A 16-year-old female with cystic fibrosis (homozygous F508del) presents to the CF clinic for a routine quarterly follow-up. She reports feeling more fatigued than usual over the past 3 months and has noted an unintentional weight loss of 8 lbs (3.6 kg) despite a voracious appetite. She admits to waking up twice a night to urinate and feeling constantly thirsty, which she attributes to the summer heat. She adheres to her airway clearance regimen and enzyme therapy. She denies fevers or acute changes in sputum production, though she feels her exercise tolerance has decreased. Her last pulmonary exacerbation treated with IV antibiotics was 14 months ago.

Her medications include elexacaftor/tezacaftor/ivacaftor, dornase alfa, hypertonic saline, azithromycin, and pancreatic enzyme replacement therapy.

She is afebrile. Her blood pressure is 110/70 mm Hg, heart rate is 88 bpm, respiratory rate is 20/min, and SpO2 is 97% on room air. Her BMI is 18.5 kg/m², which is decreased from a BMI of 21.0 kg/m² six months ago.

Spirometry is performed and shows an FEV1 at 72% of predicted, down from her baseline of 80% of predicted 6 months ago. Her FVC is 85% of predicted.