A Chemistry Panel Suggesting EDTA Contamination From an Intravenous Line
A 58-year-old woman attends the emergency department with palpitations and muscle cramps. A phlebotomist draws blood from an IV cannula in her left forearm, having first collected an EDTA tube for a complete blood count and then using the same cannula for the chemistry tube. The chemistry analyser reports sodium 138 mmol/L, potassium 6.8 mmol/L, chloride 104 mmol/L, bicarbonate 21 mmol/L, total calcium 1.72 mmol/L, albumin 40 g/L, magnesium 0.55 mmol/L, alkaline phosphatase 38 U/L (reference 40–120), and creatinine 0.9 mg/dL. The ECG shows sinus rhythm with no peaked T waves. The EDTA sample gives a platelet count of 180 × 10^9/L. A repeat potassium from a fresh venepuncture is 4.1 mmol/L. The emergency physician asks whether the first chemistry panel can be reported and whether the patient needs calcium replacement.
1. Identify the most likely pre-analytical problem and explain how each abnormal result fits.
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The pattern is typical of EDTA contamination. EDTA is a chelating anticoagulant supplied as dipotassium or tripotassium EDTA, so it both adds potassium and binds divalent cations. The spuriously high potassium of 6.8 mmol/L with a normal ECG and a repeat value of 4.1 mmol/L is the key clue. EDTA also chelates calcium and magnesium, explaining the low total calcium of 1.72 mmol/L and low magnesium of 0.55 mmol/L. Alkaline phosphatase is a zinc- and magnesium-dependent enzyme, so chelation and dilution can produce a falsely low alkaline phosphatase of 38 U/L. The normal sodium, chloride, bicarbonate, and creatinine do not argue against contamination; EDTA mainly affects potassium and divalent cations. The draw sequence is the critical history: the EDTA tube was collected first and the chemistry tube was then drawn from the same cannula, allowing EDTA to contaminate the line and the subsequent sample. This is not true hypocalcaemia or true hyperkalaemia, and calcium replacement based on this panel could be harmful.
2. What should the laboratory do with the first panel, and what should be reported to the clinician?
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The first chemistry panel should not be reported as valid. The laboratory should telephone the emergency physician, explain the suspected EDTA contamination, and request a repeat sample by fresh venepuncture, preferably from the other arm. The repeat potassium of 4.1 mmol/L supports the interpretation, but a repeat chemistry panel is still needed because calcium, magnesium, and alkaline phosphatase were also affected. The laboratory should not report the potassium of 6.8 mmol/L as a critical value without confirmation in this context, because doing so may trigger unnecessary treatment. If the patient is symptomatic or the ECG is abnormal, a repeat sample must be obtained urgently. The laboratory should document the discrepancy, the discussion with the clinician, and the reason the first result was withheld. A comment may be added to the final report stating that the initial sample was contaminated with EDTA and that the repeat results are the valid ones. The clinician should be explicitly advised not to give calcium based on the first panel.
3. What collection practice should be changed, and what other analytes could be affected?
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Blood should not be drawn through an IV cannula immediately after an EDTA tube. The standard order of draw should be followed: blood culture, citrate, serum or heparin, EDTA, then fluoride tubes. If an IV line must be used, the line should be flushed with saline and the first portion of blood discarded before the chemistry sample is collected, or a separate venepuncture should be performed. The phlebotomist should be retrained on the order of draw and on avoiding backflow from EDTA tubes. Other analytes that can be affected include calcium, magnesium, alkaline phosphatase, potassium, and sodium, because EDTA salts contain sodium or potassium. Contamination can also prolong coagulation tests by chelating calcium in citrate tubes and can cause platelet clumping and a falsely low platelet count in EDTA samples. The laboratory should review its specimen-collection instructions and audit whether chemistry samples are being drawn from lines after EDTA. If the problem is recurrent, the collection procedure and training records should be updated.