The ‘gold standard’ sample for blood gas analysis is arterial blood obtained via an indwelling arterial catheter or by arterial puncture. For a number of reasons capillary blood is an attractive substitute sample that is routinely used in some clinical settings. The purpose of this article is to examine the evidence that blood gas parameter values (pH, pCO₂ and pO₂) obtained from a capillary blood sample accurately reflect arterial blood. There is conflicting opinion that increasing local blood flow (by warming or application of vasodilating agent) prior to capillary blood sampling is necessary for most accurate results and this controversial issue will be addressed.

Note: The unit of pCO₂ and pO₂ measurement used in this article is kPa - to convert kPa to mmHg divide by 0.133.