An older adult client is taking digitalis (Digoxin) 0.25 mg…

An older adult client is taking digitalis (Digoxin) 0.25 mg twice a day and furosemide (Lasix) 40 mg daily for heart failure (HF).  She has complained of increasing lethargy and nausea over the past 2 days; however she is still able to take her medication.  Her blood pressure is 150/98 mm Hg; pulse is 110 beats/min and irregular; respiratory rate is 18 breaths/min. What laboratory information is most important for the nurse to evaluate?