Failure of internal quality control in detecting significant reagent lot shift in serum creatinine measurement

BACKGROUND: Internal quality control (IQC) in clinical laboratories is carried out to monitor analytical stability. Usually, the satisfactory results of the IQC ensure the acceptability of the examination results. Here, we reported that patients' creatinine results are unreliable, although the internal quality control is satisfactory. METHODS: Creatinine levels were analyzed from two quality control materials and twenty patients' specimens using two different lots of reagents. Lot‐to‐lot comparison was performed. The daily median values of serum creatinine levels of patients were calculated from the test results recorded in our laboratory information system. RESULTS: Although IQC was consistent, serum creatinine concentrations were higher using lot B (median: 153 μmol/L; interquartile range: 122‐522 μmol/L) than using lot A (median: 133 μmol/L; interquartile range: 76‐508 μmol/L) for 20 patients (P = .001). The Deming linear regression showed a best fit of y = 0.9394 × x + 45.66. R (2) = .8919, and mean percentage difference between two lots was 34%. The new lot was considered unacceptable. Likewise, the median serum creatinine level from the 360 patients using lot B was 102 μmol/L, which was significantly higher than the daily medians of patients using lot A (median: 66 μmol/L; range: 61‐70 μmol/L) in the previous month. CONCLUSION: The variations in creatinine concentrations proved to be due to different lots of reagents. However, IQC materials tested using both lots of reagent exhibited minimal variation. Therefore, IQC alone is insufficient for assessing laboratory analytical results. This finding prompts us to be vigilant in potential pitfall of interpreting test results based on satisfactory IQC alone.