PH
All arterial blood gas (ABG) results (FIO2, PO2, CO2, & PH) should be obtained from the same specimen. Use the PH value from the worst ABG result you have selected for a patient during their first 24 hours in ICU.
History
There have been different collection practices used when selecting the PH value, the original APACHE II user manual (version 1.0) is as above and that is how it should be collected.