An Impossible Potassium in an Asymptomatic Patient
A 58-year-old man attends his GP for a routine insurance physical. He feels well, exercises regularly, and takes no medication. Results: sodium 138 mmol/L, potassium 7.2 mmol/L, chloride 104 mmol/L, bicarbonate 22 mmol/L, creatinine 0.9 mg/dL, calcium 1.95 mmol/L (reference 2.20-2.60), magnesium 0.62 mmol/L (reference 0.70-1.00), alkaline phosphatase 465 U/L, hemolysis index 12 (slight). A repeat potassium measured from the same tube is 7.1 mmol/L. The nurse reports that blood was drawn into a full blood count tube and a coagulation tube at the same visit. An ECG performed in the office is normal. The laboratory's critical value policy requires a call within 30 minutes for potassium at or above 6.5 mmol/L.
1. How do you interpret this result pattern, and what competing explanations must you consider?
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The pattern is not physiological. A true potassium of 7.2 mmol/L in an ambulatory, asymptomatic patient with a normal ECG and normal creatinine would be exceptional, and repeat analysis of the same tube cannot discriminate a pre-analytical error.
The cluster is the clue: potassium is high while calcium and magnesium are low and alkaline phosphatase is markedly elevated. This is the classic signature of EDTA contamination - the lavender-top tube was drawn before the serum tube, or blood was poured from an EDTA tube into another tube. EDTA chelates calcium and magnesium and falsely raises potassium; alkaline phosphatase is commonly spuriously elevated in these samples.
Alternatives to exclude:
- Hemolysis - the index of 12 is only slight and cannot account for a 7.2.
- Prolonged tourniquet, fist clenching, or traumatic venepuncture - these raise potassium but do not depress calcium and magnesium.
- Thrombocytosis or marked leukocytosis releasing potassium during clotting - check the FBC and use a plasma sample.
- IV fluid contamination or a line draw - check the draw site.
- Genuine renal failure or haemolysis in vivo - excluded by a normal creatinine.
2. What are your immediate actions before any result leaves the laboratory?
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Do not report the 7.2 as a critical value. Calling it would trigger unnecessary treatment for hyperkalaemia.
Sequence:
- Hold the result and place it on a pre-analytical query.
- Inspect the sample and request form for tube type, order of draw, and draw site; ask the nurse which tube was drawn first.
- Recollect by fresh, clean venepuncture - separate tubes, EDTA last, no syringe pour-off - and request plasma (lithium heparin) potassium plus calcium, magnesium, and alkaline phosphatase.
- Where available, test for EDTA directly (EDTA/zinc assay) or check the K/Ca/Mg/ALP ratio pattern on the analyser.
- Once the repeat confirms a normal potassium, report the corrected value with a comment that the original was a pre-analytical artefact and cancel the critical-value call.
Communication: tell the GP explicitly that no treatment is indicated, that the ECG is normal, and what the true potassium is. Document the episode and retain the original result in the audit file. If the repeat potassium is still elevated, then escalate on clinical grounds.
3. What quality-management actions would you take to keep this from recurring?
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This is a systemic pre-analytical failure, not an individual error, so the response should be at the level of process and monitoring.
Detection:
- Build an autoverification rule that flags potassium above 6.0 combined with calcium below reference and alkaline phosphatase above reference for manual review before release.
- Apply delta checks against previous results.
Prevention:
- Reinforce order of draw (EDTA tubes last) with phlebotomy and nursing staff; supply visual aids at every draw station.
- Ban syringe-to-tube transfer for chemistry samples and specify separate venepunctures where a coagulation and EDTA tube are both required.
- Audit the proportion of samples with the EDTA-contamination signature monthly and report it as a quality indicator.
Governance:
- Log the event as a nonconformity with root-cause analysis, corrective action, and an effectiveness check at 3 months.
- Review the critical-value policy to allow a documented 'verify before calling' step for implausible results, so staff are not penalised for exercising judgement.