A Pap smear tests for abnormal cell changes on the cervix that could eventually develop into cervical cancer. It does not diagnose cancer directly. Instead, it catches precancerous changes early, often years before they would ever become dangerous, giving you time to monitor or treat them. Nearly all cervical cancers are caused by persistent infection with certain strains of human papillomavirus (HPV), and the Pap smear is designed to spot the cellular damage that HPV can leave behind.
What Happens to Your Cells
During a Pap smear, a healthcare provider collects a sample of cells from your cervix, specifically from an area called the transformation zone. This is the region where two types of tissue meet on the surface of the cervix, and it is the spot where precancerous changes are most likely to develop. The sample needs to contain thousands of cells to be considered adequate for analysis.
A pathologist examines those cells under a microscope, looking at nuclear size, shape, and other features that signal whether cells are developing normally or starting to change. Healthy cervical cells have predictable, uniform-looking nuclei. As cells become more abnormal, their nuclei grow larger, their internal structure becomes more irregular, and the ratio of nucleus to surrounding cell material shifts. These changes fall along a spectrum from mildly concerning to clearly precancerous.
How Results Are Reported
Pap smear results use a standardized system with specific categories, from normal to increasingly abnormal. Understanding the terminology helps you make sense of what your provider tells you.
- Normal (NILM): No abnormal cells were found. This is the result most people receive.
- ASC-US: Atypical squamous cells of undetermined significance. Some cells look slightly unusual but not clearly abnormal. This is the mildest and most common abnormal result.
- LSIL: Low-grade squamous intraepithelial lesion. Cells show mild changes, often caused by an active HPV infection. Nuclei are noticeably enlarged (three to four times normal size) and may show characteristic signs of HPV.
- HSIL: High-grade squamous intraepithelial lesion. Cells show moderate to severe changes that are more likely to progress toward cancer if left untreated. The abnormal cells take up a larger proportion of the tissue’s thickness.
- AGC: Atypical glandular cells. Some glandular cells (from deeper in the cervix or the uterus) don’t look normal. This finding is less common but taken seriously because it can signal a more advanced problem.
What Each Result Means for You
A normal result means you can follow the standard screening schedule and not think about it until your next test is due. An ASC-US result is extremely common, and your provider may simply run an HPV test on the same sample to decide whether any follow-up is needed. Many ASC-US results resolve on their own.
LSIL corresponds to what’s called CIN 1, where abnormal cells affect roughly the bottom third of the cervical tissue lining. About 99% of CIN 1 cases never progress to cancer, and most clear up without treatment. Your provider will typically recommend monitoring with repeat testing rather than any procedure.
HSIL is more serious. It corresponds to CIN 2 or CIN 3, where abnormal cells have spread through more than a third (and sometimes nearly all) of the tissue’s thickness. CIN 2 and CIN 3 are more likely to require treatment, which usually involves removing or destroying the abnormal tissue in an outpatient procedure. Catching these changes is the entire point of screening, because treating them prevents cervical cancer from developing.
What Happens After an Abnormal Result
For higher-grade findings like HSIL, ASC-H (cells that might be high-grade but aren’t certain), or atypical glandular cells, your provider will typically schedule a colposcopy. This is a closer examination of the cervix using a magnifying instrument, and it usually includes taking a small tissue biopsy. The biopsy provides a definitive answer about whether precancerous changes are present and how severe they are.
Decisions about follow-up aren’t based solely on a single Pap result. Providers now use a risk-based approach that factors in your current result alongside your previous screening history, any prior treatments, and your age. Someone with a first-time LSIL result and no history of abnormalities faces a very different risk profile than someone with repeated abnormal findings over several years.
Pap Smear vs. HPV Test
A Pap smear and an HPV test are not the same thing, though they’re often done together. The Pap looks at how your cervical cells appear under a microscope. The HPV test looks for the virus itself, specifically the high-risk strains most likely to cause cancer. You can have HPV without any cell changes, and you can (rarely) have cell changes without a detectable HPV infection.
When both tests are done at the same time, it’s called co-testing. If both come back normal, your provider may tell you that you can wait five years before your next screening. The combination gives a more complete picture of your risk than either test alone.
When and How Often to Screen
Current guidelines from the U.S. Preventive Services Task Force set clear age-based recommendations:
- Under 21: No screening recommended, regardless of sexual history.
- Ages 21 to 29: Pap smear every three years.
- Ages 30 to 65: Three options. A Pap smear alone every three years, an HPV test alone every five years, or co-testing (both together) every five years.
- Over 65: Screening can stop if you’ve had adequate prior screening with normal results and aren’t at high risk.
- After hysterectomy: If your cervix was removed and you have no history of CIN 2, CIN 3, or cervical cancer, screening is no longer needed.
What a Pap Smear Does Not Test For
A Pap smear is specifically designed for cervical cancer screening. It does not test for other gynecological cancers like ovarian or uterine cancer. It does not screen for sexually transmitted infections (though an HPV co-test checks for that one virus). It doesn’t check for fertility issues, hormonal imbalances, or infections like yeast or bacterial vaginosis, even though those might sometimes show up incidentally on a slide. A pelvic exam and a Pap smear are also not the same thing. The pelvic exam is the physical examination; the Pap is a specific lab test done during that exam.