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Methodological Evaluation

The CO-rebreathing method is one of the most accurate (see Gore et al. 2005) and least harmful dilution techniques to determine total haemoglobin mass. In a meta-analysis of measurement error for blood volume parameters Gore et al. (2005) reported a mean measurement error of 2.2% for haemoglobin mass (90% confidence interval 1.4 - 3.5%).

The “optimized CO-rebreathing method” (Schmidt & Prommer 2005)  yields an even lower typical error (TE) which is between 1-2% in experienced hands (Gore et al. 2006). However, several factors have to be considered to achieve this low TE: (i) most crucial is the reliability and precision of the blood gas analyzer to determine the percentage of carboxyhaemoglobin (COHb%).
To minimize the analyser error to <1% on an OSM3 (Radiometer, Copenhagen) ≥ 5 replicates of COHb% for one sample time point should be performed (Alexander et al. 2010), (ii) blood sampling after the rebreathing procedure should not occur before minute 6 to ensure that all inhaled CO is equally distributed in the body (Prommer & Schmidt 2007).

The authors recommend performing the sampling in healthy subjects at min 6 and 8. In clinical populations the exact sampling times are to be evaluated since differences in blood flow and CO distribution can lead to modified sampling time points. (iii) critical monitoring for CO leaks e.g. at the nose or mouth of the subject should be performed.

VO2max can be reduced by up to 4% directly after the test due to reduced oxygen carrying capacity. However, after ~2h half of the CO bound to haemoglobin is exhaled and baseline values are reached after ~12h. Therefore, the test can be performed one day before competition without any harm (Schmidt & Prommer 2005).