Matthew M. Heeney
DEFINITION
Anemia is defined as lower than normal values of hemoglobin, or hematocrit. The lower limit of the normal range is set arbitrarily at 2 standard deviations below the mean for any given age and gender.1 The observed age and gender-related differences in the “normal” hemoglobin level during the first decade of life and after puberty (see Fig. 429-4) must be considered in making a diagnosis of anemia.2 Some laboratories use only adult normal range values and will erroneously report normal pediatric levels of hemoglobin as low. Defining anemia as a hemoglobin measurement 2 standard deviations below the mean results in 2.5% of normal children being classified as being anemic. Such individuals may track at their own low level over extended periods of time but are identified by ruling out other treatable causes. Conversely, some individuals have hemoglobin values in the lower part of the normal range that may increase after treatment with iron or after the resolution of an infectious or inflammatory process. Age-related normal means and lower limits of normal hemoglobin, hematocrit, and mean corpuscular volume (MCV) are shown in Table 429-1. Figure 429-4 depicts the pattern of normal mean hemoglobin levels from birth to adult life.
PATHOPHYSIOLOGY
Normal erythrocytes survive in the circulation for 100 to 120 days, therefore approximately 1% are removed from the circulation each day, and in the steady state, about 1% new erythrocytes are released into the circulation from the bone marrow each day. Anemia is the result of a congenital or acquired imbalance in erythrocyte loss relative to the marrow’s capacity for erythrocyte production. Disorders resulting in decreased erythroid production are discussed in Chapter 432, and disorders that shorten the circulating erythrocyte lifespan are discussed in Chapter 433.
CLASSIFICATION
An erythrokinetic or pathophysiologic classification of anemia relies on the fact that the steady-state level of hemoglobin reflects a balance between production of red blood cells (RBCs) by the bone marrow and the rate of their peripheral destruction.1 Thus, most anemias can be classified as a disorder of marrow production or disorders of loss from the circulation (eg, hemolysis, sequestration, bleeding). The primary laboratory indicator that distinguishes these disorders is the reticulocyte count. If there is an appropriate increase in reticulocyte count from steady state in response to anemia, it suggests a disorder of loss. Conversely, if the reticulocyte count is inappropriately “normal” or low in response to anemia, it suggests and disorder of marrow production.
FIGURE 430-1. RBC morphology in various conditions.
A morphologic classification of anemia is based on erythrocyte size (mean corpuscular volume [MCV]) and morphology. A number of childhood anemias are associated with characteristic ERYTHROCYTE appearance, and examination of ERYTHROCYTE morphology on peripheral blood smears is an essential component of diagnosis (Fig. 430-1). A diagnostic algorithm that combines the pathophysiologic and morphologic criteria is shown in Figure 430-2.
DIAGNOSTIC APPROACH
HISTORY
Historical data to be obtained include diet, infection or chronic disease, medications and other environmental exposures, and ethnicity.3 The diet during infancy is particularly relevant to iron deficiency. Infants who have been fed only whole cow’s milk or non-iron-fortified cow’s milk formulas are at risk for developing iron deficiency.4 Although the iron present in human milk appears to be better absorbed than that in cow’s milk, it is insufficient to meet the requirements for rapid growth in the first year of life, and infants consuming only breast milk may develop iron deficiency after 9 to 12 months of age. Vitamin B12 deficiency may occur in breast-fed babies of strict vegetarian mothers (vegans).5 Infants exclusively fed goat’s milk are at risk for developing folic acid deficiency.6
The age of the child at recognition of anemia may be diagnostically important and aid in differentiation of a congenital disorder from an acquired disorder. Intrinsic abnormalities of the RBC membrane and RBC enzymopathies may present in the newborn period with anemia and jaundice, whereas major β-globin chain hemoglobin disorders such as sickle cell disease and thalassemia major usually have normal hematologic values in the neonatal period and do not become evident until 3 to 6 months of age or later.
Many hemolytic anemias are genetically determined therefore a family history is important to determine a potential pattern of inheritance. An inherited anemia such as hereditary spherocytosis may be suggested by a family history of neonatal hyperbilirubinemia, anemia, jaundice, splenomegaly, splenectomy, or gallstones. A family history indicating dominant inheritance suggests a defect of the erythrocyte membrane, whereas recessive inheritance is characteristic of many hemoglobinopathies and enzymopathies.
In the differential diagnosis of anemia in children, the relative frequencies of various etiologies should also be considered. Iron deficiency and the anemia of acute and chronic infections are by far the most common causes of anemia in children. Next in frequency are genetic conditions such as hereditary spherocytosis. Sickle cell diseases are prevalent in African Americans, and thalassemia is common in people of Mediterranean or Southeast Asian ethnicity. Other causes of anemia are relatively unusual.
PHYSICAL EXAMINATION
The most important physical finding of anemia is pallor, but this is often a subtle finding that is evident only when the degree of anemia is relatively severe (Hb < 70–80 g/L). Anemia is best appreciated by pallor of the mucous membranes and conjunctivae, particularly in dark-skinned children. The red color of the palmar creases in the hand disappears when the hemoglobin falls below 70 to 80 g/L.7 Children tolerate even fairly severe anemia quite well when it develops slowly. Some children with iron-deficiency anemia have few symptoms even when the hemoglobin is 50 to 60 g/L.8 Children with sickle cell anemia who chronically have hemoglobin levels of 65 to 80 g/L often have normal activity and few symptoms. Tachycardia is present only when anemia is severe or develops suddenly. Jaundice, best appreciated as yellow sclerae, suggests a hemolytic process, although some children with chronic hemolytic anemias are not clinically jaundiced.1 Lymphadenopathy, hepatosplenomegaly, and signs and symptoms of systemic diseases should be ascertained.
LABORATORY DIAGNOSIS
In the workup of a child with presumed anemia, it is necessary to confirm that anemia is actually present according to age- and gender-appropriate standards (Table 429-1). Hemoglobin concentrations in those of African American ancestry are about 5 g/L lower than in Caucasians, a difference that is not explained by iron deficiency or thalassemia.9
FIGURE 430-2. A diagnostic algorithm for anemia that combines the pathophysiologic and morphologic criteria. (DAT, direct antiglobulin test; DIC, disseminated intravascular coagulation; G6PD, glucose-6-phosphate dehydrogenase; HUS, hemolytic uremic syndrome; TEC, transient erythroblastopenia of childhood; TTP, thrombotic thrombocytopenic purpura.)
Next, an evaluation is made of whether the anemia is a result of decreased RBC production (aregenerative) or increased destruction/loss (hemolysis). This is most easily assessed by the reticulocyte count. Reticulocytosis reflects increased erythroid activity of the bone marrow; a sustained reticulocytosis is very suggestive of a hemolytic process, whereas reticulocytopenia is characteristic of an aregenerative process. Next, the average size and hemoglobin content of the RBCs should be determined by measuring the mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) by automated cell analyzers. Finally, the morphology of the RBCs should be assessed on a stained peripheral blood smear (Fig. 430-1). These readily available determinations permit a presumptive diagnosis of most anemias in children.
Hemoglobin, Hematocrit, and Red Blood Cell Indices
Modern automated cell analyzers accurately and directly measure white blood cell (WBC), RBC, and platelet numbers as well as hemoglobin (Hb) level, MCV, and MCH.3,10 The hematocrit (Hct) and mean corpuscular hemoglobin concentration (MCHC) are calculated from the directly measured values. Most automated cell analyzers also measure RBC distribution width (RDW), which assesses the variability of size of the RBCs, described as anisocytosis on the peripheral blood smear. For office-based screening, Hb can also be measured by point-of-care spectrophotometers, and Hct by microcentrifuges. Hematocrit values can be estimated by multiplying the Hb value by 0.3, and Hb level is estimated by dividing Hct by 0.3.
The MCV, directly measured by automated cell analyzers, provides a basis for deciding whether the RBC population is macrocytic, normocytic, or microcytic. However, age-related norms must be employed to make this decision because erythrocytes of the fetus and newborn are very macrocytic, whereas during the first 2 to 3 years of life they are distinctly microcytic compared to adult values (Table 429-1). The MCH reflects RBC hemoglobin content and gives a quantitative assessment of hypochromia evident on the blood smear. Because most microcytic anemias are also hypochromic, MCH is usually proportional to the MCV and may be even more sensitive than the MCV in the diagnosis of mild iron deficiency. The MCHC is a derived rather than a directly measured value and is not useful in assessing hypochromia. A high MCHC, however, is characteristic of membranopathies such as spherocytosis.
Reticulocyte Count
The number of new erythrocytes in peripheral blood reflects the rate at which new reticulocytes, which contain stainable RNA, are being produced and released from the erythroid bone marrow. Reticulocytosis can be recognized by polychromatophilia on the blood smear and may sometimes be accompanied by an increased MCV because reticulocytes are larger than mature RBCs. Manual reticulocyte counts can be performed on blood smear stained with new methylene blue by counting the percentage of erythrocytes with visible blue staining of RNA reticulum mesh. This rather tedious manual method has been virtually replaced by fluorescent techniques performed by automated analyzers,11 which provide accurate absolute reticulocyte counts. Reticulocytes contain stainable reticulum for about 1 to 2 days, so the normal reticulocyte count is 1.0% to 2.0%. The normal absolute reticulocyte count is 40 to 75 × 109/L. Low values (< 40 × 109/L) indicate erythroid underproduction, and increased values (> 100 × 109/L) suggest erythroid marrow hyperplasia often associated with hemolysis. Transiently high reticulocyte counts are seen after acute blood loss and after institution of specific therapy for nutritional deficiencies of iron, folic acid, or vitamin B12.
Measures of Hemolysis
Hemolysis refers to an increased rate of erythrocyte destruction leading to a survival time that is less than the normal 100 to 120 days. In acute hemolysis with a rapid onset of anemia, a compensatory increase in erythroid marrow activity mediated by erythropoietin (EPO) takes place several days before peripheral reticulocytosis appears.1 In chronic hemolytic states, anemia is usually present because the rate of erythrocyte destruction and production are not balanced. However, in some patients, the rate of hemolysis is fully compensated by increased erythrocyte production, resulting in a normal hemoglobin level. Such patients will, however, have an elevated reticulocyte count.
In most chronic hemolytic states, erythrocytes are destroyed extravascularly in the reticuloendothelial (RE) tissues of the spleen, liver, and bone marrow. Within the RE cell, hemoglobin is catabolized to amino acids, the heme metabolite bilirubin and iron recycled back to the marrow. Most patients with chronic severe hemolysis are jaundiced and have elevated serum levels of unconjugated (indirect) bilirubin. However, hepatic conjugation and biliary excretion of bilirubin may result in normal serum bilirubin levels, and hyperbilirubinemia and clinical jaundice should not be considered essential findings to consider a diagnosis of hemolysis. Chronically increased rates of bilirubin excretion, characteristic of congenital and chronic hemolysis, often result in gallstones, which are composed of calcium bilirubinate and are usually multiple, faceted, and radiopaque.
Primarily intravascular hemolysis is characteristic of some hemolytic anemias that are immune mediated, drug induced, or microangiopathic.1 Free hemoglobin is released into the plasma, where it combines with haptoglobin and is then cleared by RE tissues. When the rate of haptoglobin-hemoglobin complex clearance exceeds the rate of hepatic haptoglobin synthesis, the level of serum haptoglobin decreases below the normal range (20–200 mg/dL) and is often undetectable in patients with significant hemolysis. Low or absent levels of serum haptoglobin may also be seen in hemolytic states where RBC destruction is primarily extravascular. In acute intravascular hemolysis, the binding capacity of haptoglobin for hemoglobin may be exceeded, and free hemoglobin is excreted by the kidney, resulting in hemoglobinuria as indicated by a positive test for occult blood without erythrocytes in the urinary sediment. In chronic hemolytic states, hemosiderin may be present in the urinary sediment.
Bone Marrow Aspiration and Biopsy
Bone marrow examination is painful, and for most common forms of anemia, it should not be undertaken unless likely to provide information that cannot be readily obtained from studies of peripheral blood. For example, measurement of serum ferritin levels and other iron studies almost always obviates the need for bone marrow aspiration to assess reticuloendothelial iron stores (hemosiderin).12 Bone marrow aspiration may be necessary for the diagnosis of pure RBC aplasias or pancytopenias from marrow failure or invasion. The bone marrow should usually be evaluated when a diagnosis of leukemia is considered or when metastatic malignancy, bone marrow failure, or storage diseases such as Gaucher and Niemann-Pick disease are suspected. Overall cellularity, morphology, and maturation of the hematopoietic cell lines can be evaluated. Needle biopsy, performed at the same time as the aspirate, provides an intact specimen that is especially useful for assessing marrow architecture and cellularity. Marrow aspiration is often indicated in thrombocytopenic states to assess the number of megakaryocytes, an indicator of platelet production (Chapter 439). The most notable bone marrow finding in infants and young children, compared to older patients, is a predominance of mature lymphocytes.