Pap smear and liquid-based cytology (LBC) for cervical screening

Last edited 11/2020 and last reviewed 11/2020

Cytological abnormalities can be diagnosed by taking a cervical smear, fixing it with alcohol and then examining it applying Papanicolaou's stain (Pap smear). Cells can be obtained by scraping a spatula across the transformation zone and rotating it through 360 degrees. A variety of spatulas can be used, e.g. Aylesbury wooden spatula, but to be effective, cells from both the ecto- and the endo-cervix must be included.

However NICE have recommended that liquid-based cytology (LBC) is used as the primary means of processing samples in the cervical screening programme in England and Wales (1). LBC was implemented throughout the UK in October 2008 and is the current standard method of screening in the NHS cervical screening programme (2). In this now preferred method:

• samples are collected in the usual way, but using a brush-like device rather than a spatula
• the head of the device is rinsed or broken off into a vial of preservative fluid so that most or all of the cervical cells are retained
• samples are transported to the laboratory where they are mixed to disperse the cells - cellular debris, such as blood or mucus, is removed and a thin layer of cervical cells is deposited on a microscope slide, which is then stained

Potential advantages of the LBC method include:

• an improved means of slide preparation
  • about 8% (range of 5.9-11.0%) of Pap smear tests are inadequate - cannot be interpreted because of problems with sample collection or preparation (such as insufficient cervical cells), or the presence of inflammatory cells, mucus or blood which obscure the sample. In the economic analysis included in the NICE guidance there was an assumption of a reduction in the rate of inadequate samples from 9% with the Pap smear to 1.4% with LBC
• producing more homogeneous samples than the Pap smear (which may make slides easier to read)
• increased sensitivity and specificity
  • a meta-analysis comparing the sensitivity of LBC and the Pap smear in the detection of abnormalities of low-grade squamous intraepithelial lesions or greater demonstrated that sensitivity may be up to 12% better with LBC compared with the Pap smear. In the economic analysis included in the NICE guidance there was an assumption of sensitivity improvements with LBC relative to Pap of 13.4% for the detection of CIN1 and CIN2 combined and 4% for the detection of CIN3
• improved efficiency of handling laboratory samples, resulting in increased laboratory productivity

Reference:

• (1) NICE (November 2007). Liquid-based cytology for cervical screening
• (2) NHS Cancer Screening Programmes 2010. Colposcopy and programme management. Guidelines for the NHS cervical screening programme 2nd edition