A phrase used to describe the analysis of a sample of blood, usually arterial, for measurement of oxygen, carbon dioxide, oxygen saturation, pH (a marker of acidity), bicarbonate, and base excess (the amount of acid required to return a unit volume of the blood to normal pH). These values are vital in monitoring the severity of illness in patients receiving intensive care or who have severe respiratory illness, as they provide a guide to the effectiveness of oxygen transport between the outside air and the body tissues. Thus they are both a guide to whether the patient is being optimally ventilated, whether by breathing spontaneously or being artificially ventilated, and also a general guide to the severity of their illness.