Is arterialized capillary blood a suitable alternative to arterial blood for measurement of pH, pCO₂ and pO₂?

Arterial blood is the standard sample for measurement of pH, pCO₂ and pO₂, but collection of arterial blood can be painful. Furthermore, it is a procedure associated with some risk to the patient and can therefore only be performed by trained, medically qualified staff. 

 Capillary blood collection by finger or earlobe stab is by contrast a simple, relatively pain- and risk-free procedure that can be performed after minimal training by any healthcare professional. If heat or some equally vasodilatory agent is applied to the skin puncture site immediately prior to collection, the blood collected is "arterialized" capillary blood. 

Although collection of arterialized capillary blood is more convenient and safer than collection of arterial blood, controversy has surrounded its use because of conflicting evidence that pH, pCO₂ and pO₂ of arterialized capillary blood accurately reflect the pH, pCO₂ and pO₂ of arterial blood. The issue is addressed once again in a recently published meta-analysis study. 

Twenty-nine studies were identified which contained paired data comparing pH, pCO₂ and pO₂ derived from arterial blood with that from arterialized capillary blood (either finger or earlobe). In total, 886 data pairs (664 in which capillary blood was sampled from earlobe and 222 in which capillary blood was sampled from fingertip) were identified. 

Statistical analysis of this large database revealed that as far as pH and pCO₂ is concerned, arterialized capillary blood from either finger or earlobe accurately reflects arterial blood across a pH range of 6.77-7.74 and pCO₂ range of 10-114 mmHg (1.3-15.2 kPa). As far as pO₂ is concerned there was insufficient agreement between results on capillary blood obtained by finger stab with results from arterial blood. 

Blood obtained from earlobe showed much closer agreement, and the authors were able to conclude that sampling of arterialized capillary blood from the earlobe (but never from the fingertip) may be appropriate as a replacement for arterial pO₂, unless precision is required. Arterialized capillary blood is considered, without reservation, a suitable alternative for measurement of pH and pCO₂.