HbAS

HbAS, also known as sickle cell trait, refers to the heterozygous state for the beta-globin mutation that causes sickle cell disease. Individuals with HbAS carry one normal beta-globin allele (HbA) and one sickle beta-globin allele (HbS). It is commonly described as a carrier state and is distinct from sickle cell anemia (HbSS), where two HbS alleles are present.

The HbS mutation changes one amino acid in the beta-globin chain (glutamic acid to valine at position 6). Under low oxygen conditions, hemoglobin S can polymerize, causing red blood cells to adopt a sickled shape. In HbAS, the presence of normal HbA usually prevents widespread sickling under routine conditions, so people are typically asymptomatic. Symptoms, when they occur, are usually related to extreme dehydration, severe hypoxia, or high-intensity exercise and may include transient hematuria or very rarely splenic infarction at high altitude.

Diagnosis is commonly made by hemoglobin electrophoresis or high-performance liquid chromatography, which in HbAS shows both HbA and HbS with no or minimal HbS-related symptoms. Typical patterns include HbA about 55–60% and HbS about 35–45%, with small amounts of HbA2 and HbF. The sickling test may be positive only under deoxygenating conditions.

HbAS provides partial protection against severe Plasmodium falciparum malaria, a selective advantage in malaria-endemic regions. The trait is inherited in an autosomal recessive manner, and individuals with HbAS usually have normal life expectancy and do not require disease-specific treatment. Genetic counseling is recommended for family planning, and standard health precautions during extreme conditions (adequate hydration, avoidance of extreme altitude) are advised.