Haemoglobin-oxygen affinity


Rarely performed.


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 (25-29 mmHg).

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').


Bain et al. Dacie and Lewis Practical Haematology. 11th Ed. 2012.