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PAP Smear

How Accurate Are Pap Smear Results?

Last updated on:
23/04/2012

Contributing Author: Guy Slowik FRCS

The Pap smear is the most effective cancer screening method in medical history. But even the most conscientious laboratories sometimes classify normal cell samples as suspicious or overlook abnormalities among the half million cells on each slide.

This may be due to the fact that using the conventional Pap testing technique:

  • as much as 80 percent of every cell sample is discarded when the health care provider throws away the swab or spatula used in collecting it
  • the small fraction of a cell sample that's smeared onto a glass slide and sent to a laboratory for microscopic analysis may not include abnormalities that are present in the discarded cells
  • blood, mucus, and inflammation make it difficult or impossible for technicians to accurately analyze as many as two of every five slides
  • cells or slides can dry out and become distorted.

Therefore more accurate testing methods are being devised and used

Suspicious Test Results

The Pap smear cannot prove that a woman doesn't have cervical cancer. It can detect 95 percent of all cervical cancers and abnormalities like

  • atypical squamous cells of undetermined significance (ASCUS). Vaginal infection or human papillomavirus (HPV) causes these slight abnormalities in the flat (squamous) cells on the surface of the cervix.
  • atypical glandular cells of undetermined significance (AGUS).These slight changes can occur when cells on the surface of the cervix or in the cervical canal become infected or abnormal.
  • low-grade squamous intraepithelial lesion (LSIL). A woman who has this common cervical abnormality is likely to also have anHPV infection, even if she and her sexual partner are monogamous and she has never had any visible warts. Most cases of LSIL can be successfully treated. The condition may get better without treatment, but 30 to 50 percent of women with untreated LSIL develop cervical cancer within five years.
  • high-grade squamous intraepithelial lesions (HSIL). This highly curable condition may consist of benign (noncancerous) abnormalities or cancer cells that have not spread beyond the point where they originated. A doctor who detects HSIL performs colposcopy or a biopsy to more accurately identify the abnormality. HSIL is also known as carcinoma in situ.
  • hyperkeratosis. Dried skin cells sometimes appear on the surface of the cervix of a woman who has had a cervical infection or has used a cervical cap or diaphragm. A doctor who detects hyperkeratosis recommends that the woman have another Pap smear in six months. If hyperkeratosis is still present at that time, the doctor may recommend a third Pap smear in six months or perform colposcopy.

Nice To Know:

Understanding Why Errors Occur

Experts agree that the conventional Pap test has a certain irreducible error rate. That means that even the most conscientious laboratories will sometimes

  • classify a normal smear as suspicious, or
  • fail to detect abnormal cells.

Why do errors occur?

Factors that affect the accuracy of any Pap test include whether the healthcare provider performing the screening

  • collects cells correctly
  • collects an adequate cell sample
  • prepares the microscope slide properly

and whether the laboratory technologist analyzes the cell sample accurately.

False negatives. False negatives incorrectly indicate that the Pap smear is normal even though cancer or precancer is present. Inadequate sampling and improper slide preparation may be responsible for 90 to95 percent of all false negatives. They can also result from failure to recognize or correctly classify abnormal cells

False positives. False positives incorrectly indicate that cancer or precancer is present in a normal cell sample.

Between 10 and 60 percent of all Pap smears are incorrectly analyzed. False negatives, which are far more common than false positives, may be reported 20 and 45 percent of the time. A gynecologist or family doctor who doubts the accuracy of any Pap smear report will repeat the screening and perform any appropriate diagnostic procedures.

Newer Technologies

The United States Food and Drug Administration (FDA) has approved three new technologiesdesigned to

  • improve screening accuracy by changing the way microscope slides are prepared and analyzed
  • eliminate the inconvenience, anxiety, and expense of having to repeat tests when cell samples are compromised

These new technologies are

ThinPrep Pap Smear

The only method approved to replace the conventional Pap smear is the ThinPrep Pap Smear. Unlike rescreening technologies that reevaluate cell samples a technician has classified as abnormal, the ThinPrep Pap changes the way cell samples are placed on a slide and prepared for analysis.

After collecting cells in the usual way, the healthcare provider using the ThinPrep Pap Test

  • rinses the entire collection device into a vial of preserving solution
  • sends the vial to a laboratory for microscopic analysis

At the laboratory

  • the ThinPrep 2000 processor purifies the cell sample by reducing the amount of blood, pus, yeast, bacteria, mucus, inflammation, and other material it contains
  • deposits a thin, even layer of processed cells onto a slide that a cytotechnologist will analyze

Studies indicate that the ThinPrep Pap Test can

  • detect 65 percent more low-grade and more severe abnormalities in the general population
  • detect six percent more abnormalities in women with cervical cancer risk factors
  • reduce the number of inadequate cell samples by more than 50 percent

Cervical cancer experts are debating whether the ThinPrep Pap Test

  • is likely to have a significant impact on cervical cancer prevention
  • is the best way to improve cervical cancer screening technology

AutoPap

This system was initially used as a secondary or rescreening device to evaluate slides technologists had already examined. In 1998, the FDA approved AutoPap for use in screening smears before a doctor or technologist analyzes them.

In one study of more than 25,000 cell samples, this system identified 33 percent more suspicious cells than conventional screening.

AutoPap is the only technology approved for primary Pap screening. Technologists now provide secondary support, examining every smear AutoPap has analyzed.

PapNet

Approved by the FDA for secondary Pap smear screening, PapNet reevaluates cell samples a technologist has classified as abnormal.

This computerized system

  • focuses on areas of the slide where abnormal cells are most likely to be found
  • takes 128 pictures of the cells
  • projects magnified images onto a high-resolution video screen for review by a technician

Nice To Know:

Words of caution

The American College of Obstetricians and Gynecologists has

  • recognized that the new technologies are more sensitive than the conventional Pap test and better able to identify abnormalities
  • declined to recommend routine use of the new Pap test technologies
  • cautioned that the new technologies are not the current standard of care in cervical cancer screening
  • emphasized that it is having regular Pap tests - not the screening method used - that is most responsible for decreasing a woman's risk of developing cancer that will spread and destroy surrounding tissues (invasive cancer)

Nice To Know:

The HPV test

Approved by the FDA in 1999, the Hybrid Capture II test may

  • Identify strains of human papillomavirus (HPV) that can lead to cervical cancer.
  • clarify uncertain Pap smear results
  • provide prompt reassurance to women whose tests indicate that slight changes have occurred in the flat cells on the surface of the cervix (ASCUS)
  • eliminate the need for repeat screening and colposcopy
  • help doctors determine the best course of treatment for women with abnormal Pap smear results
 
 

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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.