Creatinine QC Shift After a Reagent Lot Change
A clinical chemistry laboratory installed a new creatinine reagent lot on Monday. Two levels of quality control are run before the morning batch: Level 1 assigned mean 88 µmol/L, SD 3.5; Level 2 mean 450 µmol/L, SD 15. Today's four consecutive Level 1 results are 95, 96, 97, and 98 µmol/L; Level 2 results are 452, 455, 456, and 458 µmol/L. The previous lot's values were at the mean. Calibration was performed with the new lot using the manufacturer's calibrator. A patient sample from yesterday gave 98 µmol/L; today's result on the same sample is 126 µmol/L. The laboratory scientist must apply Westgard rules, decide whether patient results can be released, and plan the investigation. The analyzer was serviced two weeks ago; no other analytes are affected.
1. Apply the Westgard rules to both QC levels and decide whether patient results can be released.
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Level 1 deviations are (95−88)/3.5 = +2.0; (96−88)/3.5 = +2.3; (97−88)/3.5 = +2.6; and (98−88)/3.5 = +2.9 SD. All four exceed +1 SD on the same side, so the 4-1s rule is violated and QC must be rejected. The last point approaches but does not exceed +3 SD, so 1-3s is not formally violated. Level 2 deviations are +0.13, +0.33, +0.40, and +0.53 SD; no rule is violated by Level 2 alone, but the upward trend is consistent with a positive shift. The mean bias at Level 1 is 10/88 = 11.4%, far beyond desirable bias. Because QC is rejected, patient results cannot be released.
2. What investigation should be performed before results are reported?
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Do not release patient results. Investigate calibration by checking the calibrator lot, expiry, reconstitution, and assigned values; recalibrate and rerun fresh QC. Check the reagent lot against the previous lot, look for lot-to-lot bias, and verify storage and on-board stability. Check the analyzer: photometer, lamp, tubing, temperature, water quality, and carryover. Reanalyze the previous patient sample, which rose from 98 to 126 µmol/L, a 28.6% increase far beyond analytical variation, and test a fresh sample by an alternative method. If the shift persists, open a service call and consider using the old lot or a different analyzer. Review all patient results since the last acceptable QC and place them on hold.
3. What corrective and reporting actions are required once the cause is identified?
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After recalibration and repeat QC within limits, rerun all affected patient samples and correct results; document the event and corrective action. If the new lot is confirmed biased, quarantine it, notify the manufacturer and other users, and use an alternative lot. Review whether any erroneous results were reported and initiate amended reports. Perform a risk assessment for creatinine, especially for dosing of renally cleared drugs, and update the SOP to require patient-sample comparison before and after a lot change. Monitor QC for trends and use a moving average. If the cause was calibration, verify calibrator handling and frequency; if reagent-related, evaluate lot verification before routine use.