Serum CA19-9 is a common clinical or physical screening test for cancer, and many patients and physicians are highly stressed once the test results are elevated, but the end result is often nothing after various tests. The value of CA19-9 is mainly to provide a reference for confirmed pancreatic cancer in terms of disease staging, efficacy and disease monitoring. CA19-9 is a colorectal cancer-associated antigen found in colon cancer cell line SW1116 by hybridoma monoclonal antibody technique reported by Koprowski et al[2] in 1979. However, subsequent studies found that CA19-9 expression was more common in pancreatic cancer [3], and serum CA19-9 assay is therefore used as a marker for pancreatic cancer to this day, with diagnostic criteria of >37.0 U/ml. the overall picture is as follows. 1. The sensitivity and specificity of serum CA19-9 test for the diagnosis of pancreatic cancer are 79-81% and 82-90%, respectively, when performed clinically due to abdominal symptoms. In other words, when serum CA19-9 is tested for abdominal symptoms, 79-81% of patients with pancreatic cancer are elevated and 19-21% are normal (false negative); 82-90% of patients with non-pancreatic cancer are normal and 10-18% are elevated (false positive). It can be seen that the serum CA19-9 test is valuable for the diagnosis of pancreatic cancer when abdominal symptoms appear. However, note that pancreatic cancer may not always be elevated, and those elevated may not always be pancreatic cancer. In patients with pancreatic cancer with elevated serum CA19-9, generally the higher the CA19-9, the larger the tumor and the worse the prognosis; after surgery to remove the tumor or effective chemotherapy, the serum CA19-9 decreases and then increases again when it recurs. Therefore, the detection of serum CA19-9 in patients with confirmed pancreatic cancer is helpful for disease staging, efficacy judgment, and disease monitoring [2]. 3. In the absence of symptoms and health examination tests, the positive predictive value of serum CA19-9 for pancreatic cancer is less than 1%. In other words, more than 99% of those found to have elevated serum CA19-9 on physical examination are not pancreatic cancer. In Kim et al [4], for example, 70,940 cases of health check-ups were found to have elevated serum CA 19-9, but only 4 cases (0.4%) of the latter were eventually confirmed as pancreatic cancer. This shows that CA19-9 is not of much practical value as a screening index for pancreatic cancer. 4. Under the premise that CA19-9 is used as a marker for pancreatic cancer, if the serum CA19-9 is elevated but not confirmed to be pancreatic cancer, it is called a false positive. Whether the test is due to abdominal disease or physical examination, the majority of causes of elevated false positive serum CA19-9 are benign diseases, and malignancy is only occasionally seen [2], and the specific causes are diverse, including heart failure, severe diabetes, arthritis, various biliary diseases, chronic lung disease, gynecological diseases, gastrointestinal diseases, and so on. Some people have long-term elevations without clear disease, especially those found on physical examination. In terms of the degree of elevation, it can be mildly elevated (slightly more than 37U/ml) or very high (e.g. more than 100U/ml), but the likelihood of other cancers does not increase in those with high elevation. Therefore, if CA19-9 is found to be elevated, especially during physical examination, it should be analyzed and responded calmly, and most of them only need to be reviewed periodically.