Objective To evaluate the measurement uncertainties（MU）for the determinations of serum potassium ion（K+）,sodium ion（Na+）and chlorinum ion（Cl-）by ion selective electrode（ISE）,and to improve the quality. Methods According to the Medical Laboratory-Evaluation and Expression of Uncertainty of Measurement published by China National Accreditation Service for Conformity Assessment（CNAS）in 2012,the relative expanded uncertainties（Urel）of serum K+,Na+ and Cl- were assessed by "top-down" method with the data of internal quality control（IQC）and external quality assessment（EQA）. The 2 main components [bias Ucrel（bias）and reproducibility Urel（Rw）] were set as goals. When the Ucrel（bias）or Urel（Rw）was beyond the goals,the accuracy or precision should be improved further. Results When the levels of K+ were 3.64-4.41 and 5.70-7.00 mmol/ L,the MU were 8.84% and 7.76%,respectively,the Ucrel（bias）were beyond the goals,the Urel（Rw）were not beyond the goals,and the accuracy should be improved. When the levels of Na+ were 106-122 and 130-152 mmol/L,the MU were 5.20% and 4.60%,the Ucrel（bias）and Urel（Rw）were beyond the goals,and the accuracy and precision should be improved. When the levels of Cl- were 68.9-78.3 and 96.4-107.9 mmol/L,the MU were 7.15% and 6.86%,the Ucrel（bias）and Urel（Rw）were beyond the goals,and the accuracy and precision should be improved. Conclusions In the management of determination quality,the insufficiency of determination could be found by analyzing MU and 2 main components which are assessed with the data of IQC and EQA and CNAS technical reports,which can be improved further.
