Testing the agreement of medical instruments: overestimation of bias in the Bland-Altman analysis.

OBJECTIVES The Bland-Altman method is the most popular method used to assess the agreement of medical instruments. The main concern about this method is the presence of proportional bias. The slope of the regression line fitted to the Bland-Altman plot should be tested to exclude proportional bias. The aim of this study was to determine whether the overestimation of bias in the Bland-Altman analysis is still present even when the proportional bias has been excluded. METHODS Data were collected from participants attending a workplace health screening program in a public university in Malaysia between 2009 and 2010. Variables collected were blood glucose level, body weight and systolic blood pressure (n=300 per variable). Readings from the original clinical dataset were compared with twenty randomly generated datasets for each variable. The Bland-Altman limits of agreement was used to determine the agreement. The presence of proportional bias was excluded for all datasets using the recommended method. RESULTS The range of predicted bias was higher than the simulated bias for all datasets. The overestimation of bias increased as the range of actual bias increased. CONCLUSION Testing the slope of regression line of the Bland-Altman plot does not remove the artifactual bias in the prediction.