Urinary iodine testing practices in China: Results of the 5-year external quality assessment program.

Background: This study analyzed urinary iodine (UI) external quality assessment (EQA) data from 2020 to 2024 to identify existing issues and offer recommendations for enhancing UI measurement accuracy and standardizing testing procedures.

Methods: Data were categorized based on measurement methods, and robust mean and robust coefficient of variation (CV) were calculated in accordance with ISO13528. The robust mean served as the target value, with the evaluation criteria set at ± 30 % of this target. Raw data analysis, along with comparisons of pass rates and inter-laboratory CVs, was conducted to assess the performance of various methods.

Results: The overall pass rate for UI testing was consistently high (all above 90%). The performance of the tetramethylbenzidine (TMB) colorimetric assay, though slightly lower, showed year-on-year improvement, while arsenic-cerium catalytic spectrophotometry (ACCS) maintained consistently high performance. The pass rate of all methods increased with concentration, with lower concentrations yielding a 3-7% reduction in pass rate compared to higher concentrations. Additionally, the inter-laboratory CV decreased with concentration, reaching approximately 10% at low concentrations. TMB exhibited the highest inter-laboratory CV across all concentrations. Despite the lower pass rate and higher inter-laboratory CV for TMB, no significant correlation between inter-laboratory CV and pass rate was observed. Furthermore, human errors such as incorrect decimal placement and sample misordering were identified as factors contributing to unqualified results.

Conclusion: The findings underscore the need to improve both the trueness and precision of low-concentration UI measurement. This study provides valuable insights and actionable recommendations for enhancing UI testing quality in clinical laboratories, addressing factors related to personnel, samples, and methodologies.