Improving the sampling technique of arterialized capillary samples to obtain more accurate PaO2 measurements

Arterialized earlobe capillary blood samples (ELCS) have been used as a measurement of blood gas status for over 20 years. There is general acceptance that there is a strong correlation and limits of agreement between arterial and arterialized blood samples with respect to pH and PaCO₂. Although the correlation between the arterial and arterialized PaO₂ is good, the limits of agreement poor. Our aim was to improve the accuracy of this technique in the measurement of PaO₂ by simultaneously monitoring the oxygen saturation by pulse oximetry whilst taking an ELCS. We hypothesize that significant discrepancies between the SaO₂ and SpO₂ highlight either a poorly arterialized sample or an over aerated sample from air bubbles. We compared the SpO₂ with the SaO₂ of an arterial sample from 27 inpatients. We used the limits of agreement between these samples to define the degree of discordance we would accept between SaO₂ and SpO₂ before repeat ELCS. Subsequently, 252 consecutive patients attending our respiratory physiology unit over a six-month period had an ELCS and simultaneous SpO₂.If there was a discrepancy between SaO₂ and SPO2 of > 2% the ELCS was repeated. There was a good correlation and limits of agreement between the SPO2 and arterial SaO₂ (r = 0.97, mean difference + 95% limits of agreement: 0.34 + 2.68). A difference of more than 2% between arterialized SaO₂ and SpO₂ was identified in 21 patients out of 252 (8.3%) with SaO₂ higher in two and lower in 19 (r = 0.96, mean difference +95% limits of agreement: 0.66 + 3.1). Repeat ELCS of these 21 samples reduced this discrepancy improving the concordance of the measurements (r = 0.98, mean difference + 95% limits of agreement: 0.47 + 1.0). In one case a difference of 3% remained between the saturations. We conclude that the addition of simultaneous pulse oximetry with ELCS will identify rogue measurements in about 8% of cases highlighting the need for repeat samples and thus increasing the accuracy of the measurement of PaO₂ by ELCS.