The idealised p50 value for a standard set of conditions

This chapter is most relevant to Section F7(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the oxyhaemoglobin dissociation curve and factors that may alter it". The "standard" corrected p50 value which is reported by the blood gas analyser, and the difference between this and the normal p50, can be illuminating with regards to these factors that may alter it. This topic has minimal relevance to exam-going CICM trainees and probably also to practising intensivists, and is therefore on par with the pointlessness of this online resource in a broader sense. 
In summary:
  • This is an idealised value calculated from the measured PaO2 at a standard set of conditions (pH 7.40, pCO2 40mmHg, and in the absence of dyshaemoglobins), represented as p50(st).
  • By "normalising" the potentially very abnormal environment of the critically ill patient's bloodstream, the p50(st) value eliminates the effect of everything except 2,3-DPG.
  • In the adult, the normal p50(st) should be 24-28mmHg.

Calculation of the p50(st)

Unlike the "real" p50 which at least makes some attempt to adjust the shape of the oxygen-hemoglobin dissociation curve according to measured variables, the p50(st) makes no such attempt.

Observe the equation for p50:

p50 equation

Now, the equation for p50(st) would look exactly the same, with the exception of the calculated shift.

As has been previously discussed, this is a sum of all the influences on the x-axis position of the curve, all the "Bohr's effects" (pH and pCO2) as well as the concentration of "normal" dyshaemoglobins and temperature.

If all of these were null, the only remaining measured variables required would be 2,3-DPG and weird rate dyshaemoglobins (like sulfhaemoglobin or nitrosylhaemoglobin, for example).

Therefore, the real value of p50(st) lies in the comparison between itself and p50.

Situations where there would be a difference between p50 and p50(st)

Observe, the oxygen-hemoglobin dissociation curve with a hypothetical left shift.

difference between p50 and p50st

Thus, the difference between the two values reflects the influence of Bohr's effect and temperature on the shape of the dissociation curve. This is actually a valuable piece of information, as these variables are amenable to some sort of crude adjustment. In contrast, the concentration of 2,3 -DPG is not something you can immediately address.


For my ABG analyser, one can examine this handy operations manual.

Malmberg, P. O., M. P. Hlastala, and R. D. Woodson. "Effect of increased blood-oxygen affinity on oxygen transport in hemorrhagic shock." Journal of Applied Physiology 47.4 (1979): 889-895.

Woodson, Robert D. "Physiological significance of oxygen dissociation curve shifts." Critical care medicine 7.9 (1979): 368-373.

MACDONALD, ROSEMARY. "Red cell 2, 3‐diphosphoglycerate and oxygen affinity." Anaesthesia 32.6 (1977): 544-553.

Huber, Fabienne L., et al. "Does venous blood gas analysis provide accurate estimates of hemoglobin oxygen affinity?." Annals of hematology 92.4 (2013): 517-521.