What Is a Pap Smear Test and How Does It Work?

A Pap smear is a screening test that collects cells from the surface of the cervix so a laboratory can examine them under a microscope for signs of abnormal growth. The test catches precancerous changes years before they could develop into cervical cancer, giving you and your doctor time to intervene. In a large Korean cohort study, women who had been screened had roughly a 38% lower risk of dying from cervical cancer than women who had never been screened.¹ The test is simple, takes only a few minutes in a clinic, and remains one of the most effective cancer-prevention tools in medicine.

What Happens During the Test

You lie on an exam table with your feet in stirrups. Your clinician inserts a speculum into the vagina to gently hold the walls apart, giving a clear view of the cervix. Then they use a small brush or spatula to sweep cells from the cervix’s surface. The cells are either spread directly onto a glass slide (the conventional method) or rinsed into a small vial of preservative liquid (liquid-based cytology). The whole collection takes under a minute, and while it can feel mildly uncomfortable or cause brief pressure, it is not typically painful. Afterward, the sample goes to a lab where a cytotechnologist examines the cells for anything unusual.

The Transformation Zone and Why It Matters

The cervix is not a uniform tissue. It has two distinct types of lining: a layer of flat, scale-like cells on the outer portion (the ectocervix) and a layer of column-shaped cells lining the inner canal (the endocervix). Where these two cell types meet is called the transformation zone, and this is the area the Pap smear is designed to sample. The transformation zone is the spot where nearly all cervical cancers originate, because the cells there are actively changing from one type to another through a process called squamous metaplasia. Research has found that the transformation zone has significantly higher expression of estrogen and progesterone receptors compared with the outer cervix, particularly in areas of immature metaplasia, which may help explain why this region is so vulnerable to cancer development.²

Human papillomavirus, or HPV, preferentially infects the transformation zone. The virus disrupts normal cell-cycle control in the lining cells, promoting uncontrolled division and the accumulation of genetic damage over time.³ Most HPV infections clear on their own, but persistent infection with high-risk strains can push cells through a sequence of increasingly abnormal changes. About 10% of mild abnormalities (CIN 1) progress to the most severe precancerous stage (CIN 3), and at least 12% of CIN 3 cases eventually become invasive cancer if left untreated.⁴ The Pap smear’s power lies in catching those changes along the way, typically years before invasion occurs.

What Happens in the Lab

Once the cell sample reaches the laboratory, it goes through a specialized staining process developed by George Papanicolaou, the test’s namesake. The Papanicolaou stain uses five dyes in sequence. Cells are first fixed in alcohol to make them transparent, then stained with a nuclear dye that highlights the cell’s nucleus in sharp detail. After that, cytoplasmic counterstains including orange G and a mixture of eosin and light green are applied, which allow different cell types to be distinguished by color. The result is a slide where nuclei appear crisp and dark while the surrounding cell body shows a range of hues, making it possible to spot cells with abnormally large, irregular, or dark nuclei.⁵

A trained cytotechnologist scans the slide, looking for cells whose size, shape, or staining patterns suggest something has gone wrong with normal cell division. Suspicious slides are flagged for a pathologist to review. The whole process is visual and human-driven, which is both the test’s strength and one of its limitations.

Understanding Your Results

Pap smear results are reported using a standardized system called the Bethesda System, which was designed to give every laboratory in the world a common vocabulary. Rather than vague terms, each category carries specific clinical implications backed by evidence and international consensus.⁶ Here is what the main result categories mean in plain terms:

NILM: Negative for intraepithelial lesion or malignancy. Your cells look normal. This is the result most people get.
ASC-US: Atypical squamous cells of undetermined significance. Some cells look slightly off, but not clearly abnormal. This is the most common abnormal result and usually leads to either repeat testing or HPV testing to decide if further workup is needed.
LSIL: Low-grade squamous intraepithelial lesion. Mild cell changes are present, often caused by an HPV infection that the body may clear on its own.
HSIL: High-grade squamous intraepithelial lesion. More serious changes that need prompt follow-up, usually a colposcopy with biopsy, because they carry a higher risk of progressing to cancer if untreated.
AGC: Atypical glandular cells. Abnormalities in the column-shaped cells of the endocervical canal. These warrant careful evaluation because glandular abnormalities can sometimes be harder to detect and may reflect a less common type of cervical cancer.

An abnormal result does not mean you have cancer. The overwhelming majority of abnormal Pap smears reflect changes that will either resolve on their own or can be treated easily before they ever become dangerous. The test is specifically designed to find things early, so an abnormal result is the system working as intended.

Conventional Pap Versus Liquid-Based Cytology

There are two main ways to handle the cells once they are collected. In a conventional Pap smear, the clinician spreads the cells directly onto a glass slide and sprays a fixative on them. In liquid-based cytology, the brush tip is rinsed into a vial of preservative fluid, and a machine later creates a thin, uniform layer of cells on the slide. Liquid-based preparation tends to produce cleaner slides with less blood and mucus in the background. In one study comparing the two approaches, hemorrhagic background appeared in about 35% of conventional slides but only about 13% of liquid-based slides.⁷

Despite those practical advantages, a large randomized trial found that liquid-based cytology did not actually perform better than the conventional Pap test for detecting cervical cancer precursors.⁸ The two methods catch precancerous lesions at similar rates in general screening. However, in the specific scenario of follow-up after cervical cancer treatment, liquid-based cytology showed a substantially higher diagnostic accuracy for detecting recurrence compared with conventional smears.⁹ The liquid-based method also has the advantage of allowing the leftover sample to be used for HPV testing without requiring a second collection.

Where the Test Falls Short

No screening test is perfect, and the Pap smear has a well-documented false-negative rate. A false negative means the test comes back normal even though abnormal cells are present. The reasons fall into a few broad categories: sampling errors where the brush missed the lesion or did not reach cells deep in the endocervical canal, and interpretation errors where abnormal cells were on the slide but the reader classified them as normal. Interpretation error was identified as the cause in over half of false-negative slides across multiple studies.¹⁰

Glandular lesions, which arise from the column-shaped cells of the endocervical canal rather than the flat cells of the outer cervix, pose a particular challenge. In a review of false-negative cases in women later diagnosed with endocervical adenocarcinoma, abnormal glandular cells were frequently misread as reactive or benign endometrial cells.¹¹ This is one reason screening guidelines increasingly emphasize HPV testing alongside or instead of cytology alone: the HPV test does not depend on visual interpretation of cell shape.

Current Screening Guidelines

Two major sets of recommendations guide cervical cancer screening in the United States, and they differ slightly. The U.S. Preventive Services Task Force recommends starting screening at age 21 with a Pap smear every three years. For women 30 to 65, the options expand to include Pap smears every three years, HPV testing alone every five years, or co-testing (Pap plus HPV) every five years.¹²

The American Cancer Society updated its guidelines in 2020 and now prefers starting screening at age 25 rather than 21, with primary HPV testing every five years as the preferred approach. If HPV-only testing is not available, co-testing every five years or Pap smears alone every three years are considered acceptable alternatives. Screening can stop after age 65 for people who have had adequate negative results over the prior decade and no history of significant precancerous lesions in the past 25 years.¹³ The shift toward HPV-first screening reflects the growing evidence that HPV testing catches more precancerous lesions than cytology alone.

HPV Testing and Co-Testing

HPV testing works differently from a Pap smear. Instead of looking at cell shape under a microscope, it detects the DNA or RNA of high-risk HPV strains in the sample. This matters because virtually all cervical cancers are caused by persistent HPV infection, so testing for the virus itself can flag risk before any cell changes are visible. In a cost-effectiveness analysis, primary HPV testing detected more cases of severe precancerous lesions and cost less than cytology alone over a three-year interval. Co-testing (Pap plus HPV) caught a handful of additional cases, but required over 100,000 extra cytology tests and hundreds more colposcopies per screening round to do so.¹⁴

A large population-based Chinese screening cohort reinforced these findings, showing that primary HPV testing and co-testing had comparable sensitivity for detecting severe lesions, and both dramatically outperformed cytology alone. For the most advanced precancerous changes, both HPV testing and co-testing reached 100% sensitivity in that study, while cytology alone caught only about 57%.¹⁵ HPV testing also required fewer follow-up colposcopies per case detected, making it more efficient.

What Happens After an Abnormal Result

If your Pap smear comes back abnormal, the next step depends on the severity of the finding and whether HPV testing was done. For mildly abnormal results like ASC-US, your provider may simply test the sample for HPV (if it was not already done) or ask you to come back in a year for a repeat test. For more concerning results like HSIL or atypical glandular cells, you will typically be referred for a colposcopy. During this procedure, the clinician uses a magnifying instrument to look closely at the cervix and applies a vinegar-like solution that makes abnormal areas turn white. If suspicious areas are found, a small tissue sample (biopsy) is taken for definitive diagnosis. Biopsy remains the gold standard for confirming or ruling out precancerous lesions and cancer, and colposcopy has been shown to be the most effective tool for guiding where those biopsies should be taken.¹⁶

Screening Challenges After Menopause

Pap smears become trickier to interpret in postmenopausal women. As estrogen levels drop, the cervical lining thins and the cells change in ways that can mimic abnormalities. One of the most common laboratory errors in this age group is misinterpreting an atrophic smear as ASC-US, the mildly abnormal category, when the changes are actually just the normal effects of low estrogen.¹⁷ The result is unnecessary follow-up testing and anxiety for the patient.

Hormone replacement therapy can change the picture. Women on hormone therapy show a different smear pattern, with more mature-looking cells and far fewer atrophic changes on their slides. In one study, atrophic changes appeared in nearly half of postmenopausal women not on hormones but in fewer than 2% of users.¹⁸ When the cells look more “normal,” the cytotechnologist has an easier time distinguishing true abnormalities from hormonal artifact. Even so, a study of women over 45 with a borderline-high Pap result (ASC-H) found that postmenopausal women were much more likely to have benign findings on follow-up compared with perimenopausal women, suggesting that less aggressive surveillance may be appropriate in the older group.¹⁹ These are among the reasons some experts advocate shifting to primary HPV testing for postmenopausal women, since the HPV test does not depend on how the cells look under the microscope.

How HPV Vaccination Is Changing Screening

Widespread HPV vaccination has started to affect what clinicians see on Pap smears. Because the vaccine prevents infection with the HPV strains responsible for most cervical cancers, vaccinated women have fewer abnormal results to begin with. A population-based cohort study found that among women vaccinated before age 17, the positive predictive value of a low-grade abnormal Pap for actually having a significant precancerous lesion dropped by 44% compared with unvaccinated women.²⁰ In practical terms, an abnormal Pap in a vaccinated woman is less likely to reflect a real precancerous lesion and more likely to be a false alarm.

Modeling work has projected that over the course of 25 years, a vaccination program with high coverage could reduce abnormal Pap results by more than 20% in women aged 21 to 29.²¹ This does not mean vaccinated women should skip screening. High-risk HPV strains not covered by the vaccine still exist, and screening intervals are already calibrated to be safe. But it does mean screening programs will need to adapt as the vaccinated population ages, potentially shifting further toward HPV-first testing rather than relying on visual cell analysis that was designed for a pre-vaccine era.

Self-Sampling and Reaching Underscreened Populations

One of the biggest barriers to cervical cancer screening is simply getting people through the clinic door. Embarrassment, discomfort, lack of transportation, cultural factors, and cost all keep millions of women from ever having a Pap smear. HPV self-sampling, where a woman collects her own vaginal sample at home using a swab or brush, has emerged as a powerful way to close that gap. When the sample is tested for HPV using sensitive molecular methods, self-collection has been shown to be just as accurate as clinician-collected samples. In combination with a follow-up Pap smear for anyone who tests positive, self-sampling actually detected more precancerous lesions than a Pap smear alone.²²

Women generally prefer self-sampling to clinician-collected testing, citing ease, privacy, and comfort as the main reasons. A systematic review found that roughly 59% of women preferred self-sampling over having a clinician collect the sample, and acceptability was high even among vulnerable and historically underscreened populations.²³ In low-resource settings, self-sampling has been widely accepted when providers take time to explain the process and let women choose their preferred method.²⁴ Self-sampling cannot replace the Pap smear itself, since it collects a sample suitable for HPV DNA testing but not for microscopic cell analysis. However, since HPV testing is increasingly the preferred first-line screening approach, self-collection is becoming a realistic gateway into the screening pathway for people who might otherwise go unscreened entirely.

Socioeconomic Disparities in Screening

Access to Pap smears is not evenly distributed. Across multiple countries, women with lower income and less education are consistently less likely to get screened. In one Korean study spanning more than a decade, women in the highest household income group were up to nearly three times as likely to have been screened as those in the lowest income group.²⁵ Similarly, a study that tracked screening after the introduction of national quality indicators found that women in the highest socioeconomic areas were more than three and a half times as likely to undergo screening as those in the lowest.²⁶ These gaps persist even when screening programs are free or heavily subsidized, because financial cost is only one of many barriers. Time off work, childcare, transportation, language, and mistrust of health systems all play roles. Cervical cancer is one of the most preventable cancers in existence, and the tragedy of these disparities is that the women most likely to develop and die from it are often the ones least likely to have had a Pap smear.

Anxiety Around the Test

Pre-procedural anxiety is common among women going in for cervical cancer screening, and it is an underappreciated reason people delay or avoid the test altogether. In a large survey of women seeking screening in Shenzhen, China, about three-quarters of women without prior screening experience reported anxiety symptoms, compared with roughly two-thirds of women who had been screened before.²⁷ Among first-timers, simply having heard about what the screening involves beforehand cut the odds of anxiety by more than half. Among women with previous experience, having been screened three or more times was associated with lower anxiety. However, not knowing their previous results actually increased anxiety, reinforcing that clear communication of results matters just as much as the test itself.

Artificial Intelligence in Pap Smear Analysis

Reading Pap smears is labor-intensive and subjective, and laboratories worldwide face a shortage of trained cytotechnologists. Artificial intelligence is being developed to assist with or partially automate the screening process. AI systems trained on large databases of Pap smear images can distinguish normal from abnormal slides with accuracy in the range of 90 to 95%, according to a systematic review and meta-analysis.²⁸ Some machine-learning algorithms applied to standardized datasets have achieved even higher accuracy in binary normal-versus-abnormal classification tasks.²⁹

The practical appeal is straightforward: AI does not get fatigued, does not have off days, and can pre-screen slides to flag the ones most likely to contain abnormalities so that a human expert can focus their attention where it matters most. The technology also reduces the need for large numbers of highly trained professionals, which is especially relevant in low-resource settings where cytotechnologist shortages can make conventional Pap-based screening programs impossible to sustain.³⁰ AI-assisted screening is not yet the standard of care, but it is moving quickly from research settings into clinical pilots, and the coming decade will likely see it integrated into routine laboratory workflows in many countries.