The highest potassium I have ever seen? That would be 15.5 mEq/L.

It's not real. It was pseudohyperkalemia from leukocytosis. The patient had chronic lymphocytic leukemia with a white count of 300,000. If you are not familiar with this condition, check out these posts on Renal Fellow Network: Westervelt and Nate. Nice full text references here and here (pdf).

The pseudohyperkalemia merit badge

The first time I saw this was when I was senior resident. I was sleeping in the call room my pager buzzed. It was the oncology floor with a potassium of 9. The patient had CML and was in a blast crisis. His leukocyte count around 100,000. I immediately suspected pseudohyperkalemia and ordered a whole blood potassium from the ABG lab. It was normal so I went back to sleep. The next morning I received an angry call from the Hemo-Onc fellow. The patient was coding and he was furious that I only ordered an ABG instead of treating the hyperkalemia.

I don't know if the patient coded from hyperkalemia, but I wish that I had gotten out of bed and evaluated the patient. I solved the problem the nurses alerted me to, but if I assessed him, maybe I could of averted an arrest.