Discordant Potassium in a Healthy Cat
A 9-year-old spayed female domestic shorthair cat weighing 5.1 kg is presented for a routine wellness examination. The cat is bright and has no arrhythmia on auscultation. Blood is drawn from the jugular vein into an EDTA tube and a serum tube after a 6-hour fast. The EDTA tube was filled from the same syringe after a difficult venipuncture, and the sample sat on the counter for 45 minutes before processing. The EDTA sample is moderately hemolyzed and the laboratory reports clumped platelets. Results: EDTA potassium 8.4 mmol/L, serum potassium 5.0 mmol/L, sodium 150 mmol/L, BUN 32 mg/dL, creatinine 1.6 mg/dL, glucose 110 mg/dL. The remainder of the CBC and chemistry panel is unremarkable. The owner reports the cat is eating, drinking, and urinating normally, and there have been no episodes of weakness. The owner asks whether the cat needs emergency treatment for high potassium. The clinic has a lithium heparin tube and a point-of-care blood gas analyzer.
1. Interpret the potassium results and explain the most likely cause. What is pseudohyperkalemia?
Read the worked answer
The EDTA potassium of 8.4 mmol/L is discordant with the serum potassium of 5.0 mmol/L and with the cat's normal clinical examination. This is pseudohyperkalemia: potassium is released from cells in the tube after collection, most often from hemolysis and platelet clumping, rather than reflecting true hyperkalemia. Cats are especially prone because their platelets are fragile and can release potassium when they clump. Other causes include prolonged tourniquet use, traumatic venipuncture, delayed processing, and EDTA contamination. A true emergency would be supported by weakness, bradycardia, absent P waves, or a high serum potassium. Here, the serum value is normal, so emergency treatment is not indicated. Confirm with a minimally hemolyzed lithium heparin sample and an ECG if there is any doubt.
2. What steps would you take to confirm the result before treatment, and what would you do if the repeat potassium were truly 7.5 mmol/L?
Read the worked answer
First, repeat the potassium using a lithium heparin tube or a point-of-care analyzer, avoiding hemolysis and platelet clumping. Collect with a clean venipuncture, minimal suction, and prompt processing; do not draw from an IV line. Check an ECG and assess for weakness or arrhythmia. If the repeat is truly 7.5 mmol/L, treat as hyperkalemia: calcium gluconate to protect the myocardium, insulin with dextrose to shift potassium intracellularly, and fluid therapy to promote excretion; albuterol may be used as an adjunct. Treat the underlying cause and recheck potassium frequently. The key is not to treat pseudohyperkalemia with insulin or calcium, which can cause hypoglycemia or unnecessary risk.
3. How can sample collection and handling be improved to avoid this discrepancy?
Read the worked answer
Use the correct tube, fill it to the mark, and mix by gentle inversion rather than shaking. Avoid prolonged tourniquet application and traumatic venipuncture; use a needle and syringe appropriate for the cat's size. Separate serum promptly and transport samples quickly. Label the tube and note any hemolysis or clumping on the submission form. If a result is unexpected, repeat before treating. For cats, consider a point-of-care potassium check if the sample is difficult. Consistency in collection technique matters because pseudohyperkalemia can mimic a life-threatening emergency and lead to unnecessary treatment.