Asymptomatic Dog With a Markedly Abnormal Potassium Result
A 7-year-old male castrated Labrador retriever presents for a wellness exam. He is bright and asymptomatic. A 3 mL sample is drawn from the left cephalic vein using a 22-gauge needle and syringe, and the blood is transferred into a lavender-top EDTA microtainer, a green-top lithium heparin tube, and a red-top tube. The clinic's in-house analyzer reports potassium 8.7 mmol/L (3.5-5.8), calcium 6.2 mg/dL (9.0-11.5), magnesium 1.2 mg/dL (1.6-2.4), sodium 145 mmol/L, chloride 110 mmol/L, BUN 16 mg/dL, creatinine 1.1 mg/dL, ALT 45 U/L, and glucose 96 mg/dL. The technician notes the serum sample was slightly hemolyzed. On physical exam the dog has a heart rate of 82 bpm with a regular rhythm, strong femoral pulses, and no weakness. An ECG shows a normal sinus rhythm with a QRS duration of 0.06 s. The veterinarian is about to start potassium-lowering treatment and asks you to review the results.
1. What is the most likely explanation for these results, and what should you do before treatment is started?
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The pattern points to EDTA contamination rather than true hyperkalemia. EDTA tubes contain a potassium salt of EDTA, so carryover into the chemistry sample raises potassium. EDTA also chelates divalent cations, which lowers calcium and magnesium. The dog has no weakness, a normal heart rate and rhythm, strong pulses, and a normal QRS duration, which is not consistent with a potassium of 8.7 mmol/L. The mild hemolysis can also raise potassium, but hemolysis alone would not explain the low calcium and magnesium together with a normal ECG.
I would ask the veterinarian to delay potassium-lowering treatment and redraw the sample before any intervention. Treating a laboratory artifact with calcium gluconate, insulin, or fluids exposes the dog to unnecessary risk. I would explain the specific pattern I saw and the normal ECG, and I would recommend a fresh sample collected with careful technique. If the repeat potassium is still high, then treatment and further investigation are appropriate. I would also review how the sample was handled, because the same error could affect other patients.
2. Which analytes are affected by EDTA contamination and by hemolysis, and how would you distinguish the two?
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EDTA contamination raises potassium and lowers calcium and magnesium because EDTA binds divalent cations. It can also dilute other analytes depending on how much EDTA is carried over. Hemolysis raises potassium, LDH, AST, and phosphate, and it can interfere with several other assays depending on the analyzer. Hemolysis does not typically lower calcium and magnesium the way EDTA does, so the combination of high potassium, low calcium, and low magnesium is much more consistent with EDTA than with hemolysis alone.
The normal ECG and the lack of clinical signs are the clinical counterweight. A true potassium of 8.7 mmol/L would usually produce bradycardia, weak pulses, widened QRS complexes, or absent P waves, and this dog has none of those. I would use the chemistry pattern plus the physical findings to distinguish artifact from real disease. If only potassium were high and the calcium and magnesium were normal, I would think first about hemolysis or a true renal or adrenal cause and would still redraw before treating.
3. Describe how you would collect and handle the repeat sample to avoid the same problem.
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I would collect a fresh sample with a clean syringe and needle, preferably a 20- or 21-gauge needle for a dog of this size, and avoid excessive suction or trauma to the vein. I would not transfer blood from an EDTA tube into another tube, and I would not use the same syringe for the EDTA tube and the chemistry tube. If multiple tubes are needed, I would draw the chemistry tube first or use separate syringes, and I would fill each tube to its marked volume and mix it gently by inversion.
I would label the tubes at the time of collection, allow the red-top tube to clot and separate the serum promptly, and note on the requisition that the previous sample was suspected of EDTA contamination. I would also check the analyzer's sample probes and the order of draw, because carryover can happen on the instrument as well as at the patient. If the repeat result is normal, I would document the resolution and review the blood-collection SOP with the team so the error is less likely to recur.