A 6.9 mmol/L potassium with low calcium and magnesium: spurious result or critical value?
A 72-year-old man with diabetic nephropathy (eGFR 24 mL/min/1.73 m2) attends clinic. The biochemistry is K 6.9 mmol/L, Na 132, Ca 1.98 mmol/L (2.15–2.55), Mg 0.55 mmol/L (0.70–1.00), ALP 42 U/L (previously 95) and creatinine 310 µmol/L (baseline 290). He has no weakness or palpitations and the ECG is normal. The specimen was drawn by a phlebotomist in training who had just filled an EDTA tube for a full blood count and then used the same needle and holder for the heparin tube; the request form arrived with both tubes taped together. The laboratory's delta check flagged a potassium rise of 2.1 mmol/L from three weeks earlier. The clinician phones asking whether to arrange urgent dialysis; the medical registrar has already prescribed calcium gluconate.
1. Explain the biochemical pattern and decide whether this is a critical value.
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The pattern is EDTA contamination, not true hyperkalaemia. K-EDTA is a potassium salt of a chelator: it raises potassium and chelates divalent cations, so calcium and magnesium fall. Alkaline phosphatase is a zinc/magnesium-dependent enzyme, so it also falls (42 U/L against a previous 95 U/L). Haemolysis would raise potassium but would not selectively lower calcium, magnesium and ALP, and would usually be accompanied by a raised LDH and AST. True hyperkalaemia of chronic kidney disease would not explain the simultaneous fall in calcium, magnesium and ALP.
The delta check (a 2.1 mmol/L rise in three weeks) and the phlebotomy history support contamination. However, the result cannot simply be dismissed, because this patient has CKD and could also have true hyperkalaemia. The laboratory must verify before reporting: the value is not a valid critical result until a repeat sample confirms it. The registrar's calcium gluconate was premature; calcium is given for membrane stabilisation in true hyperkalaemia with ECG changes, and here the ECG is normal and the low calcium is itself an artefact.
2. What immediate actions should the laboratory and clinician take?
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The laboratory should telephone the clinician, explain the pattern and the sampling error, and withhold a 'critical value' designation until a repeat is available. Request an urgent repeat sample with correct order of draw in separate tubes (blood culture, then citrate, serum, heparin, EDTA, fluoride), and consider a venous blood gas for potassium. Recheck potassium, calcium, magnesium and ALP. If the repeat is normal, cancel or amend the original result with a comment stating the reason. If potassium remains >6.0 mmol/L, treat as true hyperkalaemia.
The clinician should hold the calcium gluconate unless there are ECG changes or a confirmed true result, and should not arrange dialysis on the basis of this sample. Clinical review, ECG and a repeat potassium are the appropriate immediate steps. Document the event as an incident, inform the phlebotomist's supervisor, and ensure the patient is not subjected to unnecessary urgent treatment or transfer. The patient's CKD means that overtreatment of a spurious result (dialysis, unnecessary calcium) carries real harm.
3. How would you prevent recurrence, and what quality metrics would you monitor?
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Root cause: an untrained phlebotomist filled an EDTA tube first, then used the same needle and holder for the heparin tube, and the tubes were taped together on arrival. Prevention: competency-based training and sign-off for all staff who take blood; an order-of-draw poster at every phlebotomy station; a discard tube when a butterfly line is used; labels applied at the bedside with two identifiers; a rule that tubes are never decanted or taped together, and that each tube is filled in the correct sequence. Give feedback to the individual and the department.
Laboratory measures: build delta-check rules that flag a rising potassium accompanied by falling calcium, magnesium or ALP, and add a comment suggesting EDTA contamination and repeat. Audit contaminated samples per 1,000 draws, recollection rates, and documentation of critical-value call-backs; review incidents at the quality meeting and re-audit after three months. If available, measure EDTA in the sample to confirm contamination rather than relying on inference. Report the episode through the incident system and use it as a teaching case, because spurious results are a patient-safety issue, not a clerical one.