Cervical Screening Programme
Department of Health
The Government of the Hong Kong Special Administrative Region

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(i) Conventional Smear

(ii) Liquid-based Cytology

(iii) PathFinder

(iv) AutoPap Primary Screening System

(v) HPV DNA Testing

(i)

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.

 

 

(ii)

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.

 

 

(iii)

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%

 

 

(iv)

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%

 

 

(v)

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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