Definition
FHbF is the ratio between the concentrations of HbF and tHb:
The systematic symbol for arterial blood is FHbF(a). The analyzer symbol may be FHbF.
What does FHbF tell you
Fetal hemoglobin consists of two α-chains and two y-chains and has a higher oxygen affinity than adult Hb. It is also less sensitive to variations in 2,3-DPG than is adult Hb. Therefore the ODC is left-shifted when high concentrations of HbF are present.
During fetal life this ensures oxygen uptake in the placenta, and despite the left shift of the ODC, more than half of the oxygen bound will be released into the fetal tissue as the oxygen levels there are low. However, at birth the tissue oxygen levels increase, and a high FHbF may then compromise peripheral oxygen release.
Reference ranges
FHbF reference ranges (neonatal): 80 %
disclaimer
Clinical interpretations
Due to previous difficulties in measurement and few data, the clinical implications of FHbF are poorly defined. FHbF may be increased in children and adults with some hematological diseases (e.g., sickle cell anemia, thalassemias, and some leukemias).
In neonates and other patients with a very high FHbF, measuring the difference in FHbF before and after red-cell transfusion may be used to estimate total blood volume, and during exchange transfusions it may help determine the amount of blood exchanged.
Considerations
The determination of the concentration of HbF is needed for accurate determination of p50.
Be aware of the risk of preanalytical errors (inhomogeneous sample) on FHbF values.
For more information, go to Preanalytical considerations.
