Background: In 2020, the accuracy-based creatinine assay (ABCr) proficiency testing (PT) survey was transitioned from an optional to a mandatory program of the Korean Association of External Quality Assessment Service (KEQAS). This study was conducted to analyze the creatinine assay bias according to the time of ABCr PT participation.
Methods: Raw data from the ABCr PT survey conducted from 2020 (1st and 2nd round) to 2021 (1st round) by the KEQAS were used. The participating laboratories were divided into two groups (before 2019 vs. after 2020). The after 2020 group refers to the laboratory participating in ABCr PT for the first time since 2020. The creatinine assay bias between the two groups was compared for each PT round. The acceptable rate between the two groups was compared.
Results: The mean absolute percentage bias of the before 2019 group was significantly smaller than that in the after 2020 group as follows: 1st round in 2020, 3.8% vs. 5.2%, P<0.001; 2nd round in 2020, 3.3% vs. 4.8%, P<0.001; 1st round in 2021, 4.1% vs. 7.4%, P<0.001. The acceptable rates in the before and after 2019 groups were 91.9%–98.7% and 64.9%–94.3%, respectively.
Conclusions: The laboratory that participated in the ABCr PT before 2019 showed smaller bias in the creatinine assay compared to that of laboratories participating in the program for the first time after 2020. Efforts should be made to improve the quality of the creatinine assay through continuous participation in the ABCr PT survey.

Keywords: Accuracy, Creatinine, Proficiency testing