Quality Control Failure on a New Creatinine Reagent Lot
A 210-bed hospital laboratory runs a chemistry analyzer 24 hours a day. The creatinine assay uses an enzymatic method with two levels of quality control material: level 1 mean 1.00 mg/dL with a standard deviation of 0.05 mg/dL, and level 2 mean 4.00 mg/dL with a standard deviation of 0.20 mg/dL. On Tuesday morning, the night technologist reports level 1 at 1.18 mg/dL and level 2 at 4.10 mg/dL. The level 1 result is outside the acceptable range; the technologist repeats level 1 and gets 1.16 mg/dL. Both patient and quality control samples were run at 4:00 a.m. The analyzer’s creatinine reagent lot was changed yesterday, and the calibration was performed by the day shift. The night technologist wants to recalibrate and, if the repeat quality control is acceptable, report the patient results that were run before the repeat. The laboratory director is out of town. A patient in the emergency department has a creatinine of 3.9 mg/dL and a potassium of 6.1 mmol/L; the potassium is from the same sample.
1. Apply the appropriate quality control rule and state whether the run should be accepted.
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The level 1 result is 1.18 mg/dL against a mean of 1.00 and a standard deviation of 0.05, which is 3.6 standard deviations above the mean. This violates the 1-3s rule, which is a rejection rule when a single control measurement exceeds 3 standard deviations. The repeat level 1 result of 1.16 mg/dL is 3.2 standard deviations above the mean and still violates the rule. Level 2 is 4.10 mg/dL against a mean of 4.00 and a standard deviation of 0.20, which is only 0.5 standard deviation above the mean and is acceptable on its own. Because at least one control level is out by the 1-3s rule, the analytical run is rejected. Repeating the same control without investigating and correcting the cause does not make the run acceptable.
2. Can the patient results run before the repeat quality control be reported after recalibration? Explain.
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No. The patient results were generated during a rejected run, and recalibration with acceptable quality control afterward validates only the subsequent analytical run. It does not retrospectively validate the earlier patient results. The laboratory must investigate and correct the cause, recalibrate if appropriate, repeat both control levels, and then reanalyze the patient samples. Only results from the reanalyzed samples under acceptable quality control can be reported. Because the patient has a potassium of 6.1 mmol/L, the laboratory should also repeat the potassium promptly and notify the clinician of any verified critical value. If the creatinine cannot be reanalyzed in a timely manner, send it to a reference laboratory. The earlier results should be held, not reported as final.
3. What root cause and corrective actions should be investigated and documented?
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The investigation should focus on the new creatinine reagent lot and the recent calibration. Review the lot-to-lot verification, reagent storage and expiration, calibration curve, control reconstitution and storage, pipetting, and analyzer performance, including lamp intensity, tubing, and carryover. Check whether the level 1 control was properly mixed and whether the same control vial was used for both results. Compare the new reagent lot with the previous lot and review calibration data for shifts. If recalibration and fresh controls do not correct the problem, open a new reagent lot or service the analyzer. Document the QC failure, the investigation, the corrective action, and the reanalysis of patient samples. Perform a look-back to determine whether any patient results were reported during the rejected period, and notify the supervisor or director according to policy.