The values of arterial CO₂ tension (PaCO₂), arterial O₂ tension (PaO₂), and pH are highly dependent on temperature. Most blood gas analysis machines are calibrated to 37° C and, thus, run samples at 37° C. If the patient’s actual temperature is near 37° C, then the machine values are approximately the same as those of the patient in vivo. However, the further the patient’s temperature is from 37° C, the greater the difference between the numbers reported by the machine and the patient’s actual values.

When the sample is analyzed, it is heated to 37° C, which causes a decrease in the solubility and falsely elevates partial-pressure values. Conversely, if the temperature of the blood decreases, the solubility of O₂ and CO₂ increases, which consequently lowers the partial pressures of these gases (i.e., decreases PO₂ and PCO₂). PCO₂ decreases by approximately 4.5% for each 1° C temperature decrease (Table 24-1). Therefore, temperature correction—the process of correcting the machine values to the patient values in vivo at the patient’s actual temperature—is needed to determine the patient’s actual partial pressures of PO₂ and PCO₂. Changes in temperature also affect pH. As temperature decreases, less water is dissociated into OH^– and H⁺, and, thus, pH will rise, as will the pOH. The pH rises approximately 0.015 unit for each 1° C decrease in temperature. If the patient’s temperature is less than 37° C, but the blood sample is subsequently heated to 37° C in the analyzer, then elevated levels of H⁺ (and OH⁻) will be recorded, leading to a falsely decreased pH value compared with the patient’s actual pH.