Innovations in the Pap Test
In the 1940s, Dr. George Papanicolaou revolutionized women’s healthcare with the introduction of the Pap test—a test that identifies precancerous cells in the cervix. Since its introduction, this simple screening tool has been credited with a 79% reduction in deaths from invasive cervical cancer in the US.
Mechanics of the Pap Test
During a conventional pap test , which takes place during a normal pelvic exam, the physician removes cells from the cervix and/or vagina with a brush or spatula. The cells are smeared on a slide, which is then sprayed with an ethanol-based fixative to help preserve the cells. The slide is sent to a laboratory to look for any signs of abnormality. Abnormal cells can predict a higher risk of progression to invasive cervical cancer , leading to intervention strageies that can prevent this form of cancer and even save a woman's life.
Accuracy of the Pap Test
Although the Pap test is a successful screening tool, it is not accurate 100% of the time, which is normal for a screening test. A screening test is a test given to populations of people without any symptoms or signs; it casts a wide net and finds many "positives" who do in fact have the disease. In published studies reviewing the Pap test, the sensitivity (ability to detect a known cancer or precancerous condition) ranged from 50% to 80%. On the other hand, screening tests should find almost everyone who actually does have the disease. This property of a test is called "specificity."
When the Pap test fails to detect a cancer this is called a “false-negative” reading. In a false-negative reading, a woman with significant cell abnormality will have her cell sample labeled as normal. This can delay the diagnosis and treatment of a precancerous condition, with potentially dire consequences. The specificity of the Pap test is in the area of 98%-99%, which means the false negative rate is 1%-2%. This refers to missing any abnormality, of which very few are cancer. In an average population of patients, the probability of a Pap testing failing to identify an existing cervix cancer is less than 1 in 10,000.
Although some errors occur at the laboratory when the slide is read, more often errors occur when the sample is drawn and affixed to the slide. This is called a sampling error. Ways in which sampling errors are introduced include:
- Not transferring the entire specimen obtained from the patient to the slide
- Inadequate fixation of the cell sample to the slide
- Random distribution of abnormal cells in the sample
- Obscuring elements within the sample, such as blood, extraneous material, or inflammatory cells
- Variability in the quality of the smear
To address these limitations, a new technique was developed. Known as the Liquid Based Technology (ThinPrep, SurePath, AutoCyte Prep, TriPath), this method uses a liquid-based, thin-layer technology for obtaining and preparing cervical cell samples for screening.
How the Liquid-based Method Is Different
Ninety-three percent of liquid based paps done in the US use the Cytyc product, ThinPrep. During the ThinPrep test, the cervical or vaginal cells are removed in the same manner as with the standard Pap test, but the physician does not smear them on a slide. Instead, the collection device is rinsed or deposited into a vial of fixative. The vial is sent to a laboratory where it is filtered or centrifuged to remove blood and extraneous material. The cells are then mechanically mixed to create a more homogeneous sample. This means that if abnormal cells are present they will now be evenly distributed throughout the sample. The process of affixing the cells to a slide is completely automated.
A meta-analysis completed in 2001 reviewed 25 studies that compared the ThinPrep technique to the conventional Pap test. The researchers found that the ThinPrep Pap test improved sample adequacy and led to improved diagnosis of both low-grade and high-grade lesions. (High-grade lesions are potentially more serious and may need more rapid and aggressive treatments to prevent the onset of invasive cancer.)
Although the ThinPrep’s collection method addresses several of the Pap test’s limitations, the ThinPrep has some disadvantages, including:
- Preparation of the slides for screening is more labor intensive.
- Reading the slides requires retraining of the technicians and pathologists because this preparation technique causes cells to look somewhat different.
- The cost of the test is $15 to $25 higher for ThinPrep than for traditional Pap tests.
- Insurance coverage for this type of screening is variable. Some insurance companies will cover tests for all, some will only cover tests for those with a history of prior abnormal paps, and some will require the patient to cover the cost of the test if she elects this type of screening.
One important advantage of the ThinPrep is its ability to test the sample for both low- and high-risk strains of human papillomavirus (HPV) , a sexually transmitted disease that is the primary risk factor for cervical cancer.
With both methods, if abnormal cells are identified, the slide is sent to a pathologist for diagnosis. Abnormal cells are not necessarily cancerous—in fact, abnormal cells are very rarely cancerous. However, if your Pap test results are abnormal, it is important that you follow up with your physician.
Liquid Based Prep or Conventional Pap Test?
Should clinicians switch to the ThinPrep method because of its improved diagnostic ability? Although the ThinPrep does have a lower false-negative rate, this has not yet been shown to translate into a reduction in death and suffering from cervical cancer. Importantly, most women who develop cervical cancer have never had a Pap test or have not had one in three to five years. So, although accuracy is an important issue to address, it doesn’t matter how accurate a test is if a woman is not taking advantage of it.
As of November 2005, the prestigious US Preventive Services Task Force (USPSTF) does not recommend traditional Pap screening be replaced by liquid-based cytology. The USPSTF uses cost-effectiveness as a basis for their recommendations: the conventional Pap costs approximately $25-$40, while the ThinPrep costs approximately $45-$60. The USPSTF does not recommend against use of liquid prep pap, categorizing this as “I” for insufficient evidence to support a recommendation. Although the USPSTF can continue its debate, patients and their providers must decide today what screening method they want to use. The current trend in the US today is moving toward the liquid prep test.
When Should You Have a Pap Test?
The American Medical Association (AMA) and the USPSTF recommends the following guidelines for Pap tests:
- Annual Pap test starting at age 21
- Annual Pap test within 3 years of the onset of sexual activity, if earlier than age 21
- After 3 or more normal annual Pap Tests, the Pap test may be performed less frequently at 2-3 year intervals
How to Prepare
No matter which method you chose, some important things you should know before your Pap test include:
- Do not schedule your Pap test during your period. The best time to have one is between 10 and 20 days after the first day of your last period.
- Do not have sexual intercourse within 24 hours of your Pap test.
- Do not douche or use vaginal creams, suppositories, foams, or vaginal medications for two days before your Pap test.
- Do not use vaginal deodorant sprays or powders two days before your Pap test.
American Cancer Society
American College of Obstetrics and Gynecology
National Cancer Institute
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Felix JC, Amezcua C. In vitro adjuncts to the Pap smear. Obstetrics and Gynecology Clinics. 2002;29:685-99,vii.
Honebrink A. What’s a thin prep and do I need it? Women’s health issues and you. American College of Obstetricians and Gynecologists. 2002;1:2-4.
Kistner’s Gynecology and Women’s Health. 7th ed. St. Louis, MO: Mosby; 1999.
The Pap test: questions and answers. National Cancer Institute website. Available at: http://cis.nci.nih.gov/fact/5_16.htm . Accessed May 12, 2003.
Last reviewed June 2007 by Jeff Andrews, MD, FRCSC, FACOG
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
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