Cervical cancer screening is a medical screening test designed to identify risk of cervical cancer. Cervical screening may involve looking for viral DNA, and/or to identify abnormal, potentially precancerous cells within the cervix as well as cells that have progressed to early stages of cervical cancer. One goal of cervical screening is to allow for intervention and treatment so abnormal lesions can be removed prior to progression to cancer. An additional goal is to decrease mortality from cervical cancer by identifying cancerous lesions in their early stages and providing treatment prior to progression to more invasive disease.
Currently available screening tests fall into three categories: molecular, cytologic and visual inspection. Molecular screening tests includes nucleic acid amplification tests (NAAT), which identify high risk human-papilomma virus (HPV) strains. Cytologic tests include conventional Pap smear and liquid based cytology. Visual Inspection tests involve application of a solution to enhance identification of abnormal areas and can utilize the naked eye or a colposcope/magnifying camera.
Medical organizations of different countries have unique guidelines and screening recommendations. The World Health Organization has also published guidelines to increase screening and improve outcomes for all women taking into consideration differences in resource availability of regions. Management of abnormal screening results can include surveillance, biopsy, or removal of the suspicious region via surgical intervention. Diagnosis of more advanced cancer stages may require other treatment options such as chemotherapy or radiation.
General Screening Procedure
The procedures for testing women using Pap smear, liquid-based cytology, or HPV testing are similar. A sample of cells is collected from the cervix using a spatula or small brush. The cells are then checked for any abnormalities.
To take the sample of cells, the health care clinician inserts an instrument, called a speculum, inside the vagina. The speculum has two arms that spread the walls of the vagina apart in order to see the cervix. Then, they scrape the surface of the cervix with a spatula or small brush. This collects a sample of cells from the outer layer of the cervix.
Self-collection is also an option when testing by a provider is unavailable or uncomfortable for a patient. When utilizing HPV testing, self-collection has been shown to be as accurate as swabbing by a provider. This equality has not been demonstrated for other testing such as pap smear or liquid-based cytology.
With a Pap smear, cells collected using a spatula are smeared onto a slide for examination under a microscope. In liquid-based cytology, a sample of cells is taken using a small brush. The cells are put into a container of liquid, and analysed for abnormalities. Cervical cells to be tested for HPV are collected in a similar way.
Types of Screening
Molecular
Molecular testing identifies an infection called human papillomavirus, or HPV. Human papillomavirus (HPV) infection is a cause of nearly all cases of cervical cancer. Most women will successfully clear HPV infections within 18 months. Those that have a prolonged infection with a high-risk type (e.g. types 16, 18, 31, 45) are more likely to develop Cervical Intraepithelial Neoplasia, due to the effects that HPV has on DNA.
The screening process utilizes nucleic acid amplification testing to look for DNA or RNA of the virus present within cervical cells. Some tests can identify up to 14 different types of high risk strains.
Accuracy of HPV testing report: sensitivity 88% to 91% (for detecting CIN 3 or higher) to 97% (for detecting CIN2+) specificity 73% to 79% (for detecting CIN 3 or higher) to 93% (for detecting CIN2+)
Cytologic
Conventional Pap smear
In the conventional Pap smear, the collected cells are smeared on a microscope slide, and a fixative is applied. The slide is evaluated in a pathology lab to identify cellular abnormalities.
Accuracy of conventional cytology report: sensitivity: 55% to 88% specificity: 71% to 94%
Liquid-based monolayer cytology
In liquid based monolayer cytology, the collected cells are placed into a liquid medium. The sample is evaluated in a pathology lab to evaluate cellular abnormalities.
Accuracy of liquid based monolayer cytology report: sensitivity 57% to 90% specificity 64% to 97%