ctO2(x) or cx, as referred to from now on, is defined as the amount of oxygen that can be extracted per liter of arterial blood when the oxygen tension is decreased to 38 mmHg (5.1 kPa), at constant pH and pCO2. The systematic symbol for the concentration of extractable oxygen is ctO2(x). The analyzer symbol may be cx or ctO2(x).
What does cx tell you
cx is the amount of oxygen that may be extracted from arterial blood with concomitant lowering of the oxygen tension to the average normal value in venous blood (approx. 5.1 kPa). A cx below the normal range (value) indicates a decreased ability of the arterial blood to release oxygen to the tissue, due to low ctO2, high hemoglobin-oxygen affinity, or both.
cx reference range (adult): ~ 2.3 mmol/L
With an assumed normal oxygen consumption, a low cx indicates either that the actual mixed venous oxygen tension is low, or that cardiac output must be increased to maintain normal venous tension.
cx 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 cx is less reliable when sO2 > 97 %. The information provided by cx must be interpreted with this in mind.
Be aware of the risk of preanalytical errors (air bubbles, storage) on cx values.
For more information, go to Preanalytical considerations.