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Learning Objectives

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  1. Learn the different causes of anemia and their pathophysiology.

  2. Learn how to identify the specific cause of anemia in a particular patient.

  3. Learn the causes of erythrocytosis and how to distinguish among them.

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Anemia

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Definition

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Anemia refers to a deficiency in red blood cells (RBCs) and implies a decline in oxygen-carrying capacity. The complete blood count (CBC) provides several measures of red cell quantity, including RBC count, hemoglobin (Hb) concentration, and hematocrit (Hct) (see description of RBC indices later in this chapter). Hb concentration is the parameter most widely used to diagnose anemia, based on 1967 World Health Organization (WHO) recommendations (Table 10–1). This definition is not universally accepted, and numerous alternatives have been proposed over the years, usually suggesting slightly higher values and race-specific values. It is important to remember also that the normal ranges for Hb and Hct are different for infants, children, adult men, adult women, pregnant women, and the elderly. Attention to age- and gender-appropriate normal ranges is important in the evaluation of anemia.

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Anemia refers to a deficiency in red blood cells (RBCs) and implies a decline in oxygen-carrying capacity.

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Table Graphic Jump Location
Table 10–1WHO Definition of Anemia
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Anemia may present with pallor, fatigue, dyspnea, or evidence of poor tissue oxygenation (chest pain due to poor cardiac oxygenation, altered mental status due to poor cerebral oxygenation). Often, particularly when anemia is mild or the patient is otherwise healthy, anemia presents simply as an abnormal CBC.

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Anemia stimulates several compensatory mechanisms. The cardiopulmonary system compensates by attempting to make the most of the blood it has by exchanging more gases (tachypnea), and circulating more volume (tachycardia). The marrow responds with increased erythropoiesis, stimulated by an increase in renal production of erythropoietin (EPO) in response to hypoxia. If the means to create mature red cells are intact (ie, if the underlying cause of the anemia is not a production or maturation defect), then this response can usually succeed. In addition to making more erythrocytes, the marrow begins to release immature erythrocytes into the circulation. Many of these still contain a network of ribosomes and rough endoplasmic reticulum involved in the making of Hb, which identifies them morphologically as reticulocytes (see description of reticulocyte counting later in this chapter). Over the next 3 to 4 days, this endoplasmic reticulum dissolves and a mature RBC results. In very brisk marrow responses, some red cells may be released that still contain a nucleus.

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Identifying the cause of anemia is usually fairly straightforward. Examination of the peripheral smear is especially important, since numerous clues can be found there....

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