How Accurate Are Pap Smear Results?Monday, April 23, 2012 - 13:05
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.
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
squamous cellsof 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
colposcopyor a biopsyto more accurately identify the abnormality. HSIL is also known as carcinomain 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
Why do errors occur?
Factors that affect the accuracy of any Pap test include whether the healthcare provider performing the screening
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.
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
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
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.
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
Nice To Know:
The HPV test
Approved by the FDA in 1999, the Hybrid Capture II test may