Hemoglobin A2 (Hb A2) is a type of hemoglobin, the protein within red blood cells responsible for carrying oxygen throughout the body. It represents a small fraction of the total hemoglobin found in adults.
Understanding Hemoglobin A2
Hemoglobin is a protein composed of multiple globin chains. In adults, the most common type, Hemoglobin A (Hb A), consists of two alpha globin chains and two beta globin chains. Hemoglobin A2, by contrast, is formed from two alpha globin chains and two delta globin chains. Hemoglobin A makes up the vast majority of adult hemoglobin (95% to 98%), while Hb A2 is a minor component, constituting less than 3.5% of the total hemoglobin. Another type, Hemoglobin F (Hb F), is the primary hemoglobin in unborn babies and is present in very small amounts in adults.
Why Hemoglobin A2 Levels Are Measured
Measuring Hb A2 levels is a diagnostic tool, particularly for identifying certain genetic blood disorders. Its primary use is in screening for and diagnosing beta-thalassemia trait, also known as beta-thalassemia minor or carrier status. This condition involves a reduced production of beta-globin chains. The measurement of Hb A2 is often part of a broader set of tests that helps differentiate various types of anemia or other hemoglobin-related disorders.
Interpreting Hemoglobin A2 Results
The reference range for Hb A2 is between 2.0% and 3.5% of total hemoglobin. Healthcare professionals interpret Hb A2 levels, often in conjunction with other blood tests.
Elevated Hb A2
Elevated Hb A2 levels, exceeding 3.5% or 4.0%, are a characteristic indicator for diagnosing beta-thalassemia trait. In this condition, a mutation in one of the beta-globin genes leads to reduced production of beta-globin chains. The body compensates by increasing the synthesis of delta-globin chains, which then combine with alpha-globin chains to form more Hb A2. Certain genetic mutations can also contribute to increased delta-globin gene transcription, elevating Hb A2 levels. While beta-thalassemia trait is the main cause, some rare conditions, such as hyperthyroidism or megaloblastic anemia, might also show slight elevations in Hb A2.
Decreased Hb A2
Lower than normal Hb A2 levels, defined as below 2.1%, can be clinically significant. These reduced levels can occur in conditions such as severe iron deficiency anemia or rare forms of thalassemia. Iron deficiency can suppress Hb A2 levels, making iron studies an important part of interpreting low Hb A2 results. A decrease in Hb A2 may also be linked to decreased transcription or translation of the globin chains.
Normal Hb A2 with Suspected Thalassemia
A normal Hb A2 level does not exclude all forms of thalassemia. For instance, alpha-thalassemia presents with normal Hb A2 levels. If beta-thalassemia trait co-exists with iron deficiency, the Hb A2 level might appear normal, potentially masking the underlying beta-thalassemia. Therefore, a healthcare professional assesses Hb A2 results alongside a complete blood count (CBC), red cell indices, iron studies, and clinical symptoms to reach an accurate diagnosis.
The Hemoglobin A2 Testing Process
The Hb A2 test involves a blood draw from a vein in the arm. Specific preparation, such as fasting, is not required before the test. After collection, the blood sample is sent to a laboratory for analysis. Common laboratory methods used to measure Hb A2 include High-Performance Liquid Chromatography (HPLC) and capillary electrophoresis. These techniques separate and quantify the different types of hemoglobin present in the blood, providing an accurate measurement of the Hb A2 percentage.