There is a surprising degree of fluctuation in blood gas values in normal persons and in stabilized sick persons. In one study of normal persons using arterialized capillary blood, changes of at least 10% in bicarbonate or total CO2, 15% in PCO2, and 170% in base excess were required to exceed normal day-to-day variation. In another study, this time using arterial samples from stabilized ICU patients, average fluctuations within patients without known cause were found of 3.0 mm Hg (range 1-8 mm Hg) for PCO2, 0.03 pH units (range 0.01-0.08 units) for pH, and 16 mm Hg (range 1-45 mm Hg) for PO2. There were sufficient variation in repeat samples drawn at 10 minutes and at about 1 hour that a change of about 10% at 10 minutes and of about 20% at 1 hour was necessary to be considered significant. This suggests caution in making decisions based on small changes in acid-base or PO2 values.