Arterial versus capillary blood gases: A meta-analysis

A meta-analysis determined whether capillary blood gases accurately reflect arterial blood samples. A mixed effects model was used on 29 relevant studies obtained from a PubMed/Medline search. From 664 and 222 paired samples obtained from the earlobe and fingertip, respectively, earlobe compared to fingertip sampling shows that the standard deviation of the difference is about 2.5× less (or the precision is 2.5× better) in resembling arterial PO₂ over a wide range of arterial PO₂'s (21-155 mmHg). The lower the arterial PO₂, the more accurate it is when predicting arterial PO₂ from any capillary sample (p < 0.05). However, while earlobe sampling predicts arterial PO₂ (adjusted r² = 0.88, mean bias = 3.8 mmHg compared to arterial), fingertip sampling does not (adjusted r² = 0.48, mean bias = 11.5 mmHg compared to arterial). Earlobe sampling is slightly more accurate compared to fingertip sampling in resembling arterial PCO₂ (arterial versus earlobe, adjusted r² = 0.94, mean bias = 1.9 mmHg; arterial versus fingertip, adjusted r² = 0.95, mean bias = 2.2 mmHg compared to arterial) but both sites can closely reflect arterial PCO₂ (880 total paired samples, range 10-114 mmHg). No real difference between sampling from the earlobe or fingertip were found for pH as both sites accurately reflect arterial pH over a wide range of pH (587 total paired samples, range 6.77-7.74, adjusted r² = 0.90-0.94, mean bias = 0.02). In conclusion, sampling blood from the fingertip or earlobe (preferably) accurately reflects arterial PCO₂ and pH over a wide range of values. Sampling blood, too, from earlobe (but never the fingertip) may be appropriate as a replacement for arterial PO₂, unless precision is required as the residual standard error is 6 mmHg when predicting arterial PO₂ from an earlobe capillary sample.