To determine if you have sickle cell trait or sickle cell disease
Routinely as part of newborn screening; if you are in a high risk group and were born before newborn screening was mandated and want to know if you have sickle cell disease or are carrying the sickle cell trait; when you have signs and symptoms of anemia or abnormal results from a complete blood count (CBC) and your healthcare practitioner suspects sickle cell disease or trait
A blood sample drawn from a vein in your arm or, for infants, by pricking a heel or finger
None; however, if this test is used for diagnosis, the sample should not be drawn after a recent blood transfusion.
Sickle cell anemia is an inherited disorder that leads to the production of an abnormal type of hemoglobin called hemoglobin S (Hb S or Hgb S). Sickle cell tests determine the presence and relative amount of hemoglobin S in a blood sample or detect mutations in the genes that produce hemoglobin to help diagnose sickle cell anemia and/or identify people with sickle cell trait.
Hemoglobin is the protein in red blood cells (RBCs) that binds to oxygen in the lungs and carries it to tissues throughout the body. Typically, hemoglobin A (Hb A, adult hemoglobin) makes up most of the hemoglobin found in normal RBCs in adults, with small amounts of hemoglobin A2 and hemoglobin F. Before babies are born, they normally produce large amounts of hemoglobin F (Hb F, fetal hemoglobin), which is then replaced by Hb A as the predominant hemoglobin shortly after birth.
Sickle cell disease is an inherited condition, passed from parents to children. Inheriting mutations in the genes that code for the production of hemoglobin can lead to abnormal types of hemoglobin (variants), such as Hb S and hemoglobin C (Hb C). Hemoglobin Cis one of the more common hemoglobin variants and may cause no symptoms or mild illness.
Hb S can form crystals that change the shape of the RBC from a round disc to a characteristic sickle shape. This altered shape limits the RBC's ability to flow smoothly throughout the blood vessels in the body, limits the hemoglobin's ability to transport oxygen to tissues, and decreases RBC lifespan from 120 days to about 10-20 days. A person with sickle cell disease (homozygous for Hb S) can become severely anemic because the body cannot produce RBCs as fast as they are destroyed. The affected person can suffer painful episodes and a variety of complications when sickled cells become lodged in and obstruct small blood vessels (vaso-occlusion).
Sickle cell tests may be used to screen for or help diagnose sickle cell anemia (also called sickle cell disease) or to identify individuals who are genetic carriers and have sickle cell trait. Testing may be used for:
Several types of sickle cell tests are available and multiple tests may be required. The types of test used depend on the purpose of testing.
In people diagnosed with sickle cell disease, these tests may be used to measure the relative amount of Hb S and follow it over the course of treatment. For example, testing may be done after a blood transfusion or erythrocytapheresis/red cell exchange to ensure that the hemoglobin S level has been reduced. In addition, treatment with the drug hydroxyurea should result in a decrease in Hb S and an increase in fetal hemoglobin (Hb F), and so these tests can be used to monitor how well someone is responding to therapy.
Occasionally, a specialized testing called DNA sequencing may be done to help identify less common hemoglobin disorders. (To learn more, read Common Question 4 below.)
Genetic counseling and education should be provided so that patients understand test results, implications of the results, and their risk of passing genetic disorders to any children.
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:
Sickle cell tests are routinely ordered soon after birth to screen newborns for sickle cell anemia.
Testing may be done when those who were born before newborn screening was mandated want to know if they have sickle cell disease or are carrying the sickle cell trait, especially if they are in a high-risk group. In African Americans, sickle cell disease occurs in one out of every 365 births.
Carrier screening may be offered when a woman is pregnant or considering pregnancy. If a woman is found to be a carrier for sickle cell, testing should be offered to her partner. Sickle cell tests may also be ordered when a person has abnormal results on a complete blood count (CBC) and blood smear and/or has signs and symptoms that suggest the presence of sickle cell anemia.
Examples of signs, symptoms and complications of sickle cell anemia include:
To learn more about signs and symptoms, see the article on Sickle Cell Anemia.
In newborns who carry the sickle cell gene copy, most of the hemoglobin is fetal hemoglobin F, but a small amount of hemoglobin S will also be present. If a newborn has sickle cell trait, there may be a small amount of hemoglobin A and hemoglobin S. A full diagnostic evaluation should be done following positive screening results.
Hemoglobin S solubility test and sodium metabisulfite screening
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.
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 have two gene copies 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.
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.
Some examples of results that may be seen with sickle cell testing are listed in the following table.
|Slightly decreased Hb A; Moderate amount Hb S (about 40%)||Sickle cell trait||One gene copy for Hb S (heterozygous)|
|Majority Hb S; Increased Hb F (up to 10%); No Hb A||Sickle cell disease||Two gene copies for Hb S (homozygous)|
Sickle cell anemia symptoms and the complications experienced will vary greatly from person to person, even within the same family.
Recent blood transfusions, typically within the last three months of the date of testing, may cause a false-negative test result with some of the tests (e.g., Hb S solubility tests) because transfusion of normal RBCs reduce the relative amount of hemoglobin S present in an affected person's system.
People with sickle cell trait are generally healthy, but those who exercise heavily, such as athletes and those who are exposed to dehydration or altitude extremes, may sometimes experience sickle cell anemia symptoms. Sickle cell carriers produce both Hb A and some Hb S. When they are subjected to significant stresses that reduce the amount of oxygen in the body, the RBCs that contain Hb S can sickle.
Anyone can inherit Hb S gene mutations, but sickle cell disease is more prevalent among those of African ancestry and those who can trace their roots to the Mediterranean area, South and Central America, the Middle East, India, and the Caribbean.
It mirrors the areas of the world where malaria is found. Historically, sickle cell offered some protection and survival advantage against malaria. Since people from these areas have moved throughout the world, sickle cell gene mutations have become more widespread. A 2013 study noted that the rates of sickle cell anemia are increasing worldwide and are projected to affect more than 400,000 newborns by 2050, with India and sub-Saharan Africa showing the sharpest increases.
Newborn screening identifies most cases of sickle cell trait and sickle cell anemia. However, this screening was not universally performed in the U.S. until relatively recently. Many adults, and especially athletes born in other countries, may not have been tested to determine their sickle cell status. Since there is some risk of "exertional sickling" during intense training, the National Collegiate Athletic Association (NCAA) advocates testing college athletes who have not documented their sickle cell status.
Your healthcare practitioner may request that DNA sequencing be done to help determine the types of abnormal hemoglobin that may be present. It is not routine testing but may be necessary if it is suspected that you have a less common form of hemoglobin (variant), such as hemoglobin F. Rather than testing for one specific gene mutation, DNA sequencing determines the order of DNA building blocks (nucleotides) in a person's genetic code. This method may be used to help identify hemoglobin disorders caused by less common mutations.
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