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The Method

The “optimized CO-rebreathing method” facilitates the determination of total haemoglobin mass, erythrocyte and plasma volumes and, therefore, blood volume. With the measurement of total haemoglobin mass, erythrocyte, plasma and blood volume can be calculated using the actual haemoglobin concentration and haematocrit.

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. It was originally invented more than 100 years ago and recently optimized by Schmidt and Prommer (2005) and Prommer and Schmidt (2007).

Carbon monoxide (CO) is used as a marker since it strongly binds to the molecule haemoglobin (330-fold stronger than oxygen). The basic idea is to measure the changes of CO bound to haemoglobin after inhaling a defined volume of this gas. A small change of CO concentration indicates a big volume whereas a big change represents a small volume.

After having determined capillary basal COHb concentration, CO is inhaled via a specially designed Spirometer (SpiCO) for 2 min. Seven minutes after inhaling the CO bolus, a second capillary blood sample is taken to determine the increase in COHb concentration. Quantifying the difference in COHb concentration before and after rebreathing and the exact CO volume distributed in the body tHb-mass can be calculated. In total the method takes 15-20min.

spicoSchematic diagram of the optimized CO-rebreathing method
Figure: Schematic diagram of the optimized CO-rebreathing method

The administered amount of CO is very small and its effect on the COHb-concentration is within the German threshold limit value (TLV).

Kinetics of carboxyhaemoglobin (HbCO) after inhalation of a carbon monoxide bolus
Figure: Kinetics of carboxyhaemoglobin (HbCO) after inhalation of a carbon monoxide bolus

The typical error (TE = SD of the difference of 2 repeated measurements of several subjects divided by √2) which indicates the accuracy of the method is reported from qualified laboratories to be between 1.1% and 2.1% (e.g. Gore et al. 2006Prommer and Schmidt 2007) . Suggesting a normal male hemoglobin mass of 800g changes of ±9g and ±17g can be determined.
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Key benefits
The application of the optimized CO rebreathing method with SpiCO provides various advantages: 

  • accurate results (TE between 1-2%)
  • high reliability
  • no radioactive impact 
  • duration only 15 – 20min 
  • easy and mobile application 
  • cheap measurement 
  • low number of medical devices

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