In a patient with persistent hypokalemia after potassium repletion, what additional test should be considered next?

In cases of persistent hypokalemia, one important consideration is the role of magnesium in potassium homeostasis. Low levels of magnesium (hypomagnesemia) can contribute to persistent hypokalemia because magnesium is crucial for the proper functioning of the sodium-potassium pump in the renal tubules and in cells throughout the body. When magnesium is deficient, the renal handling of potassium can be impaired, leading to continued loss of potassium despite adequate repletion.

Assessing serum magnesium levels can help determine if magnesium deficiency is the underlying issue contributing to the ongoing hypokalemia. If hypomagnesemia is identified, magnesium supplementation can help correct the potassium levels.

Other tests, such as serum phosphate, complete blood count, and serum glucose, do not directly relate to the management of potassium levels and are less likely to provide valuable insight into the reason for persistent hypokalemia in this specific context. Therefore, measuring serum magnesium is the most relevant next step in addressing the patient's condition.