Time and sample site dependency of the optimized co-rebreathing method

Med Sci Sports Exerc. 2006 Jun;38(6):1187-93. doi: 10.1249/01.mss.0000222848.35004.41.


Purpose: A new method to estimate hemoglobin mass (Hbmass) requires capillary blood and rebreathing a carbon-monoxide (CO) bolus for 2 min. We hypothesized that incomplete circulatory mixing of CO could confound this method, so we compared capillary with venous blood to determine whether sampling site altered the percentage of carboxyhemoglobin (%HbCO) and the reliability and accuracy of the "2-min Hbmass." The conventional 20-min CO-rebreathing procedure was used as the Hbmass criterion.

Methods: In the first experiment (N=12), both fingertip capillary and antecubital venous blood were sampled 4 and 6 min after commencing 2 min of CO-rebreathing. Within 8 d, these subjects completed two 2-min and one 20-min CO-rebreathing periods. For the latter, capillary and venous blood were collected simultaneously after two 10-min periods of rebreathing. In a second experiment (N=6), both capillary and venous blood were sampled 4, 6, 8, 10, and 12 min after commencing 2 min of CO-rebreathing. A third experiment (N=6) evaluated the reliability of a modified 2-min CO-rebreathing test with capillary blood sampled at minutes 8 and 10.

Results: Typical error (TE) for the first two 2-min tests was 1.1% (90% confidence limits 0.9-1.8%), but the average Hbmass from 2-min capillary blood was 4.8% lower than from venous blood for the 20-min procedure. In the second experiment, peak venous %HbCO occurred at minute 6, and the difference between capillary and venous values was minimal (mean+/-SD; 0.08+/-0.07, 0.01+/-0.09) at minutes 8 and 10. TE for the third experiment was 1.2% (0.8-2.5%).

Conclusion: A modified 2-min CO-rebreathing procedure using capillary or venous blood sampled 8 and 10 min after starting CO-rebreathing allows complete circulatory mixing and provides an accurate and reliable estimate of Hbmass.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Blood Circulation / physiology
  • Blood Volume / physiology
  • Capillaries
  • Carbon Monoxide* / administration & dosage
  • Carboxyhemoglobin / analysis
  • Confounding Factors, Epidemiologic
  • Female
  • Hemoglobins / analysis*
  • Humans
  • Male
  • Phlebotomy / methods*
  • Reproducibility of Results
  • Time Factors
  • Veins


  • Hemoglobins
  • Carbon Monoxide
  • Carboxyhemoglobin