Objective:To evaluate the the effectiveness and timeliness of daily averages of patient data in identifying chloride system errors. Method: Calculate the daily arithmetic mean of the chloride ion values determined by Roche C8000 analyzer in 2018, and draw the mean change chart with the date as the abscissa. Analyze the monthly bias of internal quality control of C8000, according to the Bio-Rad Laboratory QC Worldwide Report. Evaluate the external quality assessment results and the bias between different biochemical testing systems.Compare the effectiveness and timeliness of the mean value of patient results, internal quality control, external quality assessment and different testing system comparison to identify system errors. Results: （1）From the moving average map, it can be seen that the chloride ion has been systematically low since September 28.The average value of chloride ion in 2017 was 103.1 ±1.0 mmol/L, while the average value was 99.5 ±0.5mmol/L from September 28 to December 17. （2）Comparison with equivalent group of monthly Bio-Rad Laboratory QC Worldwide Report: There was no obvious systematic bias in the results of chloride internal quality control from January to September, but there was negative bias from Octorber to December, and the standard deviation index ranges from -0.97~-1.69. （3）External quality assessment results: The SDI of chloride ion in CAP external quality assessment in June was- 0.9~+0.2SDI, and the deviation in October was-1.7~-2.9SDI.（4）Comparison in different testing system:There was no significant difference in the chloride ion result between C8000 and Beckman AU5800 in April,while the result of C8000 chloride ion is systematically low 4.0~6.0mmol/L in October. The daily averages of patient data can capture this systematically error in the occurrence of the bias in recent days,and other methods delay the detection of deviation from 1 to 2 months due to the influence of planning time.Conclusion: It is simple and effective to identify the systematic bias of chloride ion by daily averages of patient data, whose timeliness is better than external quality assessment and comparison study. The daily averages of patient data can be used as a useful supplement to the current laboratory quality control.