pO2(x) or px, as referred to from now on, is termed the oxygen extraction tension of arterial blood. It is a parameter that reflects the integrated effects of changes in arterial pO2, oxygen concentration, and changes in the hemoglobin-oxygen affinity on the ability of arterial blood to deliver oxygen to the tissues.
px is defined as the oxygen tension measured in arterial blood after extraction of 2.3 mmol of oxygen per liter of blood (at constant pH and pCO2), which corresponds to the normal arteriovenous difference in the total oxygen concentration. The systematic symbol for the arterial oxygen extraction tension is pO2(x). The analyzer symbol may be px or pO2(x).
What does px tell you
The purpose of the oxygen extraction tension is to determine whether or not a hypoxemia, an anemia, or an abnormally increased hemoglobin-oxygen affinity is inherently compensated (i.e., whether oxygen transport and release capacity are within the normal range).
pO2(x) or px reference range (adult) :
male: 35-41 mmHg (4.6-5.5 kPa)
female: 32-39 mmHg (4.2-5.2 kPa)
A px above the normal range may indicate low hemoglobin-oxygen affinity, which (if severe) may occasionally contribute to an impairment of pulmonary oxygen uptake not reflected in pO2.
A low px may be due to low ctO2, high hemoglobin-oxygen affinity, or both, and indicates that cardiac output must be elevated to maintain normal venous oxygen tension, if oxygen consumption is normal. Correction of low ctO2 and/or abnormal hemoglobin-oxygen affinity may be guided and monitored by the px.
px is a theoretical and calculated parameter based on the determination of the ODC. The ODC is quite sensitive to the quality of the measurements, especially if the ODC is based upon high sO2 values, close to 97 %. Calculation of px is unreliable when sO2 > 97 %. The information provided by px must be interpreted with this in mind.
Be aware of the risk of preanalytical errors (air bubbles, storage) on px values.
For more information, go to Preanalytical considerations.