blood gas test blood gas test a blood gas test or blood gas analysis tests blood to measure blood gas tension values, it also measures blood ph, and the level and base excess of bicarbonate. the source of the blood is reflected in the name of each test; arterial blood gases come from arteries, venous blood gases come from veins and capillary blood gases come from capillaries. the blood gas tension levels of partial pressures can be used as indicators of ventilation, respiration and oxygenation. analysis of paired arterial and venous specimens can give insights into the aetiology of acidosis in the newborn. blood gas testpurposemeasure blood gas tension values values measured