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Air bubbles and co-oximetry — a pilot study


The pre-analytical error potential of air bubbles contaminating blood gas samples has been well recognized for blood gas tension and pH measurements [1], and is thus considered in present recommendations for blood gas sample handling [2]. The effect of air contamination on the oxygen saturation of haemoglobin (HbO), measured by co-oximetry, has however, scarcely been appreciated [1]. With increased reliance on directly measured HbO for the evaluation of blood oxygen content [3,4], a closer look at possible errors of this parameter is warranted. This study was undertaken to estimate the early effect of graded air contamination in conditions simulating pre-analytical sample handling in clinical practice.


A. Serial analyses (blood gases/co-oximetric saturation, ABL505+OSM3, Radiometer Medical AS, Denmark) of fresh heparinized venous or arterialized venous blood from three study participants. Blood was drawn as batches (one batch per series), divided into graded aliquots in blood gas samplers, immediately capped with or without the inclusion of air corresponding to the syringe tip dead space (approx 0.05 ml), and analysed consecutively for 25 min following capping and storage in various conditions (table). All first samples were analysed immediately for reference; all samples were agitated for 10 s immediately before analysis.

B. Screening of arterial blood samples, referred to the ICU for analysis, for air bubbles. If present, bubble volume and air volume fraction (ie of total sample volume) was measured (as plunger displacement) on evacuating the bubble before analysis.


A. Twenty-eight series (n = 7) from each participant were analysed. Reference HbO was (mean ± SD) 0.55 ± 0.10 for venous and 0.92 ± 0.03 for arterialized venous samples. Air contamination regularly (although unpredictably) increased measured HbO within minutes after preparation; apparently, errors increased as a function of haemoglobin oxygen affinity, storage temperature, air volume fraction and sample agitation (Fig). Compared to changes in HbO. Changes in pH and blood gas tensions were small. The limited number of data for each series and the wide spread of HbO values, particularly in venous samples, precludes closer statistical analysis.

B. When present (n = 14), air bubbles had a volume of (median/lQR/range) 45/40–111/23–398 mm3 constituting a volume fraction of 7.2/3.8 15.4 /2.9 33.8%.


Present recommendations for pre-analytical blood gas sample handling may be inadequate in relation to co-oximetry. The error potential of air contamination on measured HbO in hypoxaemic blood appears much greater than errors on gas tensions and should be respected when evaluating such samples (eg mixed venous, or possibly hypoxaemic arterial samples). Optimally (ie concerning HbO), samples with subatmospheric oxygen tensions should be immediately purged of all air, stored on ice and analysed as soon as possible; improvements in blood gas samplers to minimize air trapping and facilitate purging of air might be worthwhile. Further systematic studies are ongoing.

Table Scheme of analysis series


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Faber, T., Franks, J., Hansen, R. et al. Air bubbles and co-oximetry — a pilot study. Crit Care 1 (Suppl 1), P115 (1997).

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