Haemoglobin - oxygen affinity

Last updated: Monday, 06, August, 2007

Rarely performed.

MBS Online Search Page: Click here for details

Item Process

1 mL blood in heparinised syringe (haemoximetry) or 5 mL blood in lithium heparin tube (tonometry).


Haemoximetry; tonometry.

Reference Interval

p50 O2: 3.4-3.8 kPa (25-29mm Hg).

The p50 O2 is the partial pressure of oxygen at which Hb is 50% saturated.


Investigation of patients with suspected haemoglobinopathy due to a high oxygen affinity haemoglobin variant (unexplained erythrocytosis).

Investigation of patients with a likely abnormality of oxygen carriage and release due to an acquired abnormality eg, acidosis, alkalosis, hypoxaemia, carboxyhaemoglobin.
See also Haemoximetry.


Decreased p50 O2 indicates increased oxygen affinity and decreased release ('shift to the left').

Increased p50 O2 indicates decreased oxygen affinity and increased release ('shift to the right').


Dacie JV and Lewis SM. Practical Haematology. 8th ed. Churchill Livingstone 1995.