FO₂Hb
Fraction of oxyhemoglobin

Definition
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.

Reference ranges
FO₂Hb(a) reference range (adult): 94-98 % (0.94-0.98)
disclaimer

Clinical interpretation
High (normal) FO₂Hb:

• Sufficient utilization of oxygen transport capacity provided that ctHb is normal
• Potential risk of hyperoxia (see pO₂)

Low FO₂Hb
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

Considerations
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.