Most patients hospitalized with acute heart failure (AHF) respond to intravenous (IV) loop diuretic therapy, which results in symptom improvement and discharge from the hospital after a 3-5 day stay. However, up to 20% of these patients do not respond to IV loop diuretics, and are found to be "diuretic resistant". The treatment pathway for this cohort of patients is unclear. As a result, they experience significantly longer hospital lengths of stay, consume more healthcare resources, and experience an increased proportion of adverse events. Clinicians currently do not have a reliable method of predicting who will become diuretic resistant. The diagnosis is made after hospitalization, based on poor response to diuretics resulting in escalation of therapy. The standard approach to AHF therapy is to treat all patients with IV loop diuretics, not knowing who will become diuretic resistant. When diuretics are effective in promoting diuresis and natriuresis the urinary sodium rises and the urinary potassium falls. Those patients with continued low urinary sodium despite IV diuretic administration are at risk for developing diuretic resistance. Defining diuretic resistance based on urinary electrolytes before it becomes clinically apparent would facilitate an earlier change in therapy, with an aim of preventing a prolonged hospitalization.