Cervical cytology — the Pap test — is the most successful cancer screening program in medical history. Since its introduction, cervical cancer incidence has decreased by over 70% in screened populations. The Pap test detects pre-invasive lesions (cervical intraepithelial neoplasia) that can be treated before progression to invasive cancer.
Specimen Collection
The conventional Pap smear collects cells from the transformation zone — the squamocolumnar junction of the cervix where most cervical neoplasia arises. The specimen is collected using a spatula (ectocervix) and cytobrush (endocervix) and smeared directly onto a glass slide with immediate fixation. Liquid-based cytology (ThinPrep, SurePath) uses a brush collection device rinsed into preservative fluid; the automated processor creates a thin-layer slide. LBC reduces inadequate rates, obscuring blood and inflammation, and allows HPV co-testing from the same vial.
The Bethesda System for Cervical Cytology
The Bethesda System (TBS) provides standardized terminology for reporting cervical cytology. Each report includes specimen adequacy (satisfactory, satisfactory but limited, unsatisfactory), general categorization (negative for intraepithelial lesion or malignancy, epithelial cell abnormality, other), and interpretation/result.
Squamous cell abnormalities:
- ASC-US (atypical squamous cells of undetermined significance) — cellular changes suggestive of LSIL but quantitatively or qualitatively insufficient. The most common abnormal interpretation, managed by HPV triage.
- ASC-H (atypical squamous cells, cannot rule out HSIL) — atypical cells suspicious for but not diagnostic of HSIL. Requires colposcopy.
- LSIL (low-grade squamous intraepithelial lesion) — changes associated with HPV infection (koilocytosis, nuclear enlargement, binucleation). Corresponds to CIN 1 histology.
- HSIL (high-grade squamous intraepithelial lesion) — cells with high nuclear-to-cytoplasmic ratio, hyperchromasia, irregular chromatin. Corresponds to CIN 2/3 histology.
- Squamous cell carcinoma — malignant cells with明显 nucleoli, keratinization, tumor diathesis (necrotic background).
Glandular cell abnormalities: atypical glandular cells (AGC), endocervical AGC, endometrial AGC, and adenocarcinoma. AGC carries a significant risk of malignancy and requires colposcopy, endocervical sampling, and endometrial evaluation.
HPV and Cervical Cancer
High-risk human papillomavirus (HR-HPV) types 16 and 18 cause approximately 70% of cervical cancers; types 31, 33, 45, 52, and 58 cause an additional 20%. The E6 and E7 oncoproteins of HR-HPV inactivate p53 and Rb, leading to genomic instability and malignant transformation. HPV testing is used for triage of ASC-US (HPV-positive women go to colposcopy; HPV-negative women return to routine screening), co-testing with cytology in women age 30-65, and post-treatment surveillance. HPV vaccines (quadrivalent, nonavalent) prevent infection by the most common HR-HPV types.
Screening Guidelines
Current US guidelines (ACS, ASCCP, USPSTF) recommend screening starting at age 25. Options include: primary HPV testing every 5 years; co-testing (HPV + cytology) every 5 years; or cytology alone every 3 years. Screening ends at age 65 if adequate negative prior screening. Abnormal results are managed by the ASCCP Risk-Based Management Consensus Guidelines (2019), which use current and prior results and HPV genotype to estimate the risk of CIN 3+ and recommend appropriate action (1-year follow-up, colposcopy, or treatment).
Quality Assurance
The Pap test laboratory is highly regulated. Regulations require: documented cytotechnologist and pathologist qualifications, daily workload limits (≤100 slides per 8-hour day for cytotechnologists), 10% random rescreening of negative cases, correlation of cytology with subsequent histology, and turnaround time targets. Participation in EQA programs is mandatory. Regulatory oversight includes CLIA (US), the European Guidelines for Quality Assurance in Cervical Cancer Screening, and national cervical screening program standards.
