Accuracy of gas analysis in lung function laboratories.

Fifty lung function laboratories in England and Wales analysed test gas mixtures of carbon monoxide and helium. Most of them also analysed mixtures of oxygen and carbon dioxide in nitrogen. The percentage accuracy of the results was within 1% of the expected value in only 14% of determinations of carbon monoxide concentration, 28% for carbon dioxide, 37% for helium, and 48% for oxygen. The accuracy of ratios of two concentrations of helium and carbon monoxide was better than that of the individual gas samples. Overall the variation in results between laboratories was wide, the coefficient of variation ranging from about 3% for analysis of helium to 9% for carbon dioxide. This variation affected the values calculated for carbon monoxide transfer factor, where 20% were in error by more than 5%, and for the calculated value of the respiratory exchange ratio, where the interlaboratory coefficient of variation was about 10%. Errors in analysis were due to unsatisfactory calibration of analysers; five oxygen analysers had large zero errors; five carbon monoxide analysers and one helium analyser had notably curvilinear calibration curves. Insufficient information was obtained to ascertain the nature of the errors in analysis of carbon dioxide. Given the improvements in instrumentation, these results are evidence for deterioration in analytical standards in lung function laboratories from the standards of 20 years ago.