Q. A 67-year-old male is admitted to the hospital with complaints of abdominal pain, fever, and vomiting for 2 days. The patient has a history of cirrhosis and has been admitted to the hospital several times in the past year for similar symptoms. Upon admission, the patient is found to have a fever of 101.5 degrees Fahrenheit, an elevated white blood cell count of 18,000/mm3, and abdominal tenderness. You order an ultrasound scan of his abdomen (see image), which shows evidence of ascites. You order further laboratory tests including a blood culture, and a complete blood count. You also perform abdominal paracentesis, and order analysis and culture of the ascitic fluid. The culture of the ascitic fluid shows the presence of Escherichia coli. The neutrophil count is elevated at 75 percent. The patient is started on intravenous antibiotics to treat the bacterial infection and is given pain management for this abdominal pain. The patient is also started on a nasogastric tube for fluid and electrolyte management and to prevent further vomiting. Over the next few days, the patient's fever improves, and his white blood cell count begins to decrease. The patient is able to tolerate oral intake, and the nasogastric tube is discontinued. The patient is continued on antibiotics for 7 days, and a repeat culture of the ascitic fluid is negative for bacteria. When ordering analysis of the ascitic fluid, which of the following laboratory tests is most important in diagnosing septic peritonitis?