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Anemia I

Pathophysiologically, anemia is an impaired ability to meet the body's demand for oxygen, but the most basic definition of anemia is a low hemoglobin (HGB) level. Before we begin our reader-requested discussion on anemia, let us spend a moment considering the red cell and the laboratory test that may be the first indicator of anemia.

The Complete Blood Count

Of course, the CBC is not one test, but a constellation of measurements, calculations and cell identifications and descriptions. Red cell-related measurements included in the CBC are the red blood cell (RBC) count, HGB, and mean corpuscular volume (MCV). RBCs are counted using either laser or impedance technology. HGB concentration is really a chemistry method, measured optically by the hematology analyzer. MCV is determined as RBCs are passed through an aperture and sized.

Calculated parameters usually include hematocrit (HCT), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) and red cell distribution width. (RDW describes the variation and distribution of RBC size.)

Automated CBCs do not include RBC identifications and descriptions. One must make and stain a smear, dust off the microscope, sit down, and look.

RBC Morphology

Since RBC morphology plays such a crucial role in the discussion of anemia, it is important to understand the terms that describe morphologic variations. In a healthy individual, RBCs appear in a Wright stained smear as circular, pink discs. Each disc has a central pallor caused by the cell's biconcave sides. Poikilocytosis refers to any variation in shape. Oval, teardrop, helmet, pear, saddle and irregular-shaped cells may be seen in a single case of anemia.

Target cells are thinner than normal RBCs and, upon Wright stain, show a peripheral rim of HGB and a central HGB-containing area. This 'bull's eye' effect is the result of an increased surface/volume ratio. Target cells are found in obstructive jaundice, following splenectomy, in some classes of anemia, and in hemoglobin C disease.

Spherocytes are nearly round RBCs, in contrast to the normal biconcave discs. They have a smaller diameter and lack the usual central pale area. They are found in hereditary spherocytosis, some autoimmune hemolytic anemias, and after direct physical or chemical injury to the RBCs. In all these cases, tiny bits of the surface membrane have been removed, leaving the cell with a decreased surface/volume ratio.

Normal RBCs are nearly uniform in size, ranging from 6-8 microns in diameter. Anisocytosis is a general term referring to size variation. Macrocytosis and microcytosis are more specific, describing large and small cell populations.

The pink color of RBCs is due to acidophilic HGB within the cells. The intensity of color is proportional to the amount of HGB and is described by the terms normochromic, hypochromic and hyperchromic. In hypochromia, MCH and MCHC are usually decreased; the central pallor becomes larger and paler. In hyperchromia the RBCs are larger, hence thicker, and stain more deeply with less central pallor; the MCH is higher, but the MCHC (which corrects for size) is normal. In hereditary spherocytosis the cells are also hyperchromic, but the MCH is normal and the MCHC is increased because of the reduced surface/volume ratio.

Polychromia refers to the blue-gray appearance of newly released RBCs, which retain residual RNA for the first day or two of peripheral circulation. (Basophilic RNA stains blue, acidophilic HGB stains pink.) These young polychromic cells are larger and often lack the central pallor. Basophilic stippling refers to larger bits of retained RNA. When stained supravitally, these cells prove to be reticulocytes.

Additional information on variations in RBC morphology may be found in Chapter 24, "Basic Examination of Blood," of Clinical Diagnosis and Management by Laboratory Methods, 19th ed, WB Saunders Co, 1996, as well as in other hematology texts.

Red Cell Production and Destruction

In erythropoiesis stem cells give rise to hemopoietic cells that migrate to the marrow, where maturation occurs. Mature RBCs enter the circulation through thin-walled sinusoids in the marrow. Under normal circumstances an RBC moves through the production process and is released within 6-8 days. Under maximum stress and adequate supplies of iron, B12 and folate, production rate can increase to 5-6 times normal. Effective erythropoiesis results when the number of viable and functional RBCs is appropriate to meet physiological needs. There must be a balance between the number of RBCs produced and their life span.

A reticulocyte is a young, circulating RBC that has lost its nucleus, but still contains a large amount of cytoplasmic RNA. The reticulocyte count is a measure of effective erythropoiesis and depends on three factors:

ˇ Rate of release from the marrow
ˇ Degree of maturity at release
ˇ Rate of disappearance of RNA

Reticulocytes may be counted manually after staining with supravital stain or may be counted using flow cytometry.

The spleen is the primary organ of RBC destruction. Consisting of cords and venous sinuses, passage through this branching system is the ultimate test of size, shape, and stretch. Young and viable RBCs pass through, while old or defective RBCs are culled out and destroyed.

General Features of Anemia

Reduced HGB results in decreased oxygen-carrying capacity, causing tissue hypoxia. Clinical manifestations of anemia vary with age, rapidity of onset, and concurrent clinical conditions.

Signs and symptoms include fatigue, shortness of breath, altered menses, and pallor of skin and mucous membranes. When developed over a long period of time, the patient adjusts to his/her symptoms and may be unaware of them. More serious manifestations include congestive heart failure, icterus and tachycardia.

Hints to the cause of the anemia may be gathered from a thorough patient history:

ˇ Family history
ˇ Alcohol use
ˇ Vegetarianism
ˇ Prescriptions and other drugs
ˇ Transfusion history
ˇ Menstrual history
ˇ Occupation
ˇ Hobbies
ˇ Travel
ˇ Other disorders, past and present

Classification of anemia may be made based upon pathophysiology (blood loss, decreased RBC production, or increased RBC destruction) or based on MCV (microcytic or macrocytic).

Iron Deficiency Anemia

IDA is the most common microcytic anemia and the most common nutritional deficiency in the world. Although its prevalence is decreasing in developed countries, certain populations remain at risk:

ˇ Children and adolescents
ˇ Women who are pregnant or menstruating
ˇ Individuals with blood loss due to gastrointestinal malignancy and peptic ulcer disease

In the early stages of IDA, iron stores become depleted, but HGB synthesis is not disrupted. Then HGB begins to fall and microcytosis and hypochromia become evident on the peripheral smear. Poikilocytosis, including oval, target and bizarre RBCs are present. RDW increases in the later stages.

Ferritin, an indicator of total body iron stores, is usually low. Free erythrocyte protoporphyrin (FEP) increases before iron and TIBC become affected. A bone marrow exam may be necessary to confirm the diagnosis if iron studies are inconclusive.

Serum transferrin receptor (TfR) is a transmembrane protein on virtually all cells and is required for binding iron and bringing it into the cell. The number of TfR molecules reflects iron requirement; iron deprivation induces cells to synthesize more receptors. An increase in serum TfR is a reliable and early indicator of depleted iron stores in IDA.

The α-Thalassemias

Thalassemia is an inherited disorder of globin synthesis frequently found in African, Asian, and Mediterranean populations. Initial CBC and peripheral smear findings are similar to and may be confused with IDA. RDW is normal; MCV is low. Thalassemias are subclassified according to which chain of the globin molecule is affected.

The deletion of one or more α-chain genes on chromosome 16 causes abnormal α-chain synthesis, a relative excess of β-chains and a reduction in the amount of HGB in RBCs. The result is α-thalassemia. The severity is proportional to the number of genes deleted. One deletion causes no clinical disease.

A second deletion results in mild microcytic anemia, α-thalassemia minor. Bart's HGB (tetramers of λ-chains derived from fetal Hgb) is present at birth, but disappears soon after. Older children and adults have normal HGB electrophoresis patterns. The diagnosis of α-thal minor is made on the basis of exclusion.

A third deletion results in HGB H disease and mild to moderate anemia. Occasionally the anemia is severe, but does not usually require transfusion. The peripheral smear shows microcytic, hypochromic RBCs with polychromia. Rare teardrop cells and fragmented cells are seen. Reticulocyte count is elevated. HGB electrophoresis shows fast-moving HGB H, composed of α-tetramers that form as a result of the lack of β-chains. HGB H may decrease during concomitant iron deficiency.

A fourth deletion results in the production of only Bart's HGB, which binds to oxygen so tightly that it cannot be released to the tissues. Fetal death follows.

The β-Thalassemias

Although β-thalassemia may arise from gene deletion, it is more often caused by point mutations of the β-globin gene on chromosome 11. The severity of the disease depends on the number of abnormal genes inherited.

Heterozygous β-thalassemia minor results in benign microcytic anemia. Often the MCV is lower than expected for the degree of anemia. RBC count is often elevated with a normal RDW.

Homozygous inheritance results in β-thalassemia major. The peripheral smear shows severely hypochromic, microcytic RBCs with marked aniso- and poikilocytosis. Teardrops, targets, nucleated RBCs, and basophilic stippling are seen. Patients with β-thalassemia major require transfusion.

An intermediate class of β-thalassemia is seen from homozygous inheritance with less impaired β-chain production or, conversely, severe impairment with heterozygous inheritance. Transfusion may be required.

Differential diagnosis of β-thalassemias requires HGB electrophoresis. β-thal minor shows a mild increase in HGB A2 and F. No HGB A is present in β-thal major; HGB A2 and F are markedly increased.

Our FOCUS on Anemias Continues

The discussion of microcytic anemias continues in Anemia II, were the discussion moves to the macrocytic anemias.
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