Arterial blood gases (ABGs) or normal blood gases is a collective term applied to three separate measurements—pH, Pco2, and Po2—generally made together to evaluate acid–base status, ventilation, and arterial oxygenation. Oxygen (O2) and carbon dioxide (CO2) are the most important respiratory gases, and their partial pressures in arterial blood reflect the overall adequacy of gas exchange. Pao2 is affected by age and altitude. Paco2 by altitude. Therefore, Pao2 must be individually calculated for each patient, and both determinations must be interpreted against local normal values. Hydrogen is not present in blood as a gas and, therefore, does not exert a partial pressure. However, pH, which measures hydrogen ion activity, is a conventional part of every arterial blood gas determination. The normal range for blood pH is 7.35 to 7.45.

Accurate results for ABGs depend on collecting, handling, and analyzing the specimen properly. Clinically important errors may occur at any of these steps, but ABG measurements are particularly vulnerable to preanalytic errors. The most common problems include nonarterial samples, air bubbles in the sample, either inadequate or excessive anticoagulant in the sample, and delayed analysis of an un-cooled sample.