Sickle cell trait is a heterozygous condition of the hemoglobin S (HbS) gene, making the person HbAS. It is a benign carrier condition, usually with none of the symptoms of sickle cell anemia. The erythrocytes of such individuals contain both normal adult hemoglobin (HbA) and HbS, but there is always more HbA than HbS.
Sickle cell trait is usually asymptomatic. Anemia and painful crises are exceedingly rare. Papillary necrosis may cause painless hematuria in young males, though not a common presentation. Isosthenuria is a more common manifestation of the same process. High altitude exposure and extreme dehydration or exercise can rarely lead to sudden death. Urethral obstruction due to papillary sloughing is noted in few cases. Sickle cell trait is occasionally associated with exertional rhabdomyolysis and rarely with renal medullary cancer  .
Patients with sickle cell trait are symptomless and require no treatment. They are recommended to maintain oral hydration during or in anticipation of periods of extreme physical exertion, exposure to heat or cold, emotional stress, or infection. Travel in an unpressurized cabin and high altitudes may cause sickle cell crisis.
Individuals with sickle cell trait have normal life expectancy and no anemia. They are not at excessive risk for infection and not prone to develop painful crisis under normal circumstances. If factors predisposing to sudden death or sickle cell crisis are avoided, the person has a normal life like the general population .
Sickle cell trait is an inherited condition. It is one of the sickle cell disorders, in which the homozygous (SS) sickle cell disease and many other forms of hemoglobinopathies are included. Sickle cell trait is an heterozygous carrier state which is inherited in autosomal recessive form, resulting in the occurence of a HbS gene .
In the United States, the prevalence of sickle cell trait is approximately 8 to 10% in African Americans. The group of sickle cell diseases constitutes the most common severe genetic disease in the UK and France. HbS gene is widespread throughout the world, with significant presence in Africa, Mediterranean countries, the Middle East, and parts of India. The prevalence of sickle cell trait is the highest at 25 to 30% in certain areas of western Africa. The Hispanic population in the Unites States has sickle cell trait in 1 in 180 live births. There are approximately 2.5 million people in the United States and 300 million in the world who have sickle cell trait. The presence of sickle cell trait confers some protection against severe falciparum malaria, leading to the prevalence of this gene in malaria prone areas .
Sickle cell hemoglobin results from a mutation in which the 17th nucleotide of the beta globin gene is changed from thymine to adenine and as a result the amino acid glutamic acid is replaced by valine at position 6 in the beta globin chain. Sickle cells have a reduced deformability and an increased fragility, causing respectively, occlusion of the microcirculation and anemia. HbS forms polymers under deoxy conditions and exhibits changes in solubility (gel formation) and molecular stability. The presence of normal adult hemoglobin has an inhibitory effect on gelation. Therefore, the presence of one normal and one sickle allele (HbAS) in sickle cell trait leads to absence of symptoms .
Sickle cell trait cannot be prevented because it is an inherited disease. However, passing on the gene to children can be prevented if an individual knows his hemoglobin profile. Health complications can be avoided by taking proper steps. Severe hypoxia, severe dehydration and severe physical exertion can induce sudden death. Therefore caution is advised for flying in unpressurised aircraft and visiting very high altitudes. Care must be exercised if the person undergoes general anesthesia. Exertional heat injury, painful sickling crisis, splenic infarction, and sudden death can be avoided by adequate hydration and avoidance of severe heat .
Sickle cell trait is achieved through heterozygous inheritance of hemoglobin S, in which an individual possesses hemoglobin A at approximately 60% and hemoglobin S at approximately 40%. These individuals are hematologically normal. Excessive exertion or severe hypoxia may precipitate sickle cell crisis or sudden death. Avoidance of risk factors like exposure to heat and avoidance of severe exertion are usually enough to prevent morbidity. Screening for sickle cell trait is useful to inform affected persons of health risks and to provide information that might affect an individual's reproductive decisions .