This carrier state for SCD is the most frequently encountered sickle hemoglobin variant; it is not a mild form of sickle cell disease. Approximately 8 percent of the US African-American population, or 2.5 million people, carry the gene for sickle hemoglobin. This heterozygous state is a result of autosomal dominant inheritance and results in HbAS. Each red blood cell contains both normal HbA (60 percent) and HbS (40 percent). The abundance of normal HbA prevents sickling under most physiologic circumstances. Hematologically, these patients are normal. Their red blood cells have a normal life span, so they are not anemic. Sickled red blood cells should not be seen on peripheral blood smear except under extreme hypoxia; if seen under normal circumstances, the diagnosis is incorrect.
Clinically, patients with sickle cell trait have minimal complications. The kidney is the most frequently affected organ. Microinfarcts occur in the medulla, leading to papillary necrosis and impaired concentrating ability. Hematuria can be found in 1 percent of patients with sickle cell trait. Exposure to high altitude has been associated with splenic infarction and cerebrovascular complications. Women with sickle cell trait have an increased incidence of urinary tract infections and hematuria during pregnancy. Persons with sickle cell trait have an increased incidence of sudden death during physical training, presumably due to increased sickling with extreme exertion. However, the majority of patients with sickle cell trait are asymptomatic, lead normal lives, and overall have a normal life expectancy.
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