Measurement of carboxyhaemoglobin by spectrophotometry and gas chromatography

The purpose of this study was to evaluate state-of-the-art spectrophotometry for measurement of carboxyhaemoglobin (COHb). We measured the fractional concentration of COHb in 109 blood samples from patients under investigation of anaemia or with exposure to carbon monoxide (smokers) with the OSM3 Hemoximeter and by gas chromatography. Duplicate measurements were made with both methods in 42 samples. We found only a trivial systematic difference between the two methods. There was, however, a considerable scatter of the measurements, the limits of agreement (95% confidence limits for the difference between the two methods) being -0.98 and 0.86% COHb. The poor agreement between the methods was largely explained by a large random scatter in duplicate spectrophotometric measurements, whereas the method based on gas chromatography was highly reproducible. We conclude that the low accuracy of spectrophotometric measurements of COHb precludes its use for assessment of the endogenous production of CO, but that it may be useful for assessment of exposure to exogenous CO.