Glucose QC Shift After a Reagent Lot Change
A clinical chemistry laboratory runs two levels of glucose quality-control material with each batch on an automated analyser. Over 20 days, level 1 has a mean of 100 mg/dL and a standard deviation (SD) of 3 mg/dL; level 2 has a mean of 120 mg/dL and an SD of 4 mg/dL. The laboratory uses Westgard multirule QC: 1_2s as a warning, and 1_3s, 2_2s, R_4s, 4_1s, and 10x as rejection rules. In today’s run, level 1 is 98 mg/dL and level 2 is 133 mg/dL. In yesterday’s run, level 2 was 129 mg/dL. The analyser has not been recalibrated for three days, and a new lot of glucose reagent was placed on the analyser yesterday. The laboratory has 40 patient glucose results waiting to be reported.
1. Calculate the z-scores and identify every Westgard rule violated today.
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z = (x − mean)/SD. Level 1: (98 − 100)/3 = −0.67 SD. Level 2 today: (133 − 120)/4 = +3.25 SD. Level 2 yesterday: (129 − 120)/4 = +2.25 SD. Violations: 1_3s because today’s level 2 is more than +3 SD; 2_2s because yesterday’s and today’s level 2 are both greater than +2 SD. Level 1 is within ±2 SD. R_4s is not violated: it requires one control greater than +2 SD and another less than −2 SD in the same run; today level 1 is −0.67 SD. The 4_1s and 10x rules are not triggered by the data shown.
2. Should the run be accepted? What immediate action is required?
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Reject the run. The 1_3s and 2_2s violations indicate systematic error, and the shift in level 2 after a new reagent lot points to calibration or reagent problems. Do not report the 40 patient glucose results. Repeat the QC after checking reagent lot, reconstitution, storage, expiry, and calibration. If the repeat QC passes, rerun the patient samples and review results; if it fails, recalibrate or replace the reagent and repeat QC. Document the event and notify the supervisor.
3. How would you distinguish random from systematic error here, and what follow-up would you recommend?
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Systematic error is suggested by a consistent shift in one control level over two runs (2_2s) and a +3.25 SD point, especially after a reagent lot change. Random error would show scattered points, a single outlier, or an R_4s violation with opposite directions. The level 1 control is stable, so the problem is likely specific to the level 2 material or to reagent/calibrator response at higher glucose concentrations. Review a Levey-Jennings chart, compare old and new reagent lots, and check calibration. If the shift persists, recalibrate with fresh standards and repeat both levels.