Reviewed by: HU Medical Review Board | Last reviewed: January 2017. | Last updated: May 2022
Sputum cytology is the testing of lung secretions or phlegm to look for cancerous cells. The patient coughs up a sample of mucus, which is viewed under the microscope to identify possible cancer cells. Samples are often collected early in the morning for several days.1
What is sputum cytology?
Sputum cytology is a quick and inexpensive test, however, it is not always accurate. While the false-positive rate is low (1 percent), false negatives can occur in as many as 40 percent of tests. (False-positive results are when the test indicates cancer when there is no cancer. False-negative results are when the test indicates there is no cancer when cancer is present).
Sputum cytology is more useful in diagnosing squamous cell lung cancers, which start in the central airways and may not be as effective in identifying other types of lung cancer, particularly those located in more distant areas of the lungs. Sputum cytology detects approximately 71 percent of tumors that are centrally located, but it detects less than 50 percent of tumors that are located in the periphery (outer regions) of the lungs. Due to the limitations of sputum cytology, additional testing is recommended when the test comes back with a negative result (showing no cancer) in patients where lung cancer is suspected to conclusively determine the presence or absence of lung cancer.1,2
When the results are positive for cancer, sputum cytology does not provide enough information for doctors to be able to determine exactly what type of lung cancer a patient has. A biopsy is required to get a good sample of the tumor and identify the type of lung cancer.2
Using sputum cytology for lung cancer screening
Screening is testing done before any symptoms are noticeable. The purpose of screening is to catch cancer in its earliest, and most treatable, stages. Lung cancer causes more deaths than any other type of cancer, in part because it is most often found after it has spread.3
Several studies have evaluated whether sputum cytology is an effective screening method for patients who are at high risk of developing cancer. Sputum cytology is a relatively simple and inexpensive test. However, researchers have demonstrated that sputum cytology is not reliable for screening purposes.2,4
Other tests used to diagnose lung cancer
Sputum cytology is one testing procedure that may be used during the diagnosis process of lung cancer, especially when other tests or procedures may pose too much of a risk to the individual patient. Other tests that may be used include chest x-ray, CT (computed tomography) scans, PET (positron emission tomography) scans, laboratory tests, thoracentesis, needle biopsy, bronchoscopy, thoracoscopy, and pulmonary function testing.