Methods of cervical screening. Describe.
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Methods of Cervical Screening
Cervical cancer is largely preventable because it develops from precursor lesions (CIN) over many years, giving a wide window for detection. Screening has reduced cervical cancer incidence by ~79% and mortality by ~70% since 1950. - Berek & Novak's Gynecology, p. 845
1. Conventional Cervical Cytology (Conventional Pap Smear)
The original Papanicolaou (Pap) test involves circumferentially scraping the transformation zone of the cervix with a spatula or brush, then smearing and fixing cells directly onto a glass slide for staining and microscopic examination.
Limitations:
- Only 10-20% of collected cells are transferred to the slide
- Air-drying artefact distorts cellular morphology
- Slide may be obscured by blood, mucus, or vaginal discharge
- Sensitivity for detecting CIN 2/3: 47-62%; specificity: 60-95%
- Overall false-negative rate: ~49%
- Sampling errors, preparation errors, and interpretive errors all contribute to missed cases
More than half of all invasive cervical cancer cases occur in women who were never screened or underscreened. - Berek & Novak's Gynecology, p. 845
2. Liquid-Based Cytology (LBC)
LBC was developed to overcome the limitations of conventional cytology and is now the standard in most high-income countries.
Technique:
- Cells are collected with an endocervical brush + plastic spatula, or a plastic broom
- The device is rinsed directly into a vial of liquid alcohol-based preservative (e.g., ThinPrep, SurePath)
- 80-90% of cells are transferred to the preservative (vs. 10-20% with conventional smears)
- The liquid is passed through a filter, trapping larger epithelial cells and separating them from blood and inflammatory cells
- A thin, uniform monolayer of well-preserved cells is deposited on the slide
Advantages over conventional cytology:
- Eliminates air-drying artefact
- Reduces unsatisfactory samples by 70-90%
- Residual material in the vial can be used for reflex HPV testing - no second visit required
- Berek & Novak's Gynecology, p. 845-846
3. Automated Image-Guided Slide Screening (Computer-Assisted Cytology)
An FDA-approved adjunct for primary screening and rescreening of cytology samples initially interpreted as normal.
How it works:
- An automated microscope coupled to a digital camera scans the entire slide
- Computer algorithms analyze each field of view and rank the slide by probability of containing an abnormality
- High-probability slides are then reviewed by a cytotechnologist or cytopathologist
Performance: Reduces the false-negative rate by 32% compared to unassisted cytology. - Berek & Novak's Gynecology, p. 846
4. HPV DNA/RNA Testing
Since 93-100% of squamous cell cervical cancers contain DNA from high-risk HPV strains, molecular HPV testing targets the root cause of the disease.
Characteristics:
- Higher sensitivity but lower specificity than cytology
- Not recommended as a standalone test in women under 30 (high prevalence of transient HPV infection makes results non-specific)
- HPV 16 and 18 carry the highest cancer risk and can be specifically genotyped
FDA-approved assays and their clinical roles:
| Assay | Target | Key Indications |
|---|---|---|
| Hybrid Capture 2 | DNA (genomic) | ASC-US triage, Co-test |
| Cervista | DNA (Invader Technology) | ASC-US triage, Co-test |
| Cobas HPV (PCR TaqMan) | L1 DNA | ASC-US triage, Co-test, Primary screening |
| APTIMA | E6/E7 mRNA | ASC-US triage, Co-test, Primary screening |
Berek & Novak's Gynecology, p. 846
HPV testing can be used in three clinical contexts:
- Triage of equivocal cytology (ASC-US) - to determine which women need colposcopy
- Co-testing - simultaneous cytology + HPV, particularly for women ≥30
- Primary HPV screening - standalone first-line test (Cobas/APTIMA), with reflex cytology for positives
5. Co-Testing (Cytology + HPV)
Combining Pap cytology with HPV testing provides higher sensitivity than either test alone.
- Recommended for women ≥30 years
- Women negative for both tests can extend screening interval to every 5 years
- Women with normal cytology but positive for high-risk HPV DNA should repeat cervical cytology every 6-12 months
- Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 926
6. Colposcopy (Diagnostic Follow-Up, Not Primary Screening)
When any screening test is abnormal, colposcopy is performed:
- The cervix is examined under magnification after applying acetic acid
- Abnormal epithelium appears as aceto-white areas
- Abnormal areas are biopsied for histological confirmation
- Robbins Pathologic Basis of Disease, p. 926
Cytological Appearances on Pap Smear
The following image illustrates the spectrum of cervical cytological changes:
(A) Normal exfoliated superficial squamous cells - abundant cytoplasm, small regular nuclei
(B) LSIL - koilocytes (HPV cytopathic effect): large perinuclear halo, irregular nuclear membrane
(C & D) HSIL - progressive reduction in cytoplasm, increasing nucleus-to-cytoplasm ratio, nuclear hyperchromasia - reflecting loss of cellular differentiation
The Bethesda Classification of Cervical Cytology
Screening results are reported using the Bethesda System:
- NILM - Negative for intraepithelial lesion or malignancy (normal)
- Squamous cell abnormalities:
- ASC-US - Atypical squamous cells of undetermined significance
- ASC-H - Atypical squamous cells, cannot exclude HSIL
- LSIL - Low-grade squamous intraepithelial lesion
- HSIL - High-grade squamous intraepithelial lesion
- Squamous cell carcinoma
- Glandular cell abnormalities:
- AGC (Atypical glandular cells)
- AIS (Endocervical adenocarcinoma in situ)
- Adenocarcinoma
Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed.
Screening Intervals (ACOG Guidelines)
| Age Group | Recommended Method | Interval |
|---|---|---|
| < 21 years | No screening | - |
| 21-29 years | Cytology alone | Every 2-3 years |
| 30-65 years | Co-testing (preferred) | Every 5 years |
| 30-65 years | Cytology alone | Every 3 years |
| > 65 years | Can discontinue if 3 consecutive normal Paps or 2 normal co-tests in 10 years | - |
| Post-hysterectomy (benign indication, no CIN history) | Discontinue | - |
More frequent screening (annually) is recommended for: HIV-positive women, immunosuppressed women, DES daughters, and those with history of CIN 2+. - Berek & Novak's Gynecology, p. 846-847
Summary Comparison
| Method | Sensitivity (CIN 2+) | Specificity | Key Role |
|---|---|---|---|
| Conventional Pap | 47-62% | 60-95% | Historical standard |
| Liquid-based cytology | Similar/slightly better | Similar | Current standard cytology |
| HPV DNA testing | Higher | Lower | Primary screen / co-test / ASC-US triage |
| Co-testing | Highest combined | Moderate | Gold standard for ≥30 years |
| Automated image analysis | Reduces FN by 32% | Improved | Adjunct to cytology |
Recent evidence note: A 2023 systematic review (PMID 37533074) examined DNA methylation as a triage marker for colposcopy referral in HPV-based screening, representing an emerging molecular approach that may refine triage of HPV-positive women beyond current genotyping strategies.
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