The normal value of cytokeratin 19 fragment is 2.0 μg/L, so 3.5 μg/L is abnormal. Cytokeratin 19 fragment is not an organ-specific protein, and its distribution is mainly in tissues or organs rich in epithelial cells, such as lung, breast, bladder, intestine, uterus, etc. When the tissue is malignant, the level of cytokeratin 19 fragment in blood is seen to be elevated, and cytokeratin 19 fragment is mainly used clinically for the differential diagnosis of non-small cell lung cancer, as well as for prognostic assessment. Cytokeratin 19 is the preferred tumor marker for non-small cell lung cancer and can be used in the differential diagnosis of non-small cell lung cancer, versus small cell lung cancer. The positivity rate in non-small cell lung cancer is 40%-64%, and the highest positivity rate in squamous cell carcinoma of the lung. And when cytokeratin 19 levels exceed 30 μg/L, there is a higher likelihood of having primary bronchopulmonary carcinoma. However, elevated cytokeratin 19 levels are also seen in other solid tumors, such as breast cancer, bladder cancer, colorectalized cancer, and prostate cancer. In addition, e.g., keratin 19 fragments <10 μg/L may also be seen in benign diseases, such as pneumonia, tuberculosis, chronic bronchitis, gastrointestinal diseases, gynecological diseases and urological diseases. Therefore, if a slight elevation of cytokeratin 19 fragment is detected, there is no need to worry excessively. It is recommended to improve CT and MRI examinations to clarify the diagnosis and then carry out targeted treatment.