Determining Moisture Content in Milk Powder: Challenges in the Evaluation of Performance by Proficiency Testing Using Independent Reference Values

The water content in milk powder must be controlled to ensure its stability. The analytical methods used for its determination can be verified through proficiency testing (PT). This study evaluated the reference values, their uncertainty, and the implications for nine PT rounds on moisture determination in milk powder conducted between 2017 and 2021 by a Chilean PT provider. An independent laboratory assigned the reference values, considering the analysis session as a source of uncertainty. Results showed that in 77.8% of the rounds, uncertainty did not meet the ISO 13528:2022 criterion, requiring the use of z'-score, which increased the risk of erroneous evaluations in 5.8% of cases, particularly near threshold values (z' ≈ 2.0 or 3.0). The robust standard deviation from participants' results exceeded 0.14 g water/100 g milk powder in 63.0% of PT items, suggesting that future evaluations should consider modifying the standard deviation for proficiency assessment. Reference values were found to be compatible with participants' consensus values, though their use affected 19.3% of performance classifications, reinforcing the need for independent reference values in PT schemes with a small number of laboratories. These findings highlight the importance of evaluating the dispersion trends in proficiency assessment and adapting standard deviation criteria for more accurate performance evaluation.