The Pap test
The background and evolution of the Pap test
The Pap test, or Papanicolaou test, was developed by the American anatomist, Dr. George Papanicolaou. In 1926, he reported that tumour cells could be seen in the vaginal secretions of women with cervical cancer leading to his publication of the Diagnosis of uterine cancer by the vaginal smear in 1943. The first cervical cancer screening clinic was opened in Massachusetts in 1945 .
The Pap test has been used as a screening test worldwide for more than 50 years and has since played a significant role in the dramatic decline of cervical cancer in developed countries. In Canada, screening programs have used the traditional Pap smear test, or in more recent years, liquid-based cytology as the mainstay screening method for cervical cancer .
The goal of the Pap test is to screen for precancerous lesions of the cervix and vagina that have the potential to become invasive cancer and reduce the risk of advanced cancer through the detection of asymptomatic or early-stage cancer . The test itself involves scraping a sample of epithelial cells from the surface of the cervix using a spatula, brush or other sampling device.
If the sample is clear, it will be sent to laboratories for testing. Labs in Canada use the Bethesda system to report Pap test results.
These results can be reported as being normal or abnormal. A normal, or negative, result means that there were enough cells in the sample and no cancerous cells were found. Abnormal results in the cervix and vagina are classified based on how deviant they look from normal cells :
- Normal: There is no evidence of abnormal changes in the cells sampled.
- ASCUS (Atypical Squamous Cells of Undetermined Significance): The cells are abnormal, but no definite diagnosis can be made. This test result can be caused by a yeast infection, using oral contraceptives, or problems with taking the sample. Usually doctors repeat the Pap smear in a few weeks or test for the presence of high risk types of HPV.
- LSIL (Low-grade Squamous Intraepithelial Lesion): This result means an acute infection. If it persists for at least two to three visits, it can be assumed that it could lead to cancer.
- HSIL (High-grade Squamous Intraepithelial Lesion): This result means more advanced lesions.
- AGC (Atypical Glandular Cells): these abnormal cells are the precursors of about 20 per cent of cervical cancers. These cells are very difficult to detect.
Abnormal results do not mean there is a precancerous condition or cancer present, some may return to normal on their own while others may develop into cancer over time. Every year, almost 400,000 Canadian women have an abnormal Pap test result .
If the result of a Pap test is abnormal, together with your doctor you will decide if you need to have follow-up tests, treatment or both—follow-up options another include Pap test or a colposcopy.
Limitations of the Pap test
While the Pap test can be an effective tool for preventing cervical cancer, no screening test is entirely accurate and does have a risk of giving misleading results :
- the sample may not contain abnormal cells which may still be present on the cervix;
- some samples are hard to interpret ; for example, blood or mucus may make it hard
- to see the cells
- occasionally, abnormal cells are missed under the microscope
- sometimes abnormal cells occur in cells high up in the cervix or deep in the glands of the cervix (adenocarcinoma); it is not always possible to get samples from these areas
If the results are inaccurate, it is called a false-negative result; this means the test didn’t find cancer or abnormal cells even though they are present. This can occur if the sample doesn’t have enough tissue or if abnormal cells in the sample are missed. Inaccurate results can also be false-positive, which means the test shows abnormal cells when they are not present. A false-positive result may lead to unnecessary follow-up tests, procedures and anxiety for the patient .
As the Pap test can miss precancerous changes in the cervix or mistake normal changes for precancerous ones, the Pap test needs to be repeated frequently. In fact, the Canadian Cervical Screening Trial (CCCaST) reported when using the Pap test there was only a detection rate of 55 per cent in cases in which women had precancerous changes to their cervix, compared with the HPV test which detected 95 per cent of precancerous changes .
Pap tests must be sampled properly and even then the samples are challenging to interpret. Deficiencies or errors in any one of the analytic steps may explain a false-negative result despite the presence of a dysplastic lesion (pre-cancerous condition) or cervical cancer. One study reported that sampling errors by the clinician accounted for 62 per cent of false-negative smears, while interpretive errors by the pathologist accounted for 22 per cent, and screening errors by the cytotechnician were responsible for 16 per cent .
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