p50 is the oxygen tension at half saturation (50 %) of blood and is calculated from the measured oxygen tension and oxygen saturation by extrapolation along the oxygen dissociation curve to 50 % saturation. The systematic symbol for p50 determined from arterial blood is p50(a). The analyzer symbol may be p50(act) or p50.
What does p50 tell you
p50 is the main descriptive characteristic of oxygen release to the tissue (or oxygen affinity). p50 is the pO2 at half saturation (50 %), and thereby p50 describes the position of the oxygen dissociation curve.
The position of the oxygen dissociation curve is dependent primarily on pH, but several other physical and chemical changes may affect hemoglobin affinity for oxygen.
p50(a) reference range (adult) : 24-28 mmHg (3.2-3.8 kPa)
When pO2 and ctO2 have been reviewed, p50 should be evaluated. This parameter describes the position of the ODC (i.e., the hemoglobin-oxygen affinity characteristics), which is essential for the oxygen release to the tissues. Physiologically, the affinity is altered secondary to changes in several other parameters.
Therapeutical manipulation of the affinity characteristics (i.e., the ODC position, by altering blood pH and/or blood pCO2, altering 2,3-DPG levels by phosphate supplementation, or altering body temperature) is often possible and may be indicated in selected cases. p50 serves as a useful monitor of the effects of such interventions.
The general rules
A right shift of the ODC, e.g., caused by acidosis, facilitates oxygen release to the tissues, but compromises pulmonary oxygen uptake. A left shift of the ODC, e.g., caused by FHbF, facilitates oxygen uptake in the lungs (or in the placenta), especially in situations with low pO2, but may compromise peripheral oxygen release.
When p50 is calculated from one arterial blood sample, the parameter is quite sensitive to the quality of the measurements, especially at high sO2 values. Calculation of p50 is less reliable when sO2 > 97 %.
Be aware of the risk of preanalytical errors (air bubbles, storage) on p50 values.
For more information, go to Preanalytical considerations.