FO2Hb is defined as the ratio between the concentrations of O2Hb and tHb (cO2Hb/ctHb). It is calculated as follows:
The systematic symbol for arterial blood is FO2Hb(a).
The analyzer symbol may be O2Hb or FO2Hb.
What does FO2Hb tell you
The fraction of oxygenated hemoglobin is a measure of the utilization of the potential oxygen transport capacity; i.e., the fraction of oxygenated hemoglobin in relation to all hemoglobins present (tHb), including dyshemoglobins.
FO2Hb(a) reference range (adult): 94-98 % (0.94-0.98)
High (normal) FO2Hb:
- Sufficient utilization of oxygen transport capacity provided that ctHb is normal
- Potential risk of hyperoxia (see pO2)
Common causes of low FO2Hb:
- Impaired oxygen uptake AND/OR low venous FO2Hb or ctO2 (i.e., high oxygen extraction fraction: anemia, low cardiac output, etc.)
- Presence of dyshemoglobins
- Right shift of ODC
FO2Hb is sometimes erroneously called “oxygen saturation” or “fractional saturation”; two terms which should be avoided. The relation between FO2Hb and sO2 is:
FO2Hb = sO2 × (1 - FCOHb - FMetHb)
It is important to know that “oxygen saturation”’ as measured by pulse oximeters is not FO2Hb, but sO2. The equation given above expresses the relationship between FO2Hb and sO2. Thus, if no dyshemoglobins are present, the fraction of oxygenated hemoglobin equals the oxygen saturation, expressed as a fraction.
The difference between the two can be seen from the example below. Note that this primarily is useful when used in relation to ctHb.
ctHb = 10 mmol/L
cHHb = 0.2 mmol/L
cCOHb = 3 mmol/L ~ 30 %
cO2Hb = 6.8 mmol/L
Be aware of the risk of preanalytical errors (air bubbles, storage, inhomogeneous sample) on FO2Hb values.
For more information, go to Preanalytical considerations.