出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/07/07 12:53:25」(JST)
| Pap test | |
|---|---|
| Intervention | |
| ICD-9-CM | 795.00 |
| MeSH | D014626 |
The Papanicolaou test (also called Pap smear, Pap test, cervical smear, or smear test) is a screening test used to detect potentially pre-cancerous and cancerous processes in the endocervical canal (transformation zone) of the female reproductive system. Changes can be treated, thus preventing cervical cancer. The test was invented by and named after the prominent Greek doctor Georgios Papanikolaou.
In taking a Pap smear, a speculum is used to open the vaginal canal and allow the collection of cells from the outer opening of the cervix 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. 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.
In general, in countries where Pap smear screening is routine, it is recommended that females who have had sex seek regular Pap smear testing. Guidelines on frequency vary from every three to five years.[1][2][3] If results are abnormal, and depending on the nature of the abnormality, the test may need to be repeated in six to twelve months.[4] 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. Additional biomarkers which may be applied as ancillary test with Pap test are evolving. [5]
Contents
|
In addition, an HPV test may be performed either as indicated for abnormal Pap results, or in some cases dual testing is done, where both a Pap smear and HPV test are done.
For information on other cervical screening tests and Human Papillomavirus testing, see cervical screening.
Screening guidelines vary from country to country. In general, screening starts about the age of 20 or 25 and continues until about the age of 50 or 60.[6] Screening is typically recommended every three to five years, as long as results are normal.[2][7]
Women should wait a few years after they first have intercourse before they start screening, and should not be screened before age 21. American Congress of Obstetricians and Gynecologists (ACOG) and others recommend starting screening at age 21 (since that is a few years after initial sex for most American women).[1][8] Many other countries wait until age 25 or later to start screening. For instance, some parts of Great Britain start screening at age 25.
Most women who contract HPV do so soon after becoming sexually active.[3][citation needed][dead link] It takes an average of a year, but can take up to four years, for a woman's immune system to control the initial infection. Screening during this period may show this immune reaction and repair as mild abnormalities, which are usually not associated with cervical cancer, but could cause the woman stress and result in further tests and possible treatment. Cervical cancer usually takes time to develop, so delaying the start of screening a few years poses little risk of missing a potentially precancerous lesion. For instance, screening women under age 25 does not decrease cancer rates under age 30.[9]
There is little or no benefit to screening women who have not had sexual contact. For example, United States Preventive Services Task Force (USPSTF) recommends waiting at least three years after first sex.[2] HPV can be transmitted in sex between women, so women who have only had sex with other women should be screened, although they are at somewhat lower risk for cervical cancer.[10]
Guidelines on frequency of screening vary—typically every three to five years for those who have not had previous abnormal smears.[2][7] Some older recommendations suggested screening as frequently as every one to two years, however there is little evidence to support such frequent screening; annual screening has little benefit but leads to greatly increased cost and many unnecessary procedures and treatments.[1] It has been acknowledged since before 1980 that most women can be screened less often.[11] In some guidelines, frequency depends on age; for instance in Great Britain, screening is recommended every 3 years for women under 50, and every 5 years for those over.
Screening should stop about age 65 unless there is a recent abnormal tests or disease. There is probably no benefit screening women aged 60 or over whose previous tests have been negative.[12] If a woman's last three Pap results were normal, she can stop at age 65, according to the USPSTF, ACOG, ACS and ASCP;[2][1] England's NHS says 64. There is no need to continue screening after a complete hysterectomy for benign disease.
Pap smear screening is still recommended for those who have been vaccinated against HPV,[7] since the vaccines do not cover all of the HPV types that can cause cervical cancer. Also, the vaccine does not protect against HPV exposure before vaccination.
More frequent Pap smears may be needed to follow-up after an abnormal Pap smear, or after treatment for abnormal Pap or biopsy results, or after treatment for cancer.
For best results, a Pap test should not occur when a woman is menstruating. However, 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.
Getting a pap smear should not cause pain,[13] but it can if the patient has certain untreated vaginal problems such as cervical stenosis or vaginismus, or if the person performing it is too harsh, or uses the wrong size speculum. The patient should speak up if they are in pain. Many women experience spotting or mild diarrhea afterward.
Many health care providers are under the false impression that only sterile water, or no lubricant at all, should be used to lubricate the speculum. This may result in unnecessary discomfort. A number of studies have shown that using a small amount of water-based gel lubricant does not interfere with, obscure, or distort the PAP smear. Further, cytology is not affected nor some STD testing. [14]
The health care worker begins by inserting a speculum into the woman's vagina, which spreads the vagina open and allows access to the cervix. The health care provider then collects a sample of cells from the outer opening or os of the cervix by scraping it with an Aylesbury spatula. An endocervical brush is rotated in the central opening of the cervix. The cells are placed on a glass slide and taken to the laboratory to be checked for abnormalities.
A plastic-fronded broom is sometimes used in place of the spatula and brush. The broom is not as good a collection device, since it is much less effective at collecting endocervical material than the spatula and brush.[15] The broom is used more frequently with the advent of liquid-based cytology, although either type of collection device may be used with either type of cytology.
The sample is stained using the Papanicolaou technique, in which tinctorial dyes and acids are selectively retained by cells. Unstained cells cannot be seen with a light microscope. Papanicolaou chose stains that highlighted cytoplasmic keratinization, which actually has almost nothing to do with the nuclear features used to make diagnoses now.
In some cases, a computer system may prescreen the slides, indicating those that do not need examination by a person or highlighting areas for special attention. The sample is then usually screened by a specially trained and qualified cytotechnologist using a light microscope. The terminology for who screens the sample varies according to 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.
In the last decade, there have been successful attempts to develop automated, computer image analysis systems for screening.[16] 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'.[17] Automation may improve sensitivity and reduce unsatisfactory specimens.[18] Two systems have been approved by the FDA and function in high-volume reference laboratories, with human oversight.[citation needed]
In screening a general or low-risk population, most Pap results are normal.
In the United States, about 2-3 million abnormal Pap smear results are found each year.[19] Most abnormal results are mildly abnormal (ASC-US (typically 2-5% of Pap results) or low-grade squamous intraepithelial lesion (LSIL) (about 2% of results)), indicating HPV infection.[citation needed] 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.
In a typical scenario, about 0.5% of Pap results are high-grade SIL (HSIL), and less than 0.5% of results indicate cancer; 0.2 to 0.8% of results indicate Atypical Glandular Cells of Undetermined Significance (AGC-NOS).[citation needed]
As liquid based preparations (LBPs) become a common medium for testing, atypical result rates have increased. The median rate for all preparations with low-grade squamous intraepithelial lesions using LBPs was 2.9% compared with a 2003 median rate of 2.1%. Rates for high-grade squamous intraepithelial lesions (median, 0.5%) and atypical squamous cells have changed little.[20]
Abnormal results are reported according to the Bethesda system.[21] They include:
Endocervical and endometrial abnormalities can also be detected, as can a number of infectious processes, including yeast, herpes simplex virus and trichomoniasis. However it is not very sensitive at detecting these infections, so absence of detection on a Pap does not mean absence of the infection.
Since the introduction of the Pap test, deaths caused by carcinoma of the cervix have been reduced by up to 99% in some populations wherein women are screened regularly.[22][dubious – discuss] A regular program of pap smear screening, with appropriate follow-up, can reduce cervical cancer incidence by up to 80%.[7]
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.[22] In the US, over half of all invasive cancers occur in women that have never had a Pap smear; an additional 10 to 20% of cancers occur in women that have not had a Pap smear in the preceding five years. About one-quarter of US cervical cancers were in women that had an abnormal Pap smear, but did not get appropriate follow-up (woman did not return for care, or clinician did not perform recommended tests or treatment).
Adenocarcinoma of the cervix has not been shown to be prevented by Pap tests.[22] In the UK, which has a Pap smear screening program, Adenocarcinoma accounts for about 15% of all cervical cancers[23]
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.[24]
Since the population of the UK is about 61 million, the maximum number of women who could be receiving Pap smears in the UK is around 15 million to 20 million (eliminating the percentage of the population under 20 and over 65). This would indicate that the use of Pap smear screening in the UK saves the life of 1 person for every approximately 20,000 people tested (assuming 15,000,000 are being tested yearly). If only 10,000,000 are actually tested each year, then it would save the life of 1 person for every approximately 15,000 people tested.
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.
The test was invented by and named after the prominent Greek doctor Georgios Papanikolaou. Aurel Babeş of Romania independently made similar discoveries in 1927.[25]
In the developed world result of cervical biopsy guided by colposcopy is the "gold standard" for diagnosing cervical abnormalities after an abnormal pap smear. The procedure requires a trained colposcopist and can be expensive to perform. However, Pap smears are very sensitive and some negative biopsy results may represent undersampling of the lesion in the biopsy, so negative biopsy with positive cytology requires careful follow up.[26]
Experimental visualization techniques use broad-band light (e.g., direct visualization, speculoscopy, cervicography, visual inspection with acetic acid or with Lugol's, and colposcopy) and electronic detection methods (e.g., Polarprobe and in-vivo Spectroscopy). These techniques are less expensive and can be performed with significantly less training. They do not perform as well as Pap smear screening and colposcopy. At this point, these techniques have not been validated by large-scale trials and are not in general use.
An anal Pap smear is an adaptation of the procedure to screen and detect anal cancers.
Endocervical adenocarcinoma on a pap test.
Candida organisms on a pap test.
Viral cytopathic effect consistent with herpes simplex virus on a pap test.
Normal squamous epithelial cells in premenopausal women
Atrophic squamous cells in postmenopausal women
Normal endocervical cells should be present into the slide, as a proof of a good quality sampling
the cytoplasms of squamous epithelial cells melted out; many Döderlein bacilli can be seen
Infestation by Trichomonas vaginalis
An obviously atypical cell can be seen
|
||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
| 関連記事 | 「smear」 |