What is a possible reason for the differing bilirubin mean values between two laboratories using the same analysis method?

The correct answer highlights the significance of calibrators in laboratory testing. When two laboratories use different sources of bilirubin calibrator, it can lead to variations in mean bilirubin values, even if both labs are employing the same analytical method. Calibrators are essential for ensuring that assay results are accurate and comparable across different testing environments. If one laboratory uses a calibrator that is not representative of the bilirubin levels it is meant to measure—be it due to differences in purity, preparation, or stability—this can directly impact the results obtained.

Differences in the properties of the calibrators can cause systematic biases in measurements, leading to significantly different mean values for bilirubin despite identical methodologies being used. Hence, the source of the calibrator is a critical factor that can explain discrepancies in test results between laboratories.