Sickle Cell Disease

Dec 09, 2020

Sickle cell anemia, also called sickle cell disease (SCD), is an inherited disorder that leads to the production of hemoglobin S (Hb S or Hgb S), an abnormal form of hemoglobin (hemoglobin variant). Hemoglobin is the iron-containing protein found inside red blood cells (RBCs) that carries oxygen from the lungs to all parts of the body and releases it to the body’s cells and tissues.

Hemoglobin is made up of heme, which is the iron-containing portion, and globin chains, which are proteins. The globin protein consists of chains of amino acids, the “building blocks” of proteins. There are several different types of globin chains, named alpha, beta, delta, and gamma. Normal hemoglobin types include:

  • Hemoglobin A (Hb A): makes up about 95%-98% of hemoglobin found in adults; it contains two alpha (α) chains and two beta (β) protein chains, denoted as α2β2.

  • Hemoglobin A2 (Hb A2): makes up about 2%-3% of hemoglobin found in adults; it has two alpha (α) and two delta (δ) protein chains.

  • Hemoglobin F (Hb F, fetal hemoglobin): makes up to 1%-2% of hemoglobin found in adults; it has two alpha (α) and two gamma (γ) protein chains. It is the primary hemoglobin produced by the fetus during pregnancy. Shortly after a baby is born, Hb F is replaced by hemoglobin A as the predominant hemoglobin.

Hemoglobin variants

Hemoglobin types other than normal hemoglobins F, A, and A2 – arise from either alpha or non-alpha (beta, gamma or delta) globin chains gene mutations. There are currently a few hundred hemoglobin variants identified and described. Hemoglobin Hb S, Hb C, Hb D and Hb E, resulting from mutations of beta-globin chains, are some of the most common hemoglobin variants.

Hemoglobin S results from a mutation affecting the beta-chain of hemoglobin. This mutation can affect either one of the beta chains of Hb A (heterozygote status or HS trait) or both of them (homozygote status or sickle cell disease).

  • A person who has one normal hemoglobin gene copy and one Hb S copy will produce about 40% hemoglobin S but will produce enough hemoglobin A (about 60%) so that he or she does not generally experience significant health problems. This single altered copy (heterozygous) can be passed on to the person’s children.

  • When a person has two copies of the altered gene (homozygous), the person produces 80-90% hemoglobin S, no normal hemoglobin A, and has sickle cell anemia or sickle cell disease. Symptoms and complications of sickle cell disease may also be experienced by people who have one sickle cell gene copy and one gene copy for another hemoglobin variant (doubly heterozygous), such as hemoglobin C or the variants seen with beta thalassemia, a group of blood disorders resulting from gene mutations that decrease normal hemoglobin production. People with two copies of the Hb S gene (SS), and those with one copy and a variant (SC, S beta thalassemia, SD, SOArab), are all grouped under the term “sickle cell disease.”

The hemoglobin S mutation results in hemoglobin that is less soluble within a red blood cell, which reduces the efficiency of oxygen exchange and can cause the formation of polymers in the cell during normal stages of oxygen transport. These polymers can change the shape of the RBC from a round disc to a characteristic sickle shape, especially in reduced oxygen environments. The altered shape limits the RBC’s ability to flow smoothly throughout the body. The sickled cells can become stuck and obstruct small blood vessels, causing tissue damage.

Sickled RBCs are generally short-lived, only lasting about 10-20 days instead of the normal 120 days. To compensate, affected individuals must produce more red blood cells at a much faster rate and release them into the bloodstream earlier. They may become increasingly anemic when the body cannot keep up with the rapid RBC destruction, resulting in a condition known as hemolytic anemia, smaller than normal red cells (microcytosis), and an increased number of newly produced red cells called reticulocytes (reticulocytosis).

According to the National Heart, Lung, and Blood Institute, about 100,000 people in the U.S. have sickle cell anemia. It affects about one in 365 African Americans. About one in 13 African Americans are estimated to have sickle cell trait. Other people affected by this disease have Hispanic, southern European, Middle Eastern, and Asian Indian backgrounds.

Sickle Cell Test

Sickle cell tests are used to identify the presence of hemoglobin S, to evaluate the status and number of a person’s red blood cells (RBCs) as well as hemoglobin level, and/or to determine whether a person has one or more altered hemoglobin genecopies. The presence of other abnormal hemoglobin variants may be seen but would require additional testing to identify specifically what type.

There are almost 900 hemoglobin variants of which hemoglobin S is one. To screen for and to confirm the presence of hemoglobin S, a variety of tests have been developed. Some of these tests are:

For Screening

Screening may be performed on family members of an individual who has sickle cell trait/disease. It also may be done for those who were not screened at birth because universal newborn testing was not yet implemented and who may choose to be tested if their status is not known.

  • Hemoglobin S solubility test and sodium metabisulfite test
    Both tests are used to screen for hemoglobin S by adding certain chemicals to a patient’s blood sample that reduce the amount of oxygen present. The reduced amount of oxygen will cause the abnormal sickle-shaped cells to form. Some hemoglobin S will be present in those who carry one sickle cell gene (sickle cell trait) and much more will be present in those who have sickle cell disease. This test detects the presence of hemoglobin S but does not distinguish between sickle cell disease and trait. It should not be performed on infants until they are at least 6 months old because of the presence of hemoglobin F as the predominant hemoglobin at birth. Infants with sickle cell disease or trait will not produce significant amounts of hemoglobin S until several months after birth; therefore, this test may give a false-negative result if performed too early (if hemoglobin S is <10%).

For Screening, Diagnosis, and Confirmation:

  • Hemoglobinopathy (Hb) evaluation
    There are several methods of evaluating the type and relative amounts of various normal and abnormal hemoglobin types. These methods typically separate the different types of hemoglobin that are present so that they can be identified and quantified. They include:

    • Hemoglobin electrophoresis, traditionally used as the method to identify the presence of various hemoglobins

    • Hemoglobin fractionation by HPLC, the most frequently used method to screen for hemoglobin variants, including Hb S

    • Isoelectric focusing, a highly sensitive method that is often used at large reference laboratories

  • Newborn screening for sickle cell is now mandated by all 50 states in the U.S. and the District of Columbia. It is performed via the more sensitive Hb isoelectric focusing or HPLC fractionation and identifies the specific types of hemoglobin present. As an infant with sickle cell trait/disease grows and develops, the amount of Hb S will increase as the amount of hemoglobin F decreases. At about age 2, the levels stabilize.

  • DNA analysis
    This test is used to investigate alterations and mutations in the genes that produce hemoglobin components. It may be performed to determine whether someone has one or two copies of the Hb S mutation or has two different mutations in hemoglobin genes (e.g., Hb S and Hb C). Genetic testing is most often used for prenatal testing: amniotic fluid may be tested at 14 to 16 weeks to provide a definitive answer. Genetic counseling is strongly encouraged if a positive sickle screen from one or both parents is determined. It can also be performed earlier with chorionic villus sampling.

For Monitoring Treatment

Particularly in people with sickle cell disease, the relative amount of Hb S will be measured and followed over the course of treatment, for example, after a blood transfusion to ensure that the hemoglobin S level has been reduced.

Other tests that may be used to help evaluate someone who is suspected of having or who is known to have sickle cell trait or disease include:

  • Complete blood count (CBC).
    The CBC is a snapshot of the number of cells in the bloodstream. Among other things, the CBC indicates how many red blood cells are present and how much hemoglobin is in them and will evaluate the size and shape of the RBCs present. This test is used to detect anemia.

  • Blood smear (also called peripheral smear and manual differential)
    In this test, a trained laboratorian looks at a thin, stained layer of blood on a slide under a microscope. The number and type of red blood cells are evaluated to see if they are normal. Sickle-shaped RBCs may be seen on the blood smear.

  • Iron studies
    These may include: iron, ferritin, UIBC, TIBC, and transferrin saturation. These tests measure different aspects of the body’s iron storage and usage. They are ordered to help determine whether someone has an iron deficiency anemia or an excess amount of iron (iron overload). People with sickle cell anemia who receive multiple blood transfusions may experience an iron overload.

What Does the Test Result Mean?

Newborn screening

In newborns who carry the sickle cell gene, fetal hemoglobin F will predominate but a small amount of hemoglobin S will also be present. There may be a small amount of hemoglobin A if they have sickle cell trait. A full work-up should be done after the child reaches six months of age.

Diagnostic testing

Adults with sickle cell trait will produce mostly normal hemoglobin A, while those with sickle cell disease (anemia) will produce mostly Hb S with no Hb A. People who are heterozygous for two different hemoglobin variants will usually produce varying amounts of both types. For example, they may produce both Hb S and Hb C but no Hb A.

Genetic testing

If two copies of the Hb S gene mutation are detected, then the person has sickle cell disease. If the person has one gene that codes for Hb S and one normal gene, then the person has sickle cell trait. If the person has one Hb S copy and a Hb C or beta thalassemia mutation, then the person is likely to experience some symptoms and complications associated with sickle cell disease. If the person has one Hb S gene copy and another, more rare hemoglobin variant, then the person may or may not have any symptoms or complications. See the article on Hemoglobin Abnormalities for more on this.