Macrocytosis/Macrocytic anemia

INTRODUCTION — Macrocytosis is a descriptive term for red blood cell (RBC) size larger than the normal range. It may be caused by abnormalities of RBC production in the bone marrow, altered RBC membrane composition, or an increase in the percentage of reticulocytes, which are larger than mature RBCs. This topic discusses causes of macrocytosis and macrocytic anemia. Additional topics discuss the following:

●Microcytosis/microcytic anemia – (See "Microcytosis/Microcytic anemia".)

●General anemia evaluation

•Child – (See "Approach to the child with anemia".)

•Adult – (See "Diagnostic approach to anemia in adults".)

●Vitamin B12 and folate deficiency – (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency".)

●Myelodysplastic syndromes – (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

●Hemolytic anemias – (See "Overview of hemolytic anemias in children" and "Diagnosis of hemolytic anemia in adults".)

●Alcohol-induced anemia – (See "Hematologic complications of alcohol use".)

DEFINITIONS AND CLASSIFICATION — Macrocytosis is strictly a morphologic term and does not imply a specific pathophysiology. Macrocytosis can be documented using the mean corpuscular volume (MCV; also called mean cell volume) from an automated hematology instrument, measured in femtoliters (fL; 10^-15 liter) or by observing larger-than-normal RBCs on the peripheral blood smear.

●Increased MCV – Macrocytosis is defined as an MCV above the upper limit of normal, which varies by age (table 1):

•Preterm infants born at ≤25 weeks of gestation – 119 ± 7 fL

•Term newborns (cord blood) – 106 ± 4 fL

•Infants and young children – 90 fL

•Adults – 96 to 100 fL (the higher value may be more appropriate for older adults)

The normal range is defined such that approximately 2.5 percent of the healthy population will have an MCV >96 fL; <0.5 percent will have a MCV >100 fL. The MCV is reported routinely from automated hematology instruments along with other RBC indices (table 2), and high values are flagged (often without adjustment for age).

●RDW – The RBC distribution width (RDW) is a measure of the variation in RBC sizes. A single uniform population of RBCs will have a normal RDW regardless of whether the absolute MCV is normal or abnormal. A population of RBCs with varied sizes or two populations of RBCs (eg, small RBCs plus reticulocytes) will have a large RDW (above approximately 14.5 percent when the coefficient of variation method is used).

RDW is typically increased with concomitant disorders that cause macrocytosis and microcytosis (eg, vitamin B12 or folate deficiency plus iron deficiency) [1]. This finding suggests that it may be worthwhile to measure all of these nutrients when the RDW is increased.

●Large RBCs on peripheral blood smear – The normal RBC diameter on the peripheral blood smear is 7 to 8 microns, approximately the size of the nucleus of a small lymphocyte (picture 1); RBCs that have a greater diameter are considered macrocytic. (See "Evaluation of the peripheral blood smear".)

Other findings on the peripheral blood smear that may be helpful in determining the cause of macrocytosis are discussed below. (See 'Clues from the CBC and blood smear' below.)

Megaloblastic anemia (or megaloblastic changes) describes a subset of macrocytic anemias in which the increased size of RBCs is caused by abnormal cell division in RBC precursors in the bone marrow (see 'Pathophysiology' below). The major associated finding on the blood smear is multilobed neutrophils. Typical bone marrow findings are described separately. (See "Evaluation of bone marrow aspirate smears", section on 'Megaloblastic changes'.)

PATHOPHYSIOLOGY — Red blood cells (RBCs) leave the bone marrow as reticulocytes, which are macrocytic. All other macrocytic RBCs are formed as a consequence of inherited or acquired abnormalities in RBC maturation, nucleic acid metabolism, membrane composition, cell water content, or a combination of these factors (table 3) [2-4].

●Increased reticulocytes – Reticulocytes are immature RBCs that have ejected their nuclei but retain some messenger RNA, making them appear slightly bluish/purplish (polychromatophilic) on a standard Wright-Giemsa stain. Reticulocytes in the bone marrow have a volume of 120 to 150 fL. After they enter the circulation they undergo loss of water and membrane and have a typical mean corpuscular volume (MCV) of approximately 103 to 126 fL, which is averaged into the MCV calculation by the automated instrument [4].

In severe anemia the signal from erythropoietin to increase RBC production may cause developing RBCs to skip cell divisions and may cause reticulocytes to enter the circulation sooner than they otherwise would (eg, "stress" or "shunt" reticulocytes) and to circulate for longer periods of time, which may further increase the MCV (figure 1) [5].

Reticulocytes can be recognized on the peripheral blood smear as larger RBCs that lack central pallor, are oval to irregular in outline, and have a blue tint (ie, polychromatophilia) (picture 2).

Reticulocytosis is a normal physiologic response to anemia of any cause. It will occur unless the ability of the bone marrow to respond to anemia is blunted by other factors such as cytokines in anemia of chronic disease or vitamin or iron deficiency. Increased reticulocytes are a hallmark of hemolytic anemia and bone marrow recovery following a bone marrow insult, repletion of a deficient vitamin (B12 or folate) or iron, or hematopoietic cell transplant (HCT). Because reticulocytes are larger than mature RBCs, their increasing percentage of total RBCs will raise the MCV proportionately. (See "Diagnosis of hemolytic anemia in adults", section on 'High reticulocyte count'.)

●Abnormal RBC development/megaloblastic anemia – Developing RBC precursor cells in the bone marrow divide rapidly and become progressively smaller as they mature (figure 2). In conditions in which cell division is impaired, such as lack of a nutrient required for DNA synthesis, the synchrony between nuclear and cytoplasmic maturation may be lost, resulting in large, immature nuclei relative to the cytoplasm, along with other megaloblastic changes (eg, binucleate cells). The ultimate basis for production of the megaloblast is inadequate conversion of deoxyuridine to thymidylate (thymidine), which leads to slowing of DNA synthesis and delayed nuclear maturation. (See "Causes and pathophysiology of vitamin B12 and folate deficiencies", section on 'Hematopoiesis'.)

Drugs and nutrient deficiencies that cause megaloblastic anemia will affect all developing blood cells, so similar abnormalities in white blood cell and platelet precursors can be seen in the bone marrow. The peripheral blood smear may show multilobed neutrophils, and the complete blood count may show mild pancytopenia. (See 'Clues from the CBC and blood smear' below.)

●Abnormal RBC membrane – The RBC membrane is a complex lipid bilayer with integral membrane proteins attached to an underlying circumferential cytoskeleton that gives it mechanical stability and deformability. Certain inherited and acquired conditions can alter the membrane composition, and in some cases result in corresponding increases in cell volume. Examples include liver disease with increased membrane cholesterol content, and hereditary stomatocytosis with impaired volume regulation. (See 'Causes of macrocytosis/macrocytic anemia' below and "Hematologic complications of alcohol use" and "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

The presence of macrocytosis, along with the other RBC indices, the complete blood count, and the peripheral blood smear review, can be extremely helpful in evaluating the cause of anemia, or the early stage of a condition that is likely to cause anemia if untreated. (See 'Evaluation' below.)

CAUSES OF MACROCYTOSIS/MACROCYTIC ANEMIA

Overview/common causes — Causes of macrocytic anemia include the following (table 3):

●Reticulocytosis

•Hemolytic anemia

•Bone marrow recovery after chemotherapy or hematopoietic cell transplant (HCT)

•Increased erythropoiesis following administration of erythropoietin, repletion of iron, vitamin B12, or folate

•Recovery from bleeding episode

●Megaloblastic anemia

•Vitamin B12 or folate deficiency – (See 'Megaloblastic anemia' below.)

•Medications that interfere with DNA synthesis – (See 'Drug-induced macrocytosis' below.)

•Thiamine-responsive megaloblastic anemia – (See 'Megaloblastic anemia' below.)

●Multifactorial

•Alcohol – (See 'Alcohol/liver disease' below.)

•Liver disease – (See 'Alcohol/liver disease' below.)

•HIV infection (and therapy) – (See "HIV-associated cytopenias".)

•Myelodysplastic syndrome – (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

•Hereditary stomatocytosis – (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

•Hypothyroidism – (See "Clinical manifestations of hypothyroidism", section on 'Anemia'.)

•Pregnancy – (See "Maternal adaptations to pregnancy: Hematologic changes".)

•Aplastic anemia – (See "Aplastic anemia: Pathogenesis, clinical manifestations, and diagnosis".)

•Multiple myeloma – (See "Multiple myeloma: Clinical features, laboratory manifestations, and diagnosis".)

•Bariatric surgery (or, less commonly, other gastrointestinal surgery) – (See "Bariatric surgery: Postoperative nutritional management".)

•Down syndrome – (See "Down syndrome: Clinical features and diagnosis".)

•Copper deficiency (although often causes normocytic anemia, and microcytic cells may be seen) – (See "Sideroblastic anemias: Diagnosis and management", section on 'Copper deficiency' and "Overview of dietary trace elements", section on 'Copper'.)

In the multifactorial conditions, the full mechanism may not be elucidated; there may be a combination of changes in the bone marrow, medication effects, and in some cases mild reticulocytosis.

Reticulocytosis — As noted above, an increase in the reticulocyte count and reticulocyte percentage is associated with a corresponding increase in mean corpuscular volume (MCV). (See 'Pathophysiology' above.)

Reticulocytosis can occur in any of the following settings:

●Any hemolytic anemia – (See "Overview of hemolytic anemias in children" and "Diagnosis of hemolytic anemia in adults".)

●Repletion of iron, vitamin B12, folic acid, or copper – (See "Treatment of iron deficiency anemia in adults", section on 'Response to iron supplementation'.)

●Recovery from bleeding or transient bone marrow aplasia (eg, following parvovirus infection) – (See "Diagnostic approach to anemia in adults".)

●Any other condition associated with increased erythropoietin, such as congenital heart disease, erythropoietin-secreting tumors, or "blood doping" – (See "Regulation of erythropoiesis".)

Megaloblastic anemia — As noted above, megaloblastic anemia is a form of macrocytic anemia in which nucleic acid metabolism is impaired, leading to reduced efficiency of cell division and nuclear-cytoplasmic dyssynchrony. (See 'Pathophysiology' above.)

Causes of megaloblastic anemia include deficiency of vitamin B12, folate, or copper, as well as a number of medications that interfere with DNA synthesis.

●Vitamin B12 deficiency has numerous causes related to its complex absorption (table 4). (See "Treatment of vitamin B12 and folate deficiencies" and "Causes and pathophysiology of vitamin B12 and folate deficiencies".)

●Folate deficiency has become increasingly rare in countries in which certain foods are routinely supplemented with folate. However, it may occur in countries where routine folic acid supplementation of foods does not occur, or in individuals with severe malnutrition or certain dietary practices (table 5). Folate deficiency was also common in individuals with increased folate requirement such as those with chronic eczema (21 of 28 individuals [75 percent] in one series) in the era before routine supplementation was initiated [6]. (See 'Clues from the history and examination' below.)

Relative folate deficiency is a common finding in all hemolytic conditions with increased bone marrow compensation and reticulocytosis. (See "Warm autoimmune hemolytic anemia (AIHA) in adults", section on 'Folic acid'.)

●Thiamine-responsive megaloblastic anemia syndrome (TRMA) is an exquisitely rare autosomal recessive disorder characterized by anemia, diabetes, and early-onset sensorineural hearing loss [7-9]. Ocular and cardiac manifestations have also been reported [10,11]. Various mutations in a gene encoding a thiamine transporter (SLC19A2) have been identified. Disease onset is in infancy through adolescence. The anemia improves with administration of high doses of thiamine (vitamin B1). (See "Overview of water-soluble vitamins", section on 'Vitamin B1 (thiamine)'.)

●As a result of reductions in vitamin B12 and folate deficiency, drug-induced megaloblastic anemia has accounted for a larger proportion of megaloblastic anemia in some settings [12]. (See 'Drug-induced macrocytosis' below.)

Myelodysplastic syndrome — Myelodysplastic syndrome (MDS) is characterized by anemia in almost all cases, which may be macrocytic or normocytic. The reticulocyte response is generally inappropriately low. The RDW is often increased. Details are presented separately. (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

Drug-induced macrocytosis — Medications potentially implicated in causing macrocytic anemia include the following [12-23]; many of these cause megaloblastic anemia, although some cause hemolytic anemia, and the resulting reticulocytosis increases the MCV:

●Allopurinol (megaloblastic changes)

●Aminosalicylic acid (reduces folate absorption)

●Antacids (reduce vitamin B12 absorption)

●Ampicillin and other penicillins (reduces folate absorption)

●Azathioprine (megaloblastic changes)

●Capecitabine (megaloblastic changes)

●Chloramphenicol (reduces folate absorption)

●Cladribine (megaloblastic changes)

●Cytosine arabinoside (ara-C) (megaloblastic changes)

●Dapsone (hemolysis in individuals with G6PD deficiency)

●Erythromycin (reduces folate absorption)

●Estrogens or hormonal contraceptives (reduces folate absorption)

●Fludarabine (megaloblastic changes)

●Fluorouracil (megaloblastic changes)

●Gadolinium (megaloblastic changes)

●Gemcitabine (megaloblastic changes)

●Histamine H2 receptor antagonists (eg, cimetidine, famotidine, nizatidine; reduce vitamin B12 absorption)

●Hydroxyurea (megaloblastic changes)

●Imatinib (unknown; possibly inhibition of c-kit)

●Lamivudine (megaloblastic changes)

●Leflunomide (megaloblastic changes)

●Mercaptopurine (megaloblastic changes)

●Metformin (interferes with folate metabolism)

●Methotrexate (megaloblastic changes)

●Methylene blue (hemolysis in individuals with G6PD deficiency)

●Mycophenolate mofetil (megaloblastic changes)

●Nitrofurantoin (reduces folate absorption, hemolysis in individuals with G6PD deficiency)

●Nitrous oxide (megaloblastic changes)

●Pegloticase (hemolysis in individuals with G6PD deficiency)

●Pentostatin (megaloblastic changes)

●Phenytoin (interferes with folate metabolism)

●Primaquine (hemolysis in individuals with G6PD deficiency)

●Primidone (unknown)

●Proton pump inhibitors (eg, omeprazole, lansoprazole; reduce B12 absorption)

●Stavudine (megaloblastic changes)

●Rasburicase (hemolysis in individuals with G6PD deficiency)

●Sunitinib (unknown; possibly inhibition of c-kit)

●Teriflunomide (megaloblastic changes)

●Tetracyclines (reduces folate absorption)

●Thioguanine (megaloblastic changes)

●Triamterene

●Trimethoprim (megaloblastic changes)

●Valproic acid

●Zidovudine (megaloblastic changes)

Of these, we most commonly see macrocytosis from hydroxyurea for sickle cell disease, chemotherapeutic agents, and antiretroviral therapy (ART) in patients with HIV infection. (See "Overview of antiretroviral agents used to treat HIV", section on 'Non-nucleoside reverse transcriptase inhibitors (NNRTIs)'.)

A more extensive discussion of drugs that cause hemolytic anemia is presented separately. (See "Drug-induced hemolytic anemia".)

Alcohol/liver disease — Alcohol is a common cause of macrocytosis and macrocytic anemia. Regular ingestion of 80 grams of alcohol each day (eg, one bottle of wine) is sufficient to cause this effect [2]. Even before anemia appears, chronic alcohol users have a mild macrocytosis (typical mean corpuscular volume [MCV] between 100 and 110 fL) [24-26]. (See 'Clues from the history and examination' below.)

Alcohol-induced macrocytosis occurs even in patients who are folate and cobalamin replete and do not have liver disease. Abstinence from alcohol results in resolution of the macrocytosis within two to four months. Return of the MCV to normal also confirms the diagnosis. (See "Hematologic complications of alcohol use", section on 'Anemia'.)

In a series of 168 chronic alcohol users from the 1970s, half had macrocytosis with a mean MCV of 98 fL; in those with concomitant liver disease, 64 percent had macrocytosis with a mean MCV of 100 fL [27]. In a 1990 series of 73 individuals with macrocytosis identified from a primary care practice, macrocytosis was ascribed to alcohol in 47 (64 percent) [28]. In another primary care series of 300 individuals with macrocytosis, approximately a third of the women and four-fifths of the men were alcohol abusers [29].

The mechanism of alcohol-induced macrocytosis is unknown. Acetaldehyde, a metabolic breakdown product of alcohol, is capable of inducing membrane changes in red blood cell (RBC) precursors and circulating RBCs of individuals with alcohol-associated macrocytosis, through the in vivo production of acetaldehyde adducts [30]. Acetaldehyde also interferes with cell division and may increase cell volume by this mechanism [31]. In addition, one study has shown a significant correlation between the MCV in heavy drinkers (>300 grams of alcohol per week) and the genotype of the aldehyde dehydrogenase responsible for ethanol metabolism [32]. (See "Pathogenesis of alcohol-associated liver disease".)

Other forms of liver disease not related to alcohol may cause macrocytosis by effects on the lipid composition of the RBC membrane [2]. (See "Cirrhosis in adults: Etiologies, clinical manifestations, and diagnosis", section on 'Hematologic abnormalities'.)

Vitamins and trace minerals — Gastric bypass surgery is associated with micronutrient malabsorption related to the physical bypass or removal of absorptive cells as well as reduced dietary intake and other mechanisms; the risk of nutrient deficiencies is greater with bypass procedures than restrictive procedures such as gastric banding. Patients may develop combined deficiencies of vitamin B12, folate, and iron if they do not receive supplementation. Copper deficiency can also occur. (See "Bariatric surgery: Postoperative nutritional management".)

These vitamin and mineral deficiencies may also arise in other settings such as parenteral nutrition without adequate supplementation or the use of infant formulas that lack these constituents [33]. Other malabsorption syndromes or the use of chelating agents may also lead to single or multiple deficiencies. (See "Chronic complications of short bowel syndrome in children", section on 'Nutritional complications' and "Approach to the adult patient with suspected malabsorption" and "Overview of the treatment of malabsorption in adults" and "Parenteral nutrition in infants and children".)

Excessive ingestion of zinc may lead to copper deficiency, including the common use of zinc as a denture adhesive. (See "Copper deficiency myeloneuropathy", section on 'Causes of acquired copper deficiency'.)

As noted above, nitrous oxide can cause vitamin B12 deficiency. (See 'Drug-induced macrocytosis' above.)

In addition to acquired micronutrient and trace mineral deficiencies, rare genetic conditions affecting vitamin and mineral metabolism may lead to macrocytic anemia. (See "Treatment of vitamin B12 and folate deficiencies" and "Copper deficiency myeloneuropathy".)

Hypothyroidism — Macrocytosis can occur in the setting of hypothyroidism. The mechanism is unclear and may be multifactorial in some patients.

In a series of 202 patients with hypothyroidism from 1976, anemia was present in 53 (26 percent) [1]. A subset of 53 individuals who were analyzed in more detail had macrocytosis despite normal levels of vitamin B12, folate, and iron. Of these, 13 (25 percent) had anemia that resolved upon treatment with thyroxine. Patients with autoimmune hypothyroidism may have concomitant vitamin B12 deficiency caused by autoantibodies to gastric parietal cells. In the series of 202 patients, 10 of 118 (8.5 percent) had concomitant pernicious anemia.

The typical MCV in hypothyroidism is mildly increased (in the range of 90 to 100 fL).

Macrocytosis with mild or no anemia — In addition to mild forms of the conditions discussed above, macrocytosis may be seen in newborns (in which case the upper limit of normal for the MCV is age-adjusted), during pregnancy, and in some older adults.

A full evaluation including history, physical examination, and extensive laboratory testing as discussed below often reveals no abnormalities, and the peripheral blood smear shows only macrocytic RBCs (see 'Evaluation' below). The underlying concern is that this may be a harbinger of myelodysplastic syndrome (MDS). Some consultants proceed to a bone marrow examination with cytogenetics, myelodysplasia fluorescence in situ hybridization (FISH) panel, and comprehensive somatic mutation panel. There are no data showing that this approach results in a clarifying diagnosis. Others prefer to watch and follow. Studies to clarify the benefits of each approach are needed. (See 'Age of the patient' below.)

EVALUATION

Initial considerations — Before embarking on an extensive evaluation, it is appropriate to confirm that macrocytosis is real and not artifactual. Causes of spurious macrocytosis include clumping of red blood cells (RBCs) (eg, by cold agglutinins), osmotic swelling of RBCs in the setting of severe hyperglycemia (eg, blood glucose levels above 625 mg/dL [35 mmol/L]) or prolonged storage, or high concentrations of certain forms of EDTA as an anticoagulant in the blood collection tube (although EDTA can also cause microcytosis) [34-39]. (See "Automated complete blood count (CBC)", section on 'Sample collection and processing'.)

These possibilities can generally be eliminated by repeating the complete blood count and reviewing the peripheral blood smear. If cold agglutinins are suspected, the sample can be warmed prior to analysis.

While the mean value for the MCV in adults is 88 fL, a number of studies of non-anemic ambulatory adults over age 65 have reported mean values ranging from 91 to 93 fL, with up to 10 percent having a MCV >96 fL, and 2 to 6 percent having a MCV >100 fL [40-43]. Thus, we generally do not initiate an intensive evaluation of older adults for an MCV in the range between 96 and 100 fL, unless the patient is anemic or the MCV value represents a meaningful change from the individual’s baseline. (See 'Macrocytosis with mild or no anemia' above.)

Clues from the history and examination — Patient age, medical history, alcohol use, and medications are especially important in the evaluation of macrocytosis (algorithm 1).

Age of the patient — As noted above, normal values for the MCV are higher in neonates (especially those born preterm) and infants; age-appropriate values should be consulted (table 1) [44-46]. (See "Approach to the child with anemia", section on 'Red blood cell indices'.)

In children, common causes of macrocytosis/macrocytic anemia include drugs and conditions present from birth. A 1992 retrospective series of 146 children with macrocytosis identified the following as common causes [46]:

●Medications (eg, anticonvulsants, zidovudine, immunosuppressive agents) – 51 (35 percent)

●Congenital heart disease – 20 (14 percent)

●Down syndrome – 12 (8 percent)

●Reticulocytosis – 11 (8 percent)

●Bone marrow failure/dysplasia – 6 (4 percent)

Rare causes of macrocytosis in children include inherited defects in vitamin B12 or thiamine metabolism that cause megaloblastic anemia. (See 'Megaloblastic anemia' above.)

In adults, common causes of macrocytosis/macrocytic anemia include alcohol use or abuse, liver disease, thyroid disease, and megaloblastic anemias [22,47]. Hemolytic anemias with reticulocytosis and bone marrow failure states including myelodysplastic syndromes (MDS), aplastic anemia, and paroxysmal nocturnal hemoglobinuria (PNH) are also seen. (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

Macrocytosis is a common presenting feature of MDS, especially in older adults [48]. In one series of bone marrow examinations that included mostly older adults, a score including unexplained macrocytosis, abnormal RBC distribution width (RDW), and elevated lactate dehydrogenase (LDH) identified 114 of 313 (36 percent) as having MDS [40]. Many did not have an ultimate diagnosis even after bone marrow evaluation. As noted above, there are not good data suggesting that an extensive evaluation for MDS improves outcomes in this setting. (See 'Macrocytosis with mild or no anemia' above.)

Medications — Typically, medications that cause megaloblastic anemia will be obvious from the medical history. These include hydroxyurea, certain chemotherapy agents, and combination antiretroviral therapy (ART) for HIV infection. (See 'Drug-induced macrocytosis' above.)

Other medications that may be less obvious causes of macrocytosis include those that cause hemolysis in individuals with glucose-6-phosphate dehydrogenase deficiency (G6PD), over-the-counter antacids or medications to reduce gastric acid production, which may reduce vitamin B12 absorption, and certain antibiotics. (See 'Drug-induced macrocytosis' above.)

The use of nitrous oxide anesthesia (or as a recreational drug) has also been reported to cause macrocytic anemia, possibly by inducing transient vitamin B12 deficiency.

Alcohol — Alcohol use is considered to be under-reported and should be specifically queried in the context of macrocytosis. (See 'Alcohol/liver disease' above.)

Diet — The dietary history may be helpful in identifying the use of an infant formula or formula substitute that may lack sufficient nutrients, strict vegetarians who do not take supplemental vitamin B12, poor nutritional intake, or other unusual dietary practices. Excessive intake of zinc has been reported to cause copper deficiency. (See 'Vitamins and trace minerals' above.)

Clues from the CBC and blood smear — The severity of macrocytosis, presence of other cytopenias, and specific RBC morphologies on the peripheral blood smear may be useful in narrowing the possible causes of macrocytosis and in guiding the evaluation (algorithm 1).

●Severe macrocytosis (MCV above 110 to 115 fL) is associated almost exclusively with megaloblastic anemias. This was illustrated in a series of 109 individuals with MCV >130 fL from 2014 [49]. Over 90 percent of the cases were due to one of the following: ART treatment of HIV infection, use of hydroxyurea, or a deficiency of vitamin B12 or folate. As noted above, an extremely high MCV may also be a clue to an artifactually high MCV caused by RBC agglutination. (See 'Evaluation' above.)

●Anemia plus one or more additional cytopenias (leukopenia and/or thrombocytopenia) is suggestive of a primary bone marrow problem (eg, megaloblastic anemia, myelodysplasia). If a drug-induced cause or a vitamin B12, folate, or copper deficiency is not diagnosed from the history or initial testing, bone marrow evaluation usually is indicated. (See 'Bone marrow/hematologist referral' below.)

●The following findings on the blood smear and their associated conditions may be helpful, although none are pathognomonic for a specific condition:

•Target cells (picture 3) are suggestive of liver disease.

•Macro-ovalocytes (picture 1) are suggestive of a megaloblastic process such as vitamin B12, folate, or copper deficiency; or drug-induced megaloblastic anemia; or membrane disorders such as hereditary stomatocytosis (HSt) or hereditary xerocytosis (HX). (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

•Spherocytes such as in hereditary spherocytosis or autoimmune hemolytic anemia. (See "Hereditary spherocytosis" and "Warm autoimmune hemolytic anemia (AIHA) in adults".)

•RBC agglutination as in cold agglutinin disease (CAD). (See "Cold agglutinin disease".)

•Neutrophils with more than five distinct lobes (picture 4) (referred to as multilobed or hypersegmented neutrophils) are suggestive of a megaloblastic process.

•Neutrophils with fewer than three distinct lobes (referred to as hypolobulated neutrophils or pseudo-Pelger-Huet cells) and/or reduced number of cytoplasmic granules (hypogranular neutrophils) are associated with myelodysplasia (picture 5).

Initial testing — In addition to a review of the complete blood count (CBC) including RBC indices and other cell lines, individuals for whom the history does not point to a clear diagnosis should have one or more of the following tests:

●Reticulocyte count

●Serum vitamin B12 and folate level

●Thyroid-stimulating hormone (TSH)

●Liver function tests

●Copper level, especially in individuals who have undergone gastric bypass surgery

Whether these are done simultaneously or sequentially depends on other clinical features, the degree of suspicion of one or more of the likely causes of macrocytosis, and the acuity and severity of anemia.

In a patient without anemia, reticulocytes make up 1 to 2 percent of RBCs. Hemolytic anemia typically raises the reticulocyte percentage above 4 to 5 percent as long as bone marrow function is adequate and vitamin B12, folate, and iron are available. It is also appropriate to perform other testing such as haptoglobin and lactate dehydrogenase (LDH) if the reticulocyte count is not dramatically increased but hemolysis is suspected. Additional information regarding interpretation of the absolute reticulocyte count and evaluation for the cause of hemolysis is presented separately. (See "Diagnosis of hemolytic anemia in adults".)

If the vitamin B12 or folate level is in the borderline range, it may be necessary to perform additional testing for methylmalonic acid and homocysteine levels.

A small percentage of individuals with multiple myeloma have macrocytosis, either due to concomitant vitamin B12 deficiency or from an unknown mechanism. If the patient has any findings suggestive of multiple myeloma such as back pain, hypercalcemia, or renal insufficiency, serum protein electrophoresis (SPEP) should be performed. (See "Multiple myeloma: Clinical features, laboratory manifestations, and diagnosis", section on 'Anemia'.)

Response to therapy as confirmation — Importantly, macrocytosis or macrocytic anemia should resolve with appropriate treatment based on a documented diagnosis. If macrocytosis or anemia persists despite correction of an underlying abnormality, investigation for additional causes of anemia is appropriate.

Bone marrow/hematologist referral — In cases where the cause of macrocytosis is not obvious from preliminary laboratory testing, hematologist referral is appropriate to evaluate the possibility of less-common causes of chronic hemolytic anemia and underlying bone marrow disorders such as myelodysplasia. Bone marrow evaluation for signs of dysplasia, hematologic malignancy, or abnormality of RBC maturation is appropriate if there is pancytopenia or if the initial testing listed above does not provide a diagnosis. (See 'Initial testing' above.)

Bone marrow testing should include standard morphologic analysis as well as iron staining, as well as cytogenetic analysis in cases of possible myelodysplasia or hematologic malignancy. The usefulness of genetic testing with a myeloid somatic mutation panel has not been studied. (See "Evaluation of bone marrow aspirate smears" and "Bone marrow aspiration and biopsy: Indications and technique", section on 'Preparation of samples'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anemia in adults".)

SUMMARY AND RECOMMENDATIONS

●Definitions – Macrocytosis is defined as a red blood cell (RBC) mean corpuscular volume (MCV) above the upper limit of normal, which is typically >96 to 100 femtoliters (fL) in adults. Neonates, infants, and young children have a higher upper limit of normal. Macrocytic anemia can be further classified according to the presence of reticulocytes and megaloblastic changes on the peripheral blood smear. (See 'Definitions and classification' above.)

●Mechanisms – RBCs leave the bone marrow as reticulocytes, which are macrocytic. All other macrocytic RBCs are formed as a consequence of inherited or acquired abnormalities in RBC maturation, nucleic acid metabolism, membrane composition, cell water content, or a combination of these factors. (See 'Pathophysiology' above.)

●Causes – Causes of macrocytic anemia include reticulocytosis in the setting of hemolytic anemia, bone marrow recovery, or erythropoietin; megaloblastic anemia from vitamin deficiency, drugs that interfere with nucleic acid metabolism or cell division, or rare inherited conditions; and a number of multifactorial processes such as alcohol, liver disease, hypothyroidism, myelodysplasia, and others listed above. (See 'Causes of macrocytosis/macrocytic anemia' above.)

●Evaluation – Patient age, medical history, alcohol use, and medications are especially important in the evaluation of macrocytosis (algorithm 1). Age-appropriate normal ranges for the MCV should be consulted for neonates and infants. Medications should be checked, alcohol use discussed, and dietary practices reviewed. (See 'Clues from the history and examination' above.)

●CBC and blood smear review – The severity of macrocytosis, presence of other cytopenias on the complete blood count (CBC), and specific RBC morphologies on the peripheral blood smear may be useful in narrowing the possible causes of macrocytosis and in guiding the evaluation (algorithm 1). Severe macrocytosis (MCV above 110 to 115 fL) is associated almost exclusively with megaloblastic anemias. Anemia plus one or more additional cytopenias (leukopenia and/or thrombocytopenia) is suggestive of a primary bone marrow problem (eg, megaloblastic anemia, myelodysplasia). Helpful findings on the blood smear are discussed above. (See 'Clues from the CBC and blood smear' above.)

●Additional laboratory testing – Individuals for whom the history does not point to a clear diagnosis should have a reticulocyte count; serum vitamin B12 (cobalamin), folate, and copper levels; thyroid-stimulating hormone (TSH); and liver function tests. Additional testing such as serum protein electrophoresis (SPEP) may occasionally be appropriate if the other tests are normal or there are clinical features of multiple myeloma, as myeloma is occasionally associated with macrocytosis. (See 'Initial testing' above.)

●Hematologist input and bone marrow – If the preliminary evaluation is negative or there is severe pancytopenia, referral to a hematologist and bone marrow evaluation are appropriate. In addition to standard morphology and iron staining, cytogenetic analysis and a myeloid somatic mutation panel may define myelodysplastic syndromes (MDS). (See 'Bone marrow/hematologist referral' above and "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

ACKNOWLEDGMENTS — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic review and was a founding Editor-in-Chief for UpToDate in Hematology.

The UpToDate editorial staff also acknowledges the extensive contributions of William C Mentzer, MD, to earlier versions of this and many other topic reviews.