QC Failure on a Chemistry Analyzer
A 220-bed hospital laboratory runs chemistry on two identical analyzers. On Monday at 7:00 a.m., the level 2 QC for the general chemistry panel shows glucose 118 mg/dL (mean 100, SD 5), creatinine 1.8 mg/dL (mean 1.2, SD 0.05), and sodium 130 mmol/L (mean 140, SD 2). The level 1 control is within limits. The analyzer was calibrated Friday evening and has run 412 patient samples since; the night shift reported no flags. The laboratory manager is on vacation, and the senior technologist must decide whether to release the morning results. The last acceptable QC was Friday at 11:00 p.m. On Monday, 38 patient samples have already been run before QC. The analyzer uses the same reagent lot that was installed Thursday.
1. Apply the Westgard rules to these QC results. Can the run be released?
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Apply the Westgard rules to the level 2 control: glucose is (118−100)/5 = +3.6 SD, creatinine is (1.8−1.2)/0.05 = +12 SD, and sodium is (130−140)/2 = −5 SD. Each is a 1-3s violation, and the creatinine and sodium deviations also represent a large systematic shift. The level 1 control is within limits, which suggests a level-specific problem or a drift that affects the upper part of the reportable range more than the lower part. One 1-3s violation is enough to reject the run; three analytes failing at once is a strong signal of a systematic error. Do not release the morning patient results until the problem is corrected.
2. What are the likely causes, and what immediate corrective actions should be taken?
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Consider calibration drift, a new reagent lot or deteriorated reagent, a clogged sample or reagent probe, a lamp or optical problem, or incorrectly prepared level 2 control. The fact that the analyzer was calibrated Friday and has run 412 samples makes drift or reagent exhaustion likely. Immediate actions: repeat the level 2 control with a fresh aliquot; check reagent lots, expiry, and levels; inspect probes and perform maintenance; recalibrate if QC still fails; then repeat QC at both levels. Document each step and the corrective action. If a new reagent lot is the cause, quarantine it and notify the manufacturer if indicated.
3. How should the laboratory handle patient results generated since the last acceptable QC, and what documentation is required?
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Identify all patient samples run since the last acceptable QC (Friday 11:00 p.m.), including the 38 run on Monday before QC. After the analyzer is corrected, re-run those samples or at least the affected analytes and compare with the original results using delta checks. Notify clinicians of any clinically significant differences, especially for glucose, creatinine, and sodium, and repeat critical values immediately. Document the event, the number of samples affected, the corrective action, and the clinician notifications. Perform a root-cause analysis and a CAPA; if the failure affected proficiency testing, follow the separate PT investigation policy. Do not simply repeat QC and release the old results without assessing patient impact.