CA 19-9 is elevated in some cases of gastric cancer and in those cases it is often used to monitor response to chemotherapy. Approximately one third of patients with gastric cancer will have elevated levels of CEA and one third will have elevated levels of CA 19-9; Ca 19-9 is more commonly elevated in pancreatic and biliary cancer. It can be used for detecting early relapse or to monitor therapy. Unfortinately, there is not much credible medical literature defining these used for specifically gastric cancer and many plans, including Anthem do not consider 19-9 to be a medically necessary marker to follow gastric cancer. This approach is defensible based on the literature. Although there is set of Japanese guidelines that recommends Ca 19-9, gastric cancer in Japan is a differently behaving disease than in the West, as is widely recognized, and Japanese practice in gastric cancer is not followed in the USA.
M.J. Gaspara, I. Arribasa, M.C. Cocaa, M. Díez-AlonsobPrognostic Value of Carcinoembryonic Antigen, CA 19-9 and CA 72-4 in Gastric Carcinoma Tumor Biology Vol. 22, No. 5, 2001
V Heinemann, J Stemmler, M Schermuly, P Stieber, K Niebler, R Wilkowski, T Helmberger, M Garbrecht, A Schalhorn CA 19-9: An Early Indicator of Response to Chemotherapy in Advanced Pancreatic Cancer. Proc Am Soc Clin Oncol 19: 2000 (abstr 1076)