Quantitation of carboxyhaemoglobin in blood: external quality assessment of techniques

The performance of four dedicated carbon monoxide (CO)-oximeters (AVL, Chiron, IL, Radiometer), spectrophotometry with and without dithionite, spectrophotometry by second derivative, and the Whitehead and Worthington precipitation technique for the measurement of carboxyhaemoglobin in blood was compared by a mean of 136 participants in the United Kingdom National External Quality Assessment Scheme in 21 samples formulated to contain from 4% to 48% carboxyhaemoglobin. The dedicated instruments and spectrophotometry by second derivative were of significantly higher precision than the other techniques, producing fewer measurements rejected as being > 3 standard deviations from the sample mean and having a lower standard deviation for non-rejected measurements. The AVL instrument and spectrophotometry by second derivative had a significant positive bias compared to the other techniques. The Whitehead and Worthington method was of an unacceptably low precision.