Cervical Screening Programme
Department of Health
The Government of the Hong Kong Special Administrative Region
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(i)
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Conventional Smear
George Papanicolaou introduced cervical cytology into clinical
practice in 1940. Cervical smear (Pap smear) is a simple test that
can detect early abnormal changes in the cells of the cervix. Smear
collection instruments include the wooden or plastic Ayre's spatula
and the broom type. The specimens are transferred to glass slides
and fixed with spray to avoid air drying. The specimens are then
reviewed by trained professionals. Sensitivity of cervical smear
for detection of high-grade squamous intraepithelial lesion (SIL)
is in the range of 70-80%. Specificity of cervical smear has been
reported to be higher than 90%. Factors that limit test sensitivity
include small size of the lesion, inaccessible location of the lesion,
the lesion not being sampled or the presence of inflammation and/or
blood obscuring cell visualization.
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(ii)
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Liquid-based Cytology
Thin layer smears are collected with a non-absorbent instrument
such as Cervex brush. Instead of being smeared onto glass slides,
the cells are rinsed into an alcohol-bases solution. The advantages
are elimination of drying changes, less unsatisfactory smear, speeding
up microscopic exam because blood, mucus and inflammatory cells
are removed during processing. Moreover, the cells are deposited
in a thin-layer located within a circle of 13-20mm diameter instead
of over a large area in conventional smear. Studies found that the
detection rate for SIL and cancers are superior to conventional
smear. However, it is more expensive. Examples of liquid based cytology
are Thin-prep or Autocyte prep.
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(iii)
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PathFinder
This is a computer-based device consisting of a computer and a
small monitor that are attached to the microscope. A 'map' is created
when the screener examine a smear. If the screener has a tendency
to overlook area on the slide, it could be detected and corrective
action could then be taken. This system also allows the electronic
tagging and labeling of abnormal cells for storage and later retrieval.
Screening efficiency can be improved by up to 15%
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(iv)
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AutoPap Primary Screening System
It is a system using high-resolution scanner and high-speed video
microscope to obtain cell images from conventional cervical smears.
The digitized images are processed with image interpretation software.
Slides containing high probability of abnormal cells are reviewed
by cytologist manually. This instrument may shorten the screening
time for cervical smears by about 50%
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(v)
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HPV DNA Testing
The effectiveness of HPV DNA testing in mass screening of cervical
cancer has not been established. HPV testing is more sensitive than
conventional cytology for high-grade CIN, but has lower specificity,
especially in young women. The sensitivity of HPV testing alone,
for detection of high-grade CIN or worse can be as high as 90% and
specificity at 50-70%. Detection of low-grade CIN by HPV DNA testing
has sensitivity of 73% and specificity of 62%.
High risk HPV can be found in around 50% of ASCUS and 82-85% of
LSIL. According to the HKCOG guidelines on management of abnormal
cervical smears, HPV typing in triaging patients with ASCUS is an
alternative to repeat cytology at 6 months in decision for colposcopy
referral. On the other hand, HPV typing has limited advantage in
triaging patients with LSIL for colposcopy because over 80% of LSIL
has high risk HPV. The use of HPV typing alone in primary screening
for cervical cancer is not recommended.
Therefore, HPV DNA testing can only be used as an adjunct test
to cervical smear but is not recommended as a standalone test for
cervical cancer screening.
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