Spironolactone (aldosterone/mineralocorticoid receptor antagonist)

Well-done! Spironolactone, an aldosterone antagonist, has been shown to decrease the progression of heart failure, most likely due to reduced remodeling, diminished endothelial dysfunction and reduced arrhythmogenesis.

The cardiologist goes on to explain that while spironolactone is a beneficial drug, studies have shown some potential adverse effects. He further explains that it is important to watch out for these effects through certain tests. The cardiologist looks over to you. 

Which test should be used to monitor safety of this drug?