Lung cancer cannot be ruled out when no cancer cells are detected in the fluid in the lungs. Pneumonic fluid is a symptom of lung cancer, in which a small amount of cancer cells are present, but they will be necrotic and dissolve soon after leaving the body, which means that extracting fluid to screen for lung cancer will take a longer time, during which the cancer cells may have already been dissolved, and it is therefore difficult to find normal forms of cancer cells, which will have an impact on the result. Moreover, lung fluid screening for cancer cells is only a preliminary screening method, which cannot diagnose lung cancer, therefore, just with the result that no cancer cells are detected in lung fluid, there is no way to detect lung cancer completely, and it is still necessary to improve the pathological tissue biopsy in time.