Which test is most appropriate to confirm a diagnosis of hypokalemia based on an ECG?

The serum potassium level is the most appropriate test to confirm a diagnosis of hypokalemia based on an ECG. Hypokalemia, or low potassium levels, can lead to characteristic changes on an electrocardiogram (ECG), such as flattened T waves, U waves, or ST-segment depression. To verify the diagnosis, measuring the serum potassium concentration directly assesses the potassium levels in the bloodstream, reflecting the body's potassium status and allowing for accurate diagnosis and appropriate management.

While other tests such as a complete blood count, arterial blood gas panel, or urine osmolality test provide useful clinical information, they do not directly measure potassium levels. A complete blood count assesses various components of blood but does not give any specific information about potassium. An arterial blood gas panel evaluates respiratory and metabolic function, along with blood pH, but it also does not include potassium levels. The urine osmolality test assesses the concentration of solutes in urine, which can be influenced by several factors but again does not provide direct data about serum potassium levels. Therefore, the serum potassium level is the critical test needed for confirming hypokalemia.