What is a Pap Smear Used For?
A Pap smear (Papanicolaou test) is used to screen for cervical cancer and its precursor lesions by examining cervical cells under a microscope to detect abnormal cells that may be precancerous or cancerous. 1
Primary Purpose and Mechanism
The Pap test serves two critical functions in cervical cancer prevention:
Primary goal: Detect and enable treatment of high-grade precancerous lesions (CIN2, CIN3, and adenocarcinoma in situ) before they progress to invasive cervical cancer, thereby reducing both incidence and mortality 1
Secondary goal: Identify early-stage invasive cervical cancers when they are most treatable, contributing to reduced mortality and decreased treatment-related morbidity 1
The test works by collecting cells from the cervix and examining them microscopically for abnormalities, including precancerous changes and cancer cells 1
Historical Impact and Effectiveness
Since its introduction in the 1940s, the Pap test has decreased cervical cancer mortality in the United States by over 70%, transforming cervical cancer from the leading cancer killer of women to the 13th most common cause of cancer death 1
The test has proven to be the most efficacious and cost-effective cancer screening method available, with five-year survival rates approaching 100% for preinvasive lesions and 92% for early-stage cancers 1
Approximately half of cervical cancers diagnosed in the United States occur in women who have never been screened, highlighting the importance of regular screening 1
Test Characteristics and Limitations
Important caveat: The Pap test is not perfect and has inherent limitations that clinicians must understand:
Sensitivity for high-grade cervical intraepithelial neoplasia (CIN) ranges from 70-80%, meaning false-negative results occur even in optimized screening programs 1
False-negative results stem from multiple factors: small lesion size, inaccessible lesion location, inadequate sampling, few abnormal cells on the slide, small abnormal cell size, or obscuring inflammation/blood 1
The test has high specificity, and false-positive results are rare 2
Modern Screening Context
The landscape of cervical cancer screening has evolved significantly:
Current recommendations (as of 2020): The American Cancer Society now recommends primary HPV testing every 5 years starting at age 25 as the preferred strategy, with cytology-based screening (Pap test alone or cotesting) as transitional options 1
The Pap test detects abnormalities using the Bethesda System classification: atypical squamous cells (ASC-US, ASC-H), low-grade squamous intraepithelial lesion (LSIL), high-grade squamous intraepithelial lesion (HSIL), and atypical glandular cells (AGC) 1
Liquid-based cytology has largely replaced conventional Pap smears in the United States, offering improved specimen adequacy and the ability to perform HPV testing from the same sample 1
Clinical Application
Key screening parameters:
Screening should begin at age 25 years (not earlier, even if sexually active) 1
For women aged 25-65 years, screening with cytology alone should occur every 3 years if primary HPV testing is unavailable 1
For women aged 30-65 years, cotesting (cytology plus HPV testing) every 5 years is an acceptable alternative when primary HPV testing is not available 1
Screening can cease at age 65 years in women with adequate prior screening and normal results 1
Annual screening is not recommended for any age group 1
Special Populations
Women who have received HPV vaccination still require screening using the same recommendations as unvaccinated women 1
Women who are immunocompromised (HIV-positive), exposed to diethylstilbestrol in utero, or have a history of cervical cancer may require more frequent screening than standard guidelines 1
Screening is not recommended for women under age 21 years, regardless of sexual activity 1