What is the lab test for a Pap (Papanicolaou) smear?

What is the Lab Test for a Pap Smear?

A Pap smear (Papanicolaou test) is a cervical cytology test that examines cells collected from the cervix under a microscope to detect abnormal cells that may be precancerous or cancerous. 1

Types of Pap Smear Tests

The Pap smear can be performed using two main laboratory techniques:

Conventional Pap Smear

• Cells are collected from the cervix using a spatula and brush, then smeared directly onto a glass slide and immediately fixed with spray fixative or 95% ethanol 1, 2
• The slide is sent to a cytopathology laboratory where a cytopathologist examines the cells microscopically for abnormalities 1
• This method has been the traditional screening approach since the 1940s 3

Liquid-Based Cytology (LBC)

• Cells are collected using the same sampling devices but are rinsed into a vial containing liquid preservative medium rather than smeared on a slide 1
• The laboratory then prepares a thin, single-layer slide from the liquid specimen 4
• Liquid-based cytology is now the predominant method used in the United States and is preferred because it produces fewer inadequate readings, has cleaner preparations with less blood and debris obscuring cells, and allows HPV testing from the same specimen 1, 2
• Common LBC systems include ThinPrep and SurePath 5

What the Laboratory Evaluates

The cytopathologist examines the specimen for:

• Presence and adequacy of endocervical/transformation zone cells (to confirm proper sampling of the area where cervical cancer develops) 2
• Cellular abnormalities including atypical squamous cells, low-grade squamous intraepithelial lesions (LSIL), high-grade squamous intraepithelial lesions (HSIL), and malignant cells 1
• Adequacy of the specimen (whether sufficient cells are present for interpretation) 5

Complementary Testing Options

Modern cervical cancer screening often combines cytology with additional molecular testing:

HPV DNA Testing

• Primary HPV testing detects DNA of high-risk (oncogenic) HPV types in cervical samples and is now the preferred screening method starting at age 25 according to the 2020 American Cancer Society guidelines 1
• FDA-approved primary HPV tests include cobas HPV (approved 2014) and Onclarity HPV (approved 2018) 1
• HPV testing is more sensitive than cytology alone but less specific 1

Cotesting

• Cotesting combines cytology and HPV testing on the same cervical sample 1
• Multiple FDA-approved cotest platforms exist, including Digene HC2, Cervista HPV, Aptima HPV, cobas HPV, and Onclarity HPV 1
• Cotesting every 5 years is an acceptable alternative for women aged 30-65 years when primary HPV testing is not available 1

Important Clinical Caveats

• The Pap test has a sensitivity of only 70-80% for high-grade cervical intraepithelial neoplasia (CIN), meaning false negatives occur even in optimized programs 1
• At least one-third of false-negative results are due to sampling errors where abnormal cells were never collected 2
• Approximately half of cervical cancers diagnosed in the United States occur in women who have never been screened 1
• The test specifically samples the cervical transformation zone; women who have had total hysterectomy with complete cervical removal do not require Pap testing unless the hysterectomy was performed for high-grade dysplasia 6

Specimen Quality Factors

To optimize laboratory interpretation:

• Schedule testing to avoid heavy menstrual flow when possible, as blood can obscure cellular material 2
• Patients should avoid douching, intercourse, tampons, and vaginal medications for 48 hours before testing 2
• Proper collection technique using extended-tip spatula for the ectocervix and endocervical brush for the endocervical canal provides the lowest false-negative rate 2