What is a Cytology Test?
A cytology test is a microscopic examination of cells collected from the body to detect abnormalities, and in the context of cervical cancer screening, it specifically refers to the Pap (Papanicolaou) test—a procedure that collects and examines cells from the cervix to identify precancerous changes and early cancer. 1
Purpose and Clinical Significance
The primary goal of cervical cytology is the early detection, diagnosis, and treatment of cancer precursor lesions and invasive cancer before they progress to life-threatening disease. 1 This screening method has proven to be the most successful cancer prevention strategy in modern medicine, reducing cervical cancer incidence by 75% and mortality by 74% in the United States over 50 years. 1
How the Test Works
Cell Collection Methods
Cervical cytology involves collecting cells from the cervix using specialized instruments designed to sample the transformation zone (the area where most cervical cancers develop). 2 Two main collection techniques exist:
Conventional Pap smear: The sampling device is smeared directly onto a glass slide with immediate spray fixation, though this method is subject to air-drying artifact if fixation is delayed. 3
Liquid-based cytology (LBC): The sampling device is placed directly into liquid fixative, providing immediate fixation, eliminating air-drying artifact, and allowing for more even cell distribution. 4, 3 This is now the predominant method used in the United States. 1
Laboratory Analysis
The collected cells are examined under a microscope using the 2001 Bethesda System for reporting results. 1 The laboratory evaluates cellular characteristics to identify:
- Normal cells (negative for intraepithelial lesion or malignancy)
- Precancerous changes (low-grade or high-grade squamous intraepithelial lesions)
- Invasive cancer cells 1
Clinical Advantages
The success of cervical cytology reflects several key factors:
Slow disease progression: The generally slow progression from precancerous lesions to invasive cancer provides ample opportunities for early detection. 1
Detectable abnormalities: The ability to identify cytologic abnormalities before invasive disease appears. 1
Effective treatment: Availability of minimally morbid therapy for premalignant disease. 1
Repeat testing strategy: Frequent repetition of the test increases detection rates over time. 1
Modern Screening Approach
Current guidelines recommend primary HPV testing every 5 years starting at age 25 as the preferred screening strategy, though cytology alone every 3 years or cotesting (cytology plus HPV testing) every 5 years remain acceptable alternatives during the transition period. 1 The combination of HPV testing with cytology improves screening efficacy compared to cytology alone and allows for safer screening with increased intervals. 1
Test Limitations
Despite its success, cervical cytology has important limitations:
Suboptimal single-test sensitivity: The imperfect sensitivity is estimated to be responsible for 30% of all cervical cancers. 1
Limited reproducibility: Results can vary between readings. 1
Equivocal results: Many results fall into uncertain categories requiring additional testing. 1
Higher sensitivity but lower specificity with LBC: Liquid-based cytology detects more abnormalities but also generates more false positives compared to conventional methods. 3
Critical Pitfall
Half of all women who develop cervical cancer in the United States have never been screened, and an additional 10% have not been screened within 5 years of diagnosis. 1 This underscores that the primary failure of cervical cancer prevention is not the test itself, but rather failure to reach at-risk populations who do not participate in regular screening programs.