Determination of Agreement Between Laboratory Instruments

The present study was conducted to illustrate the utility of Bland-Altman plots for use by our laboratory staff and other non-statisticians in assessing the agreement between values measured by using two different laboratory instruments. A high degree of agreement reflects acceptable interchangeability of equipment and minimal effect on clinical decision-making. We have summarized literature that suggests that the regression line and correlation coefficient used with regression analysis, although commonly employed, are not appropriate first choices for evaluating agreement. Using the ABL 500 Radiometer and i-STAT Portable Chemistry Analyzer, we evaluated pH, PCO2, and TCO2. Bland-Altman plots were simple to produce, were not mathematics-intensive, and provided an easily interpreted, graphical answer to the question of agreement between instruments. For purposes of clinical decision-making, the ABL and i-STAT machines were found to be in good agreement for the tests evaluated.