much higher unless there is coexisting iron deficiency. Circulating transferrin receptor levels are normal in ACD, but elevated in iron deficiency. Bone marrow iron is present in ACD, and a trial of oral iron therapy does not correct the anemia.

The factors linking different anemias of chronic disease are inflammatory cytokines that negatively affect erythropoiesis, in particular interferon and interleukin-6.²⁴,²⁵ They impair the proliferation and differentiation of erythroid precursors, diminish erythropoietin production, and decrease the bone marrow response to erythropoietin. Cytokines affect iron metabolism by increasing iron retention in the bone marrow and in macrophages by stimulating hepcidin synthesis, thus decreasing iron availability for production of hemoglobin in maturing erythroid precursors. In addition, RBC lifespan is moderately decreased in ACD.

In this rare disorder, a normocytic anemia with diminished reticulocytes (<1%) and absence of polychromasia on the peripheral blood smear is accompanied by a marked diminution to absence of erythroblasts in the bone marrow (<0.5% of the marrow differential count), despite the presence of normal numbers of megakaryocytes and myeloid precursors and normal overall marrow cellularity. Pure red cell aplasia may develop without apparent cause or be associated with a wide variety of systemic diseases. It may be associated with viral infections such as parvovirus B19, in which case giant erythroblasts containing viral inclusions may be seen in the bone marrow. Pure red cell aplasia is also associated with numerous conditions characterized by disturbed immunity: in about 5% of patients with thymoma (accounting for approximately 10% of cases of pure red cell aplasia), hematologic malignancies, especially chronic lymphocytic and large granular lymphocytic leukemias; rheumatologic diseases (such as Sjögren syndrome and systemic lupus erythematosus), and even solid tumors. Numerous medications have been implicated, including phenytoin, azathioprine, and isoniazid. Sometimes, pure red cell aplasia occurs during pregnancy without any apparent explanation and typically disappears following delivery. In many patients, no cause is found but an immunoglobulin G (IgG) that inhibits erythropoiesis is present in the serum.

In aplastic anemia, pancytopenia in the peripheral blood and hypocellularity of the bone marrow accompany the anemia. Clinical presentations usually are for anemia or bleeding; infections are less common until the late stages. Certain types, such as Fanconi anemia, are hereditary, whereas some acquired aplastic anemias may have identifiable causes, such as exposure to medications, (chloramphenicol) or chemicals (benzene), or infections with certain viruses (hepatitis). Aplastic anemia may develop in patients with PNH or in other bone marrow failure states (e.g., dyskeratosis congenita) or as a complication of certain rheumatic diseases, such as eosinophilic fasciitis, systemic lupus erythematosus, or Sjögren syndrome. In the hemophagocytic syndrome, most commonly associated with viral infections or certain malignancies, pancytopenia, fever, hepatosplenomegaly, and lymph node enlargement occur, and the bone marrow, often hypocellular, shows macrophages ingesting erythrocytes.

Acquired aplastic anemia in the majority of cases appears to result from an immunologic reaction against an unknown target that damages bone marrow stem cells. The use of intensive immunosuppressive therapy during the past 30 years has resulted in marked improvement in survival,²⁶ which has further improved since the introduction of the thrombopoietin agonist eltrombopag which has been able to stimulate growth of megakaryocytic and erythroid precursors.²⁷ Clonal hematopoiesis is common in aplastic anemia (documented in 30%-40% of cases) as is acquired mutations associated with MDS/acute myeloid leukemia, which, as well as PNH occur in increased frequency in patients with preceding aplastic anemia.²⁸