Curriculum · Hematology
Normal haemoglobin types and the fetal-to-adult switch
What it is
Hemoglobin is reported as a set of types, and a hemoglobin variant analysis gives how much of each type is present. In a normal adult the pattern is:
- Hemoglobin A: 95%-98%
- Hemoglobin A2: about 2.5%
- Hemoglobin F: less than 1%
- Hemoglobin S: absent
Every abnormal pattern is read against these normal values, so it is worth holding them in mind before any result is read.
Each hemoglobin is named by the globin chains it is built from, and it is the chains, not the amounts, that separate one from another. In the adult, HbA is two alpha and two beta chains at 97%, HbA2 is two alpha and two delta chains at 2.5%, and HbF is two alpha and two gamma chains at 0.5%. In the fetus two are given, HbF at 85% and HbA at 5-10%. In the embryo there are three, Hb Gower 1 (ζ2ε2) at 42%, Hb Gower 2 (α2ε2) at 24%, and Hb Portland (ζ2γ2), the two amounts being by the 5th week. So the delta chain is what marks HbA2 apart from HbA, and the gamma chain what marks HbF.