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Healthy Tips

What Is a Pap Smear?

The Science Behind This Routine Test

Its real name: Papanicolaou test. There's a good chance you know it as a Pap smear, or more appropriately, a Pap test. Discover the science behind this test ­— including what it doesn’t detect.

The Science Behind the Test

By definition, a Pap test is a test where a sample of cells is taken from the cervix, which is the lower part of the uterus neck located at the top of the vagina. Your physician will then place the cells into a liquid substance in a small jar to be tested for abnormalities under a microscope.

It’s important to note that the Pap test is a screening test and not a diagnostic test. There can be a number of possible reasons why you could have abnormal findings. These include:

“The Pap test can detect changes in cells that could be concerning for possible cervical cancer or precancerous changes,” says Northwestern Medicine Obstetrician and Gynecologist Melissa A. Simon, MD, MPH.

Abnormal test results will indicate a number of atypical squamous cells, which will then be classified as low-grade or high-grade squamous intraepithelial lesions (LSIL and HSIL, respectively). LSIL indicates that the changes are mildly abnormal and usually caused by HPV infection, and they may go away on their own. HSIL, however, suggests more serious changes.

“Cells that are determined to be atypical of undetermined significance basically reveal slightly abnormal cells, and it does not clearly mean precancer is there. Usually these test results indicate to follow up in a year,” says Dr. Simon.

These abnormal test results are usually caused by HPV. HPV is a sexually transmitted infection that enters cells and causes them to change. HPV can be prevented by getting the vaccine, which targets the HPV types that most commonly cause certain types of cancer and genital warts.  Certain high-risk types of HPV are associated with an increased risk for cervical, vulva, vagina, penis, anus, mouth and throat cancer.

Depending on your Pap test findings, your physician will determine the appropriate next steps. This can include another test, known as a colposcopy, which uses a special camera to look at your cervix. During this procedure, a biopsy of cells on your cervix may be taken for further analysis. Your physician may also opt for a loop electrosurgical excision procedure, known as LEEP, if there are more concerning findings or repeated abnormal Pap test results. This removes a larger portion of tissue using local anesthetic.

What It Can’t Determine

Although Pap tests can help detect precancerous cells, there are other types of gynecologic cancers that are not screened for by the Pap test. 

Regarding ovarian cancer, your ovaries are far removed from your cervix. Therefore, it is extremely unlikely to detect ovarian cancer through a Pap test. This would mean the cancer cells traveled away from your ovaries, through your fallopian tubes and uterus, and into the area surrounding your cervix.

The Pap test is also limited in detecting other types of sexually transmitted infections (STIs). If you are concerned you may have a STI, you should ask your care provider to screen for specific STIs.

Dr. Simon admits there are many nuances and complexities of when to start screening and how frequently screening should be done. Cervical cancer can also take a long time to develop, which could explain the varying guidelines.

The U.S. Preventive Services Task Force (USPSTF) and the American College of Obstetricians and Gynecologists  both suggest starting screening at age 21.  However the American Cancer Society suggests that cervical cancer screening begin at age 25.

Screening with a Pap test every three years during this time period is acceptable. However, alternate forms of screening include primary HPV testing every five years.  If primary HPV testing is not available, screening may be performed with contesting that combines an HPV test with a Pap test every five years.

 “If you have vaginal discharge, abnormal bleeding or are feeling pain, schedule a visit with your healthcare provider to discuss these symptoms,” says Dr. Simon. “It’s important that if you are ever in any doubt of what you are feeling or experiencing, and are questioning if you need screening or a test, always reach out to your care team so you can have that conversation.”

Although the organizations and the USPSTF all agree that women older than 65 should not be screened, Dr. Simon does clarify that women who were not adequately screened before age 65 may benefit from screening.

Bottom Line

The Pap test is a screening tool that can help detect abnormal cells on your cervix, which could be important to stop precancerous cells from progressing to cervical cancer.  

“Cervical cancer, if caught early, is highly treatable. And that’s why engaging in routine screening is very important,” says Dr. Simon. “Between the HPV vaccine and routine testing, this type of cancer really should be nonexistent.”

Melissa A. Simon, MD
Melissa A. Simon, MD
Nearest Location:
Professor, Feinberg School of Medicine
  • Primary Specialty Obstetrics and Gynecology
Does Not Schedule or Accept New Patients
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