Insulin drip and false low potassium levels?

I recently had a patient on an insulin drip (7 units/hr) whose K level came back at 2.7. Initially, the order I was given for replacement was 40 meq Klorvess pNGT (along with 40mg of furosemide for diuresis). The pt. dumped only about 400 cc from the furosemide, but I was concerned that 40 meq was not enough replacement for a K of 2.7, and the pt. has a hx of ectopy (hx MI, CHF, EF 25%). I was given an order for an additional 20 meq pNGT, and the PA said that they weren't overly concerned about the K of 2.7 because insulin gtt can cause a false low K. This same pt. had a K of 8.5 come back the day before, which was probably inaccurate because the pt's blood clots if not tested immediately (d/t cold agglutinin disease). He had gotten Kayexalate.

I remember learning in school that one tx of hyperkalemia is insulin w/dextrose, to push K into the cells, so what they're saying does make sense, but what I'm wondering is whether there's some sort of formula to estimate the amount that the serum K level will be off d/t the insulin. Normally on my unit with cardiac pts, we like a min K of 4.0

Thanks,
Hillary
(Surgical Intermediate Care Unit)