EXTERNAL QUALITY ASSESSMENT


protein material also highlights errors in electrolyte analysis that cannot be detected by other aqueous controls. Despite these advantages, aqueous- based material has poor buffering capacity and ability to dissolve gases, compared to fresh whole blood. This has most impact on pO2


measurements at low


concentration, with factors such as tubing length, chamber material and test time contributing to the variation between analysers. Thus, although this material allows comparison of pO2


performance


within peer groups, it is unsuitable for between-instrument comparisons. For this reason, Weqas also distribute a sample comprising tonometered bovine haemolysate on a yearly basis, to assess the accuracy of pO2


performance. This


material has similar oxygen saturation characteristics to fresh human blood and allows comparison of performance between different analysers.5


A 20-year retrospective study A retrospective study, covering over 20 years of pO2


distributions (2001-2023),


was conducted to compare the pO2 performance of laboratory and POCT


analysers, using both aqueous material and fresh bovine haemolysate. The use of POCT analysers has grown significantly over the past 25 years, with the proportion rising from just 7% of analysers in 2001, to 43% in 2010, reaching 95% in 2023 (Table 1). The performance, expressed as the bias against the all-method average was calculated for each analyser for the haemolysate and concentration matched aqueous samples and compared with the allowable analytical performance specification, the Weqas Total Allowable Error (TAE). The all-method average was used in preference to the overall mean to allow equal weighting for each method group. The Weqas TAE is based on Milan Model 3, ‘state of the art’ and is reviewed every two years.


The intra-variation (mean ± 2 SD) is shown as an error bar for each analyser for, 2011, 2015 and 2021 respectively, for the haemolysate matrix (Fig 2).


Study findings


The study showed that there was little change in the overall precision profile for pO2


<15 kPa for analysers prior


between 2001 and 2021 using the aqueous material, however, a much lower overall method CV was observed at a pO2


to 2000, reflecting use of laboratory- based blood gas analysers, rather than POC instruments. There was significant improvement in the inter-laboratory variation for the haemolysate compared with aqueous material at low pO2


4


Fig 2. Performance of blood gas analysers for pO2 material.


assessed using fresh tonometered haemolysate


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