Potassium 7.4 mmol/L in an Asymptomatic Patient: Pre-Analytical Error on the Ward Round
At 07:15, during a busy medical ward round, a laboratory technician is asked to collect a sample for electrolytes and calcium from a 55-year-old woman admitted with uncontrolled hypertension. Only two tubes remain on the trolley: a lavender-top EDTA tube meant for a complete blood count and a plain red-top tube. The technician draws blood into the EDTA tube first and then, using the same needle and syringe, fills the red-top tube. The ward clerk labels both tubes at the nursing station, and they reach the laboratory by 08:00.
Serum results at 09:30 are: potassium 7.4 mmol/L, calcium 0.98 mmol/L (reference 2.15–2.55), magnesium 1.62 mmol/L, alkaline phosphatase 28 U/L, sodium 136 mmol/L, creatinine 0.9 mg/dL and urea 22 mg/dL. The patient is alert, passing normal volumes of urine, and her ECG shows normally shaped T waves with no QT shortening. The duty doctor, seeing the potassium of 7.4, telephones the laboratory and asks the technician to confirm the value before starting treatment.
1. Which reported values are almost certainly spurious, and what single mechanism explains the whole pattern? Explain why the patient's clinical state supports your conclusion.
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The pattern - potassium 7.4 mmol/L with calcium 0.98 mmol/L, magnesium 1.62 mmol/L, alkaline phosphatase 28 U/L, and entirely normal sodium, urea and creatinine in an alert patient with a normal ECG - is the signature of EDTA carry-over, not true hyperkalaemia.
Mechanism: the lavender-top tube was drawn first, so residual K₂/K₃-EDTA in the needle hub and syringe was flushed into the red-top tube. EDTA is a chelating agent, so it strips calcium from the serum and the measured calcium collapses. The EDTA salt used in blood tubes is a potassium salt, so potassium rises. Magnesium and alkaline phosphatase (a zinc- and magnesium-dependent enzyme) are likewise distorted.
Clinical corroboration: a genuine potassium of 7.4 mmol/L would almost always produce peaked T waves, and would usually be accompanied by renal impairment or by haemolysis - which raises LDH and AST but does not depress calcium to 0.98 mmol/L while raising magnesium. A properly drawn repeat sample gave potassium 4.2 mmol/L and calcium 2.30 mmol/L, confirming that the first result described the tube, not the patient.
2. What must the technician do at the bench and in communication with the ward, before and while the repeat sample is obtained?
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- Do not suppress or delay the critical value. Hospital policy requires every potassium above the critical limit to be telephoned, whatever the suspected cause; the technician reports 7.4 mmol/L and simultaneously states that the calcium, magnesium and alkaline phosphatase pattern strongly suggests EDTA contamination.
- Attach an interpretive comment to the report: 'Results discordant; probable EDTA contamination - repeat requested.' This protects the patient from an unnecessary intravenous calcium or insulin-dextrose infusion.
- Request a fresh sample from the opposite arm, drawn with the correct order of draw, ideally by a different phlebotomist; label at the bedside.
- Check the LIS and the tube for a short-fill or a visibly lavender-tinged serum, and confirm which tube was drawn first - the draw order is the decisive piece of history.
- Record an incident note in the laboratory register and inform the ward sister, so the pre-analytical error is documented and the doctor's decision not to treat is defensible. Repeated EDTA contamination is a training and audit issue, not merely a single bad result.
3. State the correct order of draw for the tubes involved and give one procedural safeguard that would have prevented this error.
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Standard (CLSI) order of draw is: blood culture bottles → light-blue sodium citrate tube → plain red or serum separator (gold/SST) tube → green heparin tube → lavender EDTA tube → grey fluoride-oxalate tube. Additive carry-over matters most in the direction of EDTA and clot activators, so the serum tube must always precede the EDTA tube when both are required.
Safeguards, any one of which would have prevented the incident: • Draw the red-top (serum) tube before the EDTA tube on the same venepuncture. • Never fill a second tube from a syringe that has already delivered blood into an EDTA tube; use a fresh needle or, better, a closed evacuated system. • When using a butterfly set for the first tube, draw and discard a small 'discard' tube to clear additive from the tubing. • Re-label tubes at the bedside and record the draw order on the request form.
A useful bench rule: whenever potassium is high and calcium is low in the same sample, suspect EDTA before you suspect disease.