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Pap smear
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The Papanicolaou test (also called Pap smear, Pap test, cervical smear, or smear test) is a screening test used in gynecology to detect premalignant and malignant(cancerous) processes in the ectocervix. Significant changes can be treated, thus preventing cervical cancer. The test was invented by and named after the prominent Greek doctor Georgios Papanikolaou, but was also independently invented by Aurel Babes. An anal Pap smear is an adaptation of the procedure to screen and detect anal cancers.
In taking a Pap smear, a tool is used to gather cells from the outer opening of the cervix (Latin for "neck") of the uterus and the endocervix.
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Encyclopedia
The Papanicolaou test (also called Pap smear, Pap test, cervical smear, or smear test) is a screening test used in gynecology to detect premalignant and malignant(cancerous) processes in the ectocervix. Significant changes can be treated, thus preventing cervical cancer. The test was invented by and named after the prominent Greek doctor Georgios Papanikolaou, but was also independently invented by Aurel Babes. An anal Pap smear is an adaptation of the procedure to screen and detect anal cancers.
In taking a Pap smear, a tool is used to gather cells from the outer opening of the cervix (Latin for "neck") of the uterus and the endocervix. The cells are examined under a microscope to look for abnormalities. The test aims to detect potentially pre-cancerous changes (called cervical intraepithelial neoplasia (CIN) or cervical dysplasia), which are usually caused by sexually transmitted human papillomaviruses (HPVs). The test remains an effective, widely used method for early detection of pre-cancer and cervical cancer. The test may also detect infections and abnormalities in the endocervix and endometrium.
It is generally recommended that females who have had sex seek regular Pap smear testing. Guidelines on frequency vary, from annually to every five years. If results are abnormal, and depending on the nature of the abnormality, the test may need to be repeated in three to twelve months. If the abnormality requires closer scrutiny, the patient may be referred for detailed inspection of the cervix by colposcopy. The patient may also be referred for HPV DNA testing, which can serve as an adjunct to Pap testing.
Results
In the United States, most Pap results are normal, however about 2-3 million abnormal Pap smear results are found each year. Most abnormal results are mildly abnormal (ASC-US (typically 2-5% of Pap results) or LSIL (about 2% of results)), indicating HPV infection. Although most low grade cervical dysplasias spontaneously regress without ever leading to cervical cancer, dysplasia can serve as an indication that increased vigilance is needed. Endocervical and endometrial abnormalities can also be detected, as can a number of infectious processes, including yeast and Trichomonas vaginalis. A small proportion of abnormalities are reported as of "uncertain significance".
Typically about 0.5% of Pap results are HSIL, and less than 0.5% of results indicate cancer; typically 0.2 to 0.8% of results indicate AGCUS.
Abnormal results are reported according to the Bethesda system. They include:
- Atypical glandular cells (AGC)
- Atypical squamous cells of undetermined significance (ASCUS)
- Atypical squamous cells of undetermined significance (ASC-US)
- Atypical squamous cells - cannot exclude HSIL (ASC-H)
- Low grade squamous intraepithelial lesion (LGSIL or LSIL)
- High grade squamous intraepithelial lesion (HGSIL or HSIL)
- Squamous cell carcinoma
Effectiveness
Prior to the introduction of the Pap test, carcinoma of the cervix was a leading cause of death in women. Since the introduction of the Pap test, deaths caused by carcinoma of the cervix have been reduced by up to 99% in some populations where women are screened regularly.
Failure of prevention of cancer by the Pap test can occur for many reasons, including not getting regular screening, lack of appropriate follow up of abnormal results, and sampling and interpretation errors. Adenocarcinoma of the cervix has not been shown to be prevented by Pap tests. In the UK, which has a Pap smear screening program, Adenocarcinoma accounts for about 15% of all cervical cancers
Estimates of the effectiveness of the United Kingdom's call and recall system vary widely, but it may prevent about 700 deaths per year in the UK. A medical practitioner performing 200 tests each year would prevent a death once in 38 years, while seeing 152 women with abnormal results, referring 79 for investigation, obtaining 53 abnormal biopsy results, and seeing 17 persisting abnormalities lasting longer than two years. At least one woman during the 38 years would die from cervical cancer despite being screened.
HPV vaccine may offer better prospects in the long term.
Technical aspects
Samples are collected from the outer opening or os of the cervix using an Aylesbury spatula and an endocervical brush, or (more frequently with the advent of liquid-based cytology) a plastic-fronded broom. The broom is not as good a collection device, since it is much less effective at collecting endocervical material than the spatula and brush. The cells are placed on a glass slide and checked for abnormalities in the laboratory.
The sample is stained using the Papanicolaou technique, in which tinctorial dyes and acids are selectively retained by cells. Unstained cells cannot be visualized with light microscopy. The stains chosen by Papanicolaou were selected to highlight cytoplasmic keratinization, which actually has almost nothing to do with the nuclear features used to make diagnoses now.
The sample is then screened by a specially trained and qualified cytotechnologist using a light microscope. The terminology for who screens the sample varies according the country; in the UK, the personnel are known as Cytoscreeners, Biomedical scientists (BMS), Advanced Practitioners and Pathologists. The latter two take responsibility for reporting the abnormal sample which may require further investigation.
Studies of the accuracy of conventional cytology report:
- sensitivity 72%
- specificity 94%
Liquid based monolayer cytology
Since the mid-1990s, techniques based around placing the sample into a vial containing a liquid medium which preserves the cells have been increasingly used. The media are primarily ethanol based. Two of the types are Sure-Path (TriPath Imaging) and Thin-Prep (Cytyc Corp).
Once placed into the vial, the sample is processed at the laboratory into a cell thin-layer, stained, and examined by light microscopy. The liquid sample has the advantage of being suitable for low and high risk HPV testing and reduced unsatisfactory specimens from 4.1% to 2.6%. Proper sample acquisition is crucial to the accuracy of the test; clearly, a cell that is not in the sample cannot be evaluated.
Studies of the accuracy of liquid based monolayer cytology report:
- sensitivity 61% to 66%
- specificity 82% to 91%
Some, but not all studies, report increased sensitivity from the liquid based smears.
Human papillomavirus testing
The presence of HPV indicates that the person has been infected; the majority of women who get infected will successfully clear the infection within 18 months. It is those who have an infection of prolonged duration with high risk types (e.g. types 16,18,31,45) that are more likely to develop Cervical Intraepithelial Neoplasia due to the effects that HPV has on DNA.
Studies of the accuracy of HPV testing report:
- sensitivity 88% to 91% (for detecting CIN 3 or higher) to 97% (for detecting CIN2+)
- specificity 73% to 79% (for detecting CIN 3 or higher) to 93% (for detecting CIN2+)
By adding the more sensitive HPV Test, the specificity may decline. However, the drop in specificity is not definite. If the specificity does decline, this results in increased numbers of false positive tests and many women who did not have disease having colposcopy and treatment. A worthwhile screening test requires a balance between the sensitivity and specificity to ensure that those having a disease are correctly identified as having it and equally importantly those without the disease are not identified as having it. Due to the liquid based pap smears having a false negative rate of 15-35%, the American College of Obstetricians and Gynecologists and have recommended the use of HPV testing in addition to the pap smear in all women over the age of 30.
Regarding the role of HPV testing, randomized controlled trials have compared HPV to colposcopy. HPV testing appears as sensitive as immediate colposcopy while reducing the number of colposcopies needed. Randomized controlled trial have suggested that HPV testing could follow abnormal cytology or could precede cervical cytology examination.
A study published in April 2007 suggested the act of performing a Pap smear produces an inflammatory cytokine response, which may initiate immunologic clearance of HPV, therefore reducing the risk of cervical cancer. Women who had even a single Pap smear in their history had a lower incidence of cancer. "A statistically significant decline in the HPV positivity rate correlated with the lifetime number of Pap smears received."
Automated analysis
In the last decade there have been successful attempts to develop automated, computer image analysis systems for screening. Although, on the available evidence automated cervical screening could not be recommended for implementation into a national screening program, a recent NHS Health technology appraisal concluded that the 'general case for automated image analysis ha(d) probably been made' . Automation may improve sensitivity and reduce unsatisfactory specimens. One of these has been FDA approved and functions in high volume reference laboratories, with human oversight.
Practical aspects
The physician or operator collecting a sample for the test inserts a speculum into the patient's vagina, to obtain a cell sample from the cervix. Pap smears can be performed during a woman's menstrual period, especially if the physician is using a liquid-based test; if bleeding is extremely heavy, endometrial cells can obscure cervical cells, and it is therefore inadvisable to have a pap smear if bleeding is excessive. The patient's perception of the procedure ranges from no discomfort at all to severe discomfort (especially in women with cervical stenosis). Many women experience spotting or mild cramping afterward.
The endocervix may be partially sampled with the device used to obtain the ectocervical sample, but due to the anatomy of this area, consistent and reliable sampling cannot be guaranteed. As abnormal endocervical cells may be sampled, those examining them are taught to recognize them.
The endometrium is not directly sampled with the device used to sample the ectocervix. Cells may exfoliate onto the cervix and be collected from there, so as with endocervical cells, abnormal cells can be recognised if present but the Pap Test should not be used as a screening tool for endometrial malignancy.
Gallery
External links
- - UK's leading cervical cancer charity.
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- - Resource about screening, including that of Cervical Cancer. There are digital atlases of coloscopy, histology and cytology, the rationale of screening and setting up of a screening programme.
- — from the U.S.'s National Cancer Institute
- — from eMedicineHealth
- — from MedlinePlus
- The , held April 30 - May 2, 2001, reviewed issues regarding terminology and reporting of cervical cytology
- , with 349 images of different Pap smear morphologic findings — from the American Society of Cytopathology
- - Society of Obstetricians and Gynaecologists of Canada
- The UK's
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- on cellular pathology
- — from the UK's National Health Service
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