Pap Smear

  • Medical Author:
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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How is a Pap smear read (analyzed)?

Pap smear analysis and reports are all based on a medical terminology system called The Bethesda System. The system was developed (at the National Institutes of Health (NIH) in Bethesda, Maryland) to encourage all medical professionals analyzing Pap smears to use the same reporting system. Standardization reduces the possibility that different laboratories might report different results for the same smear.

The Bethesda System was the outcome of a National Cancer Institute workshop that was held in 1988 in an effort to standardize Pap smear reports. In 2001, the guidelines were revised and improved. Acceptance of the Bethesda reporting system in the United States is virtually universal.

The major categories for abnormal Pap smears reported in the Bethesda Systems are as follows (described in further detail in the "What are the possible recommendations for follow-up after a Pap smear?" section):

  • ASC-US: This abbreviation stands for atypical squamous cells of undetermined significance.
  • LSIL: This abbreviation stands for low-grade squamous intraepithelial lesion. Under the old system of classification, this category was called CIN grade I.
  • HSIL: This abbreviation stands for high-grade squamous intraepithelial lesion. Under the old system of classification, this category was called CIN grade II, CIN grade III, or CIS.
  • ASC-H: This means atypical cells are present and HSIL cannot be excluded.

The word "squamous" describes the thin, flat cells that lie on the surface of the cervix. "Intraepithelial" indicates that the surface layer of cells is affected. A "lesion" means that abnormal tissue is present.

An abnormal or inadequate Pap smear report may contain the following diagnoses.

Absence of endocervical cells on the Pap smear

There is a particular area inside the opening of the cervix where the cells lining the vagina change to the cells that characterize the inside of the cervix (endocervical cells). This is called the "transition zone". The Pap smear sample from the cervical opening attempts to sample these cells. However, this area may be so far up inside the cervix that the Pap smear sampling instrument simply cannot reach that high. Sometimes, the transition zone may be less accessible to the Pap brush or the cervical opening cannot be seen well enough to obtain an adequate sample.

Unreliable Pap smear due to inflammation

If severe inflammation is present, its cause(s) are investigated. The physician's goals are to identify the cause of inflammation and to treat and resolve the condition, if possible. Untreated inflammation can have consequences for the woman as well as her sexual partner(s).

The vaginal irritation may be caused by a lack of estrogen, such as occurs after menopause when the ovaries stop producing this hormone. This lack of estrogen tends to make the vaginal walls irritated and red. If a woman has this condition and it is related to an estrogen deficiency (called "atrophic vaginitis" and usually described on the Pap smear report as "atrophic changes"), her physician may recommend a trial of topical (locally- applied) vaginal estrogen (cream, vaginal estrogen tablets, vaginal estrogen ring) to heal the inflammation. The Pap smear is then repeated.

Atypical squamous cells of undetermined significance (ASC-US)

Sometimes, atypical squamous cells of undetermined significance (also called "ASC-US"), is the Pap report. This is the mildest form of cellular abnormality on the spectrum of cells ranging from normal to cancerous. ASC-US means that the cells appear abnormal, but are not malignant.

"Of undetermined significance" means that the atypical-appearing cells may be the end result of a number of different types of injuries to the cervix. For example, the human papilloma virus (HPV) could be the cause of ASC-US. Most instances of ASC-US (80%-90%) resolve spontaneously (by themselves without specific medical intervention or treatment).

Low-grade squamous intraepithelial lesion (LSIL)

A reading of LSIL is a reason for immediate further investigation because it is more abnormal than ASC-US. Fifteen to 30% of women who have this abnormality on Pap testing will have a more serious abnormality on biopsy of the cervix. Thus, all women with LSIL are recommended to undergo colposcopy (an examination of the cervix using a special visualizing scope, often accompanied by a cervical biopsy, see below). Even LSIL spontaneously returns to normal without therapy in many women within several months.

High-grade squamous intraepithelial lesion (HSIL)

The most severe cellular abnormality that is not actually cancer is high-grade squamous intraepithelial lesion (HSIL). A finding of HSIL unquestionably requires prompt evaluation and treatment.

Medically Reviewed by a Doctor on 1/6/2016

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