Cytokeratin 19 fragment, a protein-based tumor marker, is the tumor marker of choice for clinical detection of non-small cell lung cancer. If there is no clinical symptom and simply an elevated value is present, its severity cannot be discussed and further examination can be done to clarify the cause. Cytokeratin 19 is a class I keratin (acidic protein) with a molecular weight of about 40,000 and is the smallest member of the keratin family. It has an ELISA value of serum <3.30ng/ml. The sensitivity of cytokeratin 19 fragment for the diagnosis of various types of lung cancer is in the following order: squamous carcinoma＞adenocarcinoma＞large cell carcinoma＞small cell carcinoma. However, the diagnosis of lung tumor cannot be based on cytokeratin 19 fragment alone, but also needs to be combined with symptoms, chest imaging examination, pathological tissue sections, etc. to confirm the diagnosis. Therefore, when cytokeratin 19 fragment is elevated in the examination, further imaging examination such as chest CT should be done, and if there is no abnormality in chest CT, regular follow-up observation should be made. If the chest CT shows occupation, nodules, etc., further pathological tissue section examination is needed to clarify the diagnosis and deal with it as early as possible.