We read the article of Khan et al, “Changes in serum potassium levels during hospitalization in patients with worsening heart failure and reduced ejection fraction,” published in the American Journal of Cardiology , 2015. The authors of the article state that serum potassium changes during admissions for acute decompensated heart failure (ADHF) are not associated with long-term (9 months) prognosis. We would like to ask for some clarifications because we think that the subject of potassium changes during ADHF hospitalization merits more clinical and scientific attention. First, the authors themselves state in the discussion that “potassium concentrations are expected to decrease during hospitalization for worsening HF” because of increased renin-angiotensin-aldosterone system activation and aggressive diuretic therapy. Could it be that further subdivision of serum potassium decreases (with larger decreases of >0.5 mEq/l) may shed more light as larger serum potassium decreases may be clinically more relevant than small decreases? Second, we are curious why there was no formal testing of discharge potassium level as risk stratifier. Was this perhaps not done because potassium changes in itself were not prognostic or because there were many measurements at 7 days but not at discharge? Still, discharge potassium analysis in relation to the change and the influence of a low discharge potassium (<4.0 mEq/l) on prognosis would have been informative, and results may change depending on baseline and discharge levels of serum potassium; if a low discharge potassium in the present study is relevant for prognosis, then a change of 0.1 mEg/l in potassium in a patient with baseline 4.1 mEq/l is more relevant than a decrease of 0.9 mEq/l in a patient with a baseline potassium level of 5.2 mEq/l. Baseline serum potassium levels were not presented in the baseline characteristics of the 4 categories of potassium changes. Similarly, it may be that a high discharge potassium is prognostically relevant, in conjunction with the change in potassium leading to such discharge levels. These are all unknowns in the study and make the results and interpretation difficult to understand. Third, we found different numbers of patients in the 4 categories of patients with potassium changes, but they were analyzed as “quartiles” of potassium changes (as stated in the Methods section). Some sort of selection seems to have entered here. Equally, different total numbers of patients are found in Figure 1 and Table 1 (1829 pts), Figure 2 (1753 pts), and Figure 4 (1907 pts) without explanation.