A 40-year-old man with long-standing alcohol abuse complains of abdominal swelling, which has been progressive over several months. He has a history of gastrointestinal bleeding. On physical examination, there are spider angiomas and palmar erythema. Abdominal collateral vessels are seen around the umbilicus. There is shifting dullness, and bulging flanks are noted. Which of the following is the most important first step in the patient’s evaluation?