Sputum cytology is the reason for diagnosing lung cancer as follows: sputum cytology is a test in which doctors ask patients suspected of lung cancer to keep sputum specimens for smear, and then look for cancer cells under a microscope. The specificity of sputum cytology test is 99%, so sputum test can basically confirm the cancer diagnosis when cancer cells are found. However, the sensitivity of sputum cytology examination is low, only 66%, which means that only 66 out of 100 patients with lung cancer have positive sputum test, and it is also related to the number and quality of specimens sent for examination, therefore, those who do not find cancer cells in sputum test cannot exclude the possibility of lung cancer and need further examination. In conclusion: finding cancer cells in sputum can diagnose lung cancer, but not finding cancer cells in sputum does not necessarily mean that the patient does not have lung cancer.