Interpreting results: Bland-Altman

Difference vs. average

The first page of Bland-Altman results shows the difference and average values and is used to create the plot.

Bias and 95% limit of agreement

The second results page shows the average bias, or the average of the differences. The bias is computed as the value determined by one method minus the value determined by the other method. If one method is sometimes higher, and sometimes the other method is higher, the average of the differences will be close to zero. If it is not close to zero, this indicates that the two assay methods are producing different results.

This page also shows the standard deviation (SD) of the differences between the two assay methods. The SD value is used to calculate the limits of agreement, computed as the mean bias plus or minus 1.96 times its SD.

For any future sample, the difference between measurements using these two assay methods should lie within the limits of agreement approximately 95% of the time.

Interpreting the Bland-Altman results

Bland-Altman plots are generally interpreted informally, without further analyses. Ask yourself these three questions:

•	How big is the average discrepancy between methods (the bias)? You must interpret this clinically. Is the discrepancy large enough to be important? This is a clinical question, not a statistical one.

•	Is there a trend? Does the difference between methods tend to get larger (or smaller) as the average increases?

•	Is the variability consistent across the graph? Does the scatter around the bias line get larger as the average gets higher?