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 Pap test can detect changes in cells that could be concerning for possible cervical cancer or precancerous changes," says Melissa A. Simon, MD, an obstetrician and gynecologist at Northwestern Medicine. 

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 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.

Abnormal Pap test results are typically caused by HPV. HPV is a sexually transmitted infection (STI) that enters cells and changes them. 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 the following types of cancer:

• Cervical
• Vulva
• Vagina
• Penis
• Anus
• Mouth and throat cancer

Depending on your Pap test results, 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. A LEEP removes part of your cervix tissue for diagnosis and/or treatment.

What It Can't Determine

Although Pap tests can help detect precancerous cells on the cervix, there are other types of gynecologic cancers that Pap tests cannot detect. 

Because your ovaries are far away from your cervix, it is extremely unlikely that a Pap test will  detect ovarian cancer. For that to happen, the cancer cells would have to travel 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 STIs. If you are concerned you may have a STI, you should ask your clinician to screen for specific STIs.

Pap Tests Are Part of Routine Screening

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

Dr. Simon explains that 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.

Screening with a Pap test every three years during this time period is acceptable. However, other types of screening include primary HPV testing every five years. If primary HPV testing is not available, screening may be performed with co-testing 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 clinician 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."

The USPSTF, ACOG and American Cancer Society agree that people older than 65 do not need to be screened if adequate prior screening has been completed with normal results and if there is no history or high risk of cervical cancer. Dr. Simon recommends talking to your primary care clinician about your individual risks and recommendations for screening.

Bottom Line

The Pap test is a screening tool that can help detect abnormal cells on your cervix. This early detection can be important in stopping 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."