What Does a Pap Smear Test For?
A Pap smear (cervical cytology) screens for precancerous cervical lesions and cervical cancer by detecting abnormal cervical cells before they progress to invasive disease. 1
Primary Purpose
The Pap test identifies cytologic abnormalities in cervical cells that indicate:
- Precancerous lesions (cervical intraepithelial neoplasia - CIN grades 1,2, and 3) 1
- Invasive cervical cancer 1
- Cellular changes associated with human papillomavirus (HPV) infection, which causes virtually all cervical cancers 1
The test works by collecting exfoliative cervical cells and examining them microscopically for abnormalities ranging from atypical squamous cells of undetermined significance (ASC-US) to high-grade squamous intraepithelial lesions (HSIL) and invasive carcinoma. 1
What the Pap Smear Does NOT Test For
Important caveat: Pap smears and HPV tests should not be considered screening tests for sexually transmitted infections like chlamydia or gonorrhea. 1 While inflammation from infections may be noted on cytology, the test is not designed to diagnose STIs.
Clinical Impact
The Pap test has been extraordinarily successful as a public health intervention:
- Reduced U.S. cervical cancer incidence by 75% and mortality by 74% over 50 years following its introduction 1
- Decreases cervical cancer rates by 60-90% within 3 years when screening programs are implemented in previously unscreened populations 1
- Approximately 500,000 precancerous lesions (CIN 2 and 3) are diagnosed annually in the United States, with 50-60% attributable to HPV types 16 and 18 1
How It Works in Practice
The test capitalizes on the slow progression from precancerous lesions to invasive cancer, typically taking years to decades. 1 This provides multiple opportunities for early detection and treatment of abnormalities before cancer develops. 1 When abnormal cells are detected, colposcopy with directed biopsies is performed to determine the extent of disease and guide treatment. 1
Key limitation: The single-test sensitivity of cytology is imperfect, estimated to be responsible for 30% of cervical cancers that occur despite screening, with provider follow-up errors accounting for another 10%. 1 This is why current guidelines recommend HPV co-testing for women aged 30-65 years to improve detection rates. 1