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Evaluation of Anemia


Evan M. Braunstein

, MD, PhD, Johns Hopkins University School of Medicine

Last full review/revision Sep 2020| Content last modified Sep 2020

Anemia is a decrease in the number of red blood cells (RBCs—as measured by the red cell count, the hematocrit, or the red cell hemoglobin content). In men, anemia is defined as hemoglobin < 14 g/dL (140 g/L), hematocrit < 42% (< 0.42) , or RBC < 4.5 million/mcL (< 4.5 × 1012/L). In women, hemoglobin < 12 g/dL (120 g/L), hematocrit < 37% (< 0.37), or RBC < 4 million/mcL (< 4 × 10 12/L) is considered anemia. For infants, normal values vary with age, necessitating use of age-related tables (see table Age-Specific Values for Hemoglobin and Hematocrit).

Anemia is not a diagnosis; it is a manifestation of an underlying disorder (see Etiology of Anemia). Thus, even mild, asymptomatic anemia should be investigated so that the primary problem can be diagnosed and treated.

Anemia is usually suspected based on the history and physical examination. Common symptoms and signs of anemia include

  • General fatigue
  • Weakness
  • Dyspnea on exertion
  • Pallor

History and physical examination are followed by laboratory testing with a complete blood count and peripheral smear. The differential diagnosis (and cause of anemia) can then be further refined based on the results of testing.

Pearls & Pitfalls

  • Anemia is not a diagnosis; it is a manifestation of an underlying disorder. Thus, even mild, asymptomatic anemia should be investigated so that the primary problem can be diagnosed and treated.


The history should address

  • Risk factors for particular anemias
  • Symptoms of anemia itself
  • Symptoms that reflect the underlying disorder

Risk factors for anemia

Anemia has many risk factors. For example, a vegan diet predisposes to vitamin B12 deficiency anemia, whereas alcoholism increases the risk of folate deficiency anemia. A number of hemoglobinopathies are inherited, and certain drugs and infections predispose to hemolysis. Cancer, rheumatic disorders, and chronic inflammatory disorders can suppress red cell production.

Symptoms of anemia

The symptoms of anemia are neither sensitive nor specific and do not help differentiate between types of anemias. Symptoms reflect compensatory responses to tissue hypoxia and usually develop when the hemoglobin level falls well below the patient's individual baseline. Symptoms are generally more pronounced in patients with limited cardiopulmonary reserve or in whom the anemia developed very rapidly.

Symptoms such as weakness, fatigue, drowsiness, angina, syncope, and dyspnea on exertion can indicate anemia. Vertigo, headache, pulsatile tinnitus, amenorrhea, loss of libido, and gastrointestinal (GI) complaints may also occur. Heart failure or shock can develop in patients with severe tissue hypoxia or hypovolemia.

Symptoms that suggest cause of anemia

Certain symptoms may suggest the cause of the anemia. For example, melena, epistaxis, hematochezia, hematemesis, or menorrhagia indicates bleeding. Jaundice and dark urine, in the absence of liver disease, suggest hemolysis. Weight loss may suggest cancer. Diffuse severe bone or chest pain may suggest sickle cell disease, and stocking-glove paresthesias may suggest vitamin B12 deficiency.

Physical Examination

A complete physical examination is necessary. Signs of anemia itself are neither sensitive nor specific; however, pallor is common with severe anemia.

Signs of underlying disorders are more diagnostically accurate than are signs of anemia. Heme-positive stool identifies gastrointestinal bleeding. Hemorrhagic shock (eg, hypotension, tachycardia, pallor, tachypnea, diaphoresis, confusion) may result from acute bleeding. Jaundice may suggest hemolysis. Splenomegaly may occur with hemolysis, a hemoglobinopathy, connective tissue disease, myeloproliferative disorder, infection, or cancer. Peripheral neuropathy suggests vitamin B12 deficiency. Abdominal distention in a patient with blunt trauma suggests acute hemorrhage or splenic rupture. Petechiae develop in thrombocytopenia or platelet dysfunction. Fever and heart murmurs suggest infective endocarditis. Rarely, high-output heart failure develops as a compensatory response to anemia-induced tissue hypoxia.


  • CBC with WBC and platelets
  • RBC indices and morphology
  • Reticulocyte count
  • Peripheral smear
  • Sometimes bone marrow aspiration and biopsy

Laboratory evaluation begins with a complete blood count (CBC), including white blood cell (WBC) and platelet counts, RBC indices and morphology (mean corpuscular volume [MCV], mean corpuscular hemoglobin [MCH], mean corpuscular hemoglobin concentration [MCHC], red blood cell distribution width [RDW]), and examination of the peripheral smear. The reticulocyte count demonstrates how well the bone marrow is compensating for the anemia. Subsequent tests are selected on the basis of these results and on the clinical presentation. Recognition of general diagnostic patterns can expedite the diagnosis (see table Characteristics of Common Anemias).

Characteristics of Common Anemias

Etiology or Type

Morphologic Changes

Special Features

Blood loss, acute

Normochromic-normocytic, with polychromatophilia

If severe, possible nucleated RBCs and left shift of WBCs



Blood loss, chronic

Same as iron deficiency

Same as iron deficiency

Folate deficiency

Same as vitamin B12 deficiency

Serum folate < 5 ng/mL (< 11 nmol/L)

RBC folate < 225 ng/mL RBCs (< 510 nmol/L)

Nutritional deficiency and malabsorption (in sprue, pregnancy, infancy, or alcoholism)

Hereditary spherocytosis

Spheroidal microcytes

Normoblastic erythroid hyperplasia


Increased mean RBC Hb level

Increased RBC osmotic fragility

Shortened survival of labeled RBCs




Marrow erythroid hyperplasia

Increased serum indirect bilirubin and lactate dehydrogenase

Increased stool and urine urobilinogen

Hemoglobinuria in fulminating cases


Infection, cancer, or chronic inflammation

Normochromic-normocytic early, then microcytic

Normoblastic marrow

Normal iron stores

Decreased serum iron

Decreased total iron-binding capacity

Normal serum ferritin

Normal marrow iron content

Iron deficiency

Microcytic, with anisocytosis and poikilocytosis


Hyperplastic marrow, with delayed hemoglobination

Possible achlorhydria, smooth tongue, and spoon nails

Absent stainable marrow iron

Low serum iron

Increased total iron-binding capacity

Low serum ferritin

Marrow failure

Normochromic-normocytic (may be macrocytic)


Failed marrow aspiration (often) or evident hypoplasia of erythroid series or of all elements

Idiopathic (> 50%) or secondary to exposure to toxic drugs or chemicals (eg, chloramphenicol, quinacrine, hydantoins, insecticides)

Marrow replacement (myelophthisis)

Anisocytosis and poikilocytosis

Nucleated RBCs

Early granulocyte precursors

Marrow aspiration possibly failing or showing leukemia, myeloma, or metastatic cells

Marrow infiltration with infectious granulomas, tumors, fibrosis, or lipid histiocytosis

Possible hepatomegaly and splenomegaly

Possible bone changes

Cold agglutinin disease

Red cell agglutination


Follows exposure to cold

Results from a cold agglutinin or hemolysin

Sometimes postinfectious

Paroxysmal nocturnal hemoglobinuria




Dark morning urine



Sickle cell disease

Anisocytosis and poikilocytosis

Some sickle cells in peripheral smear

Sickling of all RBCs in preparation with hypoxia or hyperosmolar exposure


Largely limited to blacks in the United States

Urinary isosthenuria

Hb S detected during electrophoresis

Possibly painful vaso-occlusive crises and leg ulcers

Bone changes on x-ray

Sideroblastic anemia

Usually hypochromic but dimorphic with normocytes and macrocytes

Hyperplastic marrow, with delayed hemoglobination

Ringed sideroblasts

Inborn or acquired metabolic defect

Stainable marrow iron (plentiful)

Some congenital forms respond to vitamin B6 administration

Can be part of a myelodysplastic syndrome (MDS)



Target cells

Basophilic stippling

Anisocytosis and poikilocytosis

Nucleated RBCs in homozygotes


Decreased RBC fragility

Elevated Hb A2and Hb F (in beta-thalassemia)

Mediterranean ancestry (common)

In homozygotes, anemia from infancy


Bone changes on x-ray

Vitamin B12 deficiency

Oval macrocytes



Hypersegmented WBCs

Megaloblastic marrow

Serum B12 < 180 pg/mL (< 130 pmol/L)

Frequent GI and CNS involvement

Elevated serum bilirubin

Increased LDH

Antibodies to intrinsic factor in serum (pernicious anemia)

Sometimes absent gastric intrinsic factor secretion

CNS = central nervous system; GI = gastrointestinal; Hb = hemoglobin; LDH = lactic dehydrogenase; RBC = red blood cell; WBC = white blood cell.

Complete blood count and RBC indices

The automated CBC directly measures hemoglobin, RBC count, WBC count. and number of platelets, plus mean corpuscular volume (MCV, a measure of RBC volume). hematocrit (a measure of the percentage of blood made up of RBCs), mean corpuscular hemoglobin (MCH, a measure of the hemoglobin content in individual RBCs), and mean corpuscular hemoglobin concentration (a measure of the hemoglobin concentration in individual RBCs) are calculated values.

The diagnostic criterion for anemia is

  • For men: Hemoglobin < 14 g/dL (140 g/L), hematocrit < 42% (< 0.42), or RBC < 4.5 million/mcL (< 4.5 × 10 12/L)
  • For women: Hemoglobin < 12 g/dL (120 g/l), hematocrit < 37% (< 0.37), or RBC < 4 million/mcL (< 4 × 10 12/L)

For infants, normal values vary with age, necessitating use of age-related tables (see table Age-Specific Values for Hemoglobin and Hematocrit). RBC populations are termed microcytic (small cells) if MCV is < 80 fL, and macrocytic (large cells) if MCV is > 100 fL. However, because reticulocytes are also larger than mature red cells, large numbers of reticulocytes can elevate the MCV and not represent an alteration of RBC production.

Automated techniques can also determine the degree of variation in RBC size, expressed as the RBC volume distribution width (RDW). A high RDW may be the only indication of simultaneous microcytic and macrocytic disorders; such a pattern may result in a normal MCV, which measures only the mean value. The term hypochromia refers to RBC populations in which MCH is < 27 pg/RBC or MCHC is < 30%. RBC populations with normal MCH and MCHC values are normochromic.

The RBC indices can help indicate the mechanism of anemia and narrow the number of possible causes. Microcytic indices occur with altered heme or globin synthesis. The most common causes are iron deficiency, thalassemia, and related hemoglobin-synthesis defects. In some patients with anemia of chronic disease, the MCV is microcytic or borderline microcytic. Macrocytic indices occur with impaired DNA synthesis (eg, due to vitamin B12 or folate deficiencies or chemotherapeutic drugs such as hydroxyurea and antifolate agents) and in alcoholism because of abnormalities of the cell membrane. Acute bleeding may briefly produce macrocytic indices because of the release of large young reticulocytes. Normocytic indices occur in anemias resulting from deficient erythropoietin (EPO) production or inadequate response to it (hypoproliferative anemias). Hemorrhage, before iron deficiency develops, usually results in normocytic and normochromic anemia unless the number of large reticulocytes is excessive.

Peripheral smear

The peripheral smear is highly sensitive for excessive RBC production and hemolysis. It is more accurate than automated technologies for recognition of altered RBC structure, thrombocytopenia, nucleated RBCs, or immature granulocytes and can detect other abnormalities (eg, malaria and other parasites, intracellular RBC or granulocyte inclusions) that can occur despite normal automated blood cell counts. RBC injury may be identified by finding RBC fragments, portions of disrupted cells (schistocytes), or evidence of significant membrane alterations from oval-shaped cells (ovalocytes) or spherocytic cells. Target cells (thin RBCs with a central dot of hemoglobin) are RBCs with insufficient hemoglobin or excess cell membrane (eg, due to hemoglobinopathies or liver disorders). The peripheral smear can also reveal variation in RBC shape (poikilocytosis) and size (anisocytosis).

Reticulocyte count

The reticulocyte count is expressed as the percentage of reticulocytes (normal range, 0.5 to 1.5%) or as the absolute reticulocyte count (normal range, 50,000 to 150,000/mcL, or 50 to 150 × 10 9/L). The reticulocyte count is a crucial test in the evaluation of anemia because it informs about the response of the bone marrow and facilitates differentiation between deficient erythropoiesis (RBC production) and excessive hemolysis (RBC destruction ) as the cause of anemia. For example, higher values indicate excessive production (reticulocytosis); in the presence of anemia, reticulocytosis suggests excessive RBC destruction. Low numbers in the presence of anemia indicate decreased RBC production.

Reticulocytes are best visualized when blood is stained with a supravital stain, but because red cell reticulin is composed of RNA, which is present only in young red cells, they will have a bluish appearance in a Wright-stained blood smear (polychromatophilia or polychromasia), which can provide a rough estimate of reticulocyte production on a routine blood smear.

Bone marrow aspiration and biopsy

Bone marrow aspiration and biopsy provide direct observation and assessment of RBC precursors. The presence of abnormal maturation (dyspoiesis) of blood cells and the amount, distribution, and cellular pattern of iron content can be assessed. Bone marrow aspiration and biopsy are usually not indicated in the evaluation of anemia and are only done when one of the following conditions is present:

  • Unexplained anemia
  • More than one cell lineage abnormality (ie, concurrent anemia and thrombocytopenia or leukopenia)
  • Suspected primary bone marrow disorder (eg, leukemia, multiple myeloma, aplastic anemia, myelodysplastic syndrome, metastatic carcinoma, myelofibrosis)

Cytogenetic and molecular analyses can be done on aspirate material in hematopoietic or other tumors or in suspected congenital lesions of RBC precursors (eg, Fanconi anemia). Flow cytometry can be done in suspected lymphoproliferative or myeloproliferative states to define the immunophenotype. Bone marrow aspiration and biopsy are not technically difficult and do not pose significant risk of morbidity. These procedures are safe and helpful when hematologic disease is suspected. Both usually can be done as a single procedure. Because biopsy requires adequate bone depth, the sample is usually taken from the posterior (or, less commonly, anterior) iliac crest.

Other tests for evaluation of anemia

Serum bilirubin and lactate dehydrogenase (LDH) can sometimes help differentiate between hemolysis and blood loss; both are elevated in hemolysis and normal in blood loss. Other tests, such as vitamin B12 and folate levels, iron and iron binding capacity, are done depending on the suspected cause of anemia. Other tests are discussed under specific anemias and bleeding disorders.

Drugs Mentioned In This Article

Drug Name Select Trade
chloramphenicol No US brand name
hydroxyurea HYDREA

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