A 52-year-old man from Delhi presents with a 3-month history of progressive jaundice, pale stools, and dark urine. On examination, he has hepatomegaly and a palpable gallbladder. Abdominal ultrasound shows a 4 cm hypoechoic lesion in the pancreatic head with dilated intrahepatic bile ducts. Serum bilirubin is 8.2 mg/dL (direct 6.8 mg/dL), and alkaline phosphatase is markedly elevated. His serum CA 19-9 level is 320 U/mL (normal <37 U/mL). Which statement best describes the clinical utility of CA 19-9 in this patient?